Olivier Capra
email: olivier.capra@univ-lille.fr

Florent Berthaut
email: florent.berthaut@univ-lille.fr

Laurent Grisoni
email: laurent.grisoni@univ-lille.fr

Have a SEAT on Stage : Restoring Trust with Spectator Experience Augmentation Techniques

Keywords: interactions, spectator experience, visual augmentations, design for public interactions, human machine collaboration, CCS Concepts, Human-centered computing → Human computer interaction (HCI), Laboratory experiments, Mixed / augmented reality, •Applied computing

When the collaboration between humans and machines happens in public, the audience can face difficulties in distinguishing the actual human contribution from the contribution of autonomous processes. In music concerts involving digital interfaces doubts about the performer's contribution can drastically hinder the audience interest. The disappearing of the direct physical link between actions and effects is one of the reasons of this confusion. Consequently both artists and researchers have explored techniques to augment the experience of spectators. However their respective impact on the multiple aspects of audience experience has not yet been formally compared. In this controlled study, we compare two techniques : pre-performance explanations and visual augmentations. Despite contradictory results on comprehension tasks, we show that contrary to pre-performance explanations, visual augmentations improve the audience experience, increase their subjective comprehension and restore the trust in performers by reversing the doubt in their favour.

INTRODUCTION

When the collaboration between humans and machines happens in public, the audience can face difficulties in distinguishing the actual human contribution from the contribution of autonomous processes. In artistic performances involving digital interfaces, and especially in music concerts, doubts about the performer's involvement can drastically hinder the audience interest [START_REF] Andrew | Using contemporary technology in live performance: The dilemma of the performer[END_REF][START_REF] Stuart | The object of performance: Aural performativity in contemporary laptop music[END_REF]. In the HCI community, the question of the audience experience during such performances is growing First, the continuous technological improvements in Digital Musical Instruments (DMIs) dissolved the physical constraints in the production of sound, offering musicians almost infinite creative possibilities. With the multiplication of these software instruments, musicians became their own instrument makers, amplifying the diversity of interfaces and gesture to sound mappings leading to better intimacy or expressivity for the musicians [START_REF] Jordà | Digital instruments and players: part I-efficiency and apprenticeship[END_REF][START_REF] Wessel | Problems and prospects for intimate musical control of computers[END_REF]. Furthermore, the arrival of computers on stage relieved the artists of the physical requirements, allowing for expressive gestures with less amplitude, if not hidden [START_REF] Gurevich | Digital musical interactions: Performer-system relationships and their perception by spectators[END_REF][START_REF] Cadoz | Gesture-music[END_REF][START_REF] Sm Astrid Bin | Hands where we can see them! investigating the impact of gesture size on audience perception[END_REF]. From an audience perspective, these mutations of the intuitive link between actions and effects [START_REF] Reeves | Designing interfaces in public settings: Understanding the role of the spectator in Human-Computer Interaction[END_REF] have multiple consequences. A disturbed perception can lower the transmission of emotions [START_REF] Dahl | Visual perception of expressiveness in musicians' body movements[END_REF], disrupt the anticipation and its related pleasure [START_REF] Brian | Sweet anticipation: Music and the psychology of expectation[END_REF] and therefore hinder the experience of live music. Thus, less constrained by the laws of physics, musicians face new challenges to better expose their skills and intentions when on stage.

A second issue lies in the very interest of attending a live artistic performance : the singularity of seeing and hearing artists expressing themselves doing things we are not able to do. If a performance with digital devices gives the impression that everything could flow the same way whether or not the performer is involved, the audience may doubt their actual contribution. The risk is to make the performance as interesting as watching people "doing their taxes" (David Zicarelli, founder of Cycling '74, a major music software company, reported in [START_REF] Andrew | Using contemporary technology in live performance: The dilemma of the performer[END_REF]). Beyond the simple interest, the experience of flow, that is the total immersion in the performance [START_REF] Csikszentmihalyi | Flow: The Psychology of Optimal Experience[END_REF], also requires to perceive the artistic demand of the performance and the skills of the artist [START_REF] Csikszentmihalyi | Flow: The Psychology of Optimal Experience[END_REF][START_REF] Kennedy | The cognitive antecedents and motivational consequences of the feeling of being in the zone[END_REF][START_REF] Rheinberg | Flow experience in a computer game under experimentally controlled conditions[END_REF].

Artists and researchers alike have explored solutions to the degradation of the audience experience. In particular, they have proposed techniques to augment performances without constraining the design of performance interfaces. These techniques range from pre-performance demos [START_REF] Sma Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF] to augmentedreality displays [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF] and provide various types of information about the instrument, its mechanisms and the ongoing inter-actions of the musician. While they constitute a promising solution to the issue of audience experience with digital performances, these Spectator Experience Augmentation Techniques (SEATs) have not yet been formally compared. Therefore, their respective effect on the many aspects of the spectators experience remains unclear.

In this paper, we compare two of these SEATs, i.e. preperformance demo and visual augmentations, through a controlled study, in the context of musical performances. We investigate their effect on multiple aspects of the audience experience, including subjective and objective comprehension, especially looking at the perception of the performer's contribution. From the results, we derive insights which can inform both the design of future techniques and the evaluation of the audience experience with digital performances.

Related work Spectator experience augmentation techniques (SEATs)

A first strategy for improving the audience experience with digital performances is to design the performers interfaces, e.g. musical instruments, to maximise their propensity to make the observers intuitively understand their behaviour and potentialities, aka the transparency [START_REF] Fels | Mapping transparency through metaphor: towards more expressive musical instruments[END_REF]. However, this may require the performers to restrain their expressivity with respect to all potential combinations of gestures, sensors, mappings and content. These restrictions can be lifted by audiovisual interfaces designed to expose the instrument capabilities and facilitate the comprehension of both musicians and audience. This approach shows interesting benefits in the case of DMIs [START_REF] Jordà | Interactive music systems for everyone: exploring visual feedback as a way for creating more intuitive, efficient and learnable instruments[END_REF][START_REF] Nuno | AVUI: Designing a toolkit for audiovisual interfaces[END_REF] but it adds constraints to instrument design outside the sole musician's expressive needs. A second strategy explored by both artists and researchers is to augment the performance with information that complements the audience experience, without modifying the performer's interface. This augmentation can be done for various goals regarding the audience experience, such as increasing the audience enjoyment, engagement or comprehension, e.g. turning magical interfaces into expressive ones [START_REF] Reeves | Designing interfaces in public settings: Understanding the role of the spectator in Human-Computer Interaction[END_REF]. It may also take many forms.

Visual feedback can be projected behind the performers [START_REF] Perrotin | Visualizing Gestures in the Control of a Digital Musical Instrument[END_REF], overlapped with the instrument using an AR display [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF], or provided both on stage and on mobile devices [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF]. In a precedent work, we proposed a pipeline based on the integration of physiological data to select relevant visual augmentation [START_REF] Capra | Toward Augmented Familiarity of the Audience with Digital Musical Instruments[END_REF]. Haptic feedback can also be used to amplify musicians' gestures [START_REF] Armitage | Revealing timelines: Live coding and its gestures[END_REF][START_REF] Turchet | Smart Mandolin and Musical Haptic Gilet: effects of vibro-tactile stimuli during live music performance[END_REF]. Other approaches involve pre-concert explanations of the instrument [START_REF] Sma Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF][START_REF] Cavan Fyans | Examining the Spectator Experience[END_REF] or of the performer's intention [START_REF] Cavan Fyans | Examining the Spectator Experience[END_REF]. Finally, common techniques developed by artists include tilting their interface towards the audience and projecting a close-up of their gestures or their graphical user interface.

Although these techniques may definitely benefit the spectators of digital performances, to our knowledge, their efficiency has not been formally compared and no insights have been provided about the components of the audience experience that they respectively affect.

Measuring the audience experience

In the study of audience experience, most research has relied on an "in the wild" approach [START_REF] Chamberlain | Research in the wild: understanding'in the wild'approaches to design and development[END_REF][START_REF] Benford | Performance-led research in the wild[END_REF], utilising observations, post-performance questionnaires and guided interviews of spectators. Different approaches are proposed in terms of methodology (how they measure) and of the dimensions they address (what they measure).

Most contributions rely on pre/post-performance interviews and questionnaires to address components of the audience experience such as enjoyment [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF][START_REF] Sm Astrid Bin | In-the-moment and beyond: Combining post-hoc and real-time data for the study of audience perception of electronic music performance[END_REF], comprehension [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF], error perception [START_REF] Sm Astrid Bin | In-the-moment and beyond: Combining post-hoc and real-time data for the study of audience perception of electronic music performance[END_REF], emotional and social aspects [START_REF] Brown | Assessing the intrinsic impacts of a live performance[END_REF] and also to gather improvement suggestions [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF]. Additionally, real time measurements can be used to provide feedback during the performance [START_REF] Sm Astrid Bin | In-the-moment and beyond: Combining post-hoc and real-time data for the study of audience perception of electronic music performance[END_REF].

Similarly, augmentation techniques have mostly been evaluated "in the wild" by organising performances to study the impact of pre-concert explanations on error perception and enjoyment [START_REF] Sma Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF] or to better understand how different visual augmentations affect audience understanding [START_REF] Perrotin | Visualizing Gestures in the Control of a Digital Musical Instrument[END_REF][START_REF] Nuno N Correia | The role of live visuals in audience understanding of electronic music Performances[END_REF]. While this method in the field provides valuable information on some aspects of the audience experience, it also contains biases, such as the inherent differences between two runs of the same performance or the effect of the social context, which makes it difficult to precisely compare two techniques.

In contrast, few "in the lab" experiments have been conducted on audience experience. They all rely on videos of performances to address components of the experience. Questionnaires can be used to investigate the perceived causality and the attributed agency (the spectator's appreciation of the level of control of the musician) [START_REF] Berthaut | Liveness through the lens of agency and causality[END_REF], to evaluate the error perception [START_REF] Cavan Fyans | Spectator understanding of error in performance[END_REF] or to rate the perceived tension [START_REF] Mehmet Aydin Baytas | The Perception of Live-sequenced Electronic Music via Hearing and Sight[END_REF]. Beyond such subjective reports, physiological measurement can provide objective insights like the level of engagement of spectators watching dance performances [START_REF] Latulipe | Love, hate, arousal and engagement: exploring audience responses to performing arts[END_REF]. The integration of physiological sensors is a promising way to gather objective data about the audience experience [START_REF] Wang | The Play Is a Hit: But How Can You Tell?[END_REF][START_REF] Capra | Toward Augmented Familiarity of the Audience with Digital Musical Instruments[END_REF][START_REF] Stevens | Methods for measuring audience reactions[END_REF]. However, the time resolution, the many sources of interference and the variety of interpretation of physiological data are still obstacles to robust applications.

Lab experiments are by definition far away from the concert hall. Thus the expression "audience experience" may be a bit misleading, especially when the experiment involves one participant at a time. The full experience of standing with dozens of people around, participating in the same event in synchronisation, is an important part of the experience that is de facto neglected in lab studies. We believe that these rich aspects are more grounded in the field of sociological studies, the interested reader is invited to read the related literature [START_REF] Páez | Collective emotional gatherings. Five studies of identity fusion, shared beliefs and social integration[END_REF][START_REF] Duffett | Applying durkheim to elvis[END_REF][START_REF] Brown | Assessing the intrinsic impacts of a live performance[END_REF] on the subject. Following these observations, we believe that the formal comparison of the effects of augmentation techniques on the audience experience can benefit from a controlled approach. We advocate for an "in-the-lab" methodology as it allows for a balanced evaluation of experimental conditions, precise measurements of the comprehension dimension and consequent refined analyses.

Contribution

In this work, we evaluate the impact of two augmentation techniques, preliminary explanations and visual augmenta- tions, on the experience of spectators watching a performance with a Digital Musical Instrument (DMI). Our approach draws from field studies and addresses the effects of what we define as Spectator Experience Augmentation Techniques (SEATs) on multiple aspects of audience experience, from subjective ratings to objective comprehension tasks. From the results, we provide insights on the design of SEATs and their effect on experience and comprehension. We also propose a new design challenge which can be used to evaluate the spectators experience.

STUDY : COMPARISON OF TWO AUGMENTATION TECH-NIQUES FOR MUSIC PERFORMANCES

In this section, we describe the protocol used in this study. The video stimuli and the experimental conditions are presented, as well as the components of the experience we address. For the sake of clarity, the presentation of the tasks is structured by experience components rather than the chronological order. Following the general presentation of the protocol, methodological details of each component and the associated results are presented and briefly discussed one by one.

Overview

In this study we evaluate two Spectator Experience Augmentation Techniques (SEATs) : preliminary explanations of the instrument and visual augmentations. For the participants, it mostly consists in watching videos and answering subsequent questions. The experiment lasts one hour including consents signature, equipment setup and interviews. As shown on Figure 1, the experiment is composed of a familiarisation phase and four experimental blocks. Throughout the blocks, objective and subjective aspects of the spectator experience are addressed.

Hypotheses

Following the work of Berthaut [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF][START_REF] Berthaut | Liveness through the lens of agency and causality[END_REF] on visual augmentations, and the conclusions of Bin [START_REF] Sma Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF] that even if explanations can familiarise the audience with a new digital instrument, they may not suffice to increase their experience, we hypothesised a greater impact of the visual augmentations on the spectator subjective experience and their objective comprehension. We also hypothesised that this impact is due to the provided information, not only to the increased visual richness of graphical material inclusion.

Decomposing the experience

To handle the elusive concept of experience, we split it into three measurable features, the objective comprehension, the subjective experience and the subjective comprehension. Each aspect is evaluated through one or more tasks and distributed across blocks of our protocol. This methodology maintains the participants attention with short and diversified tasks, and by addressing the experience under multiple aspects prevents from too narrow results [START_REF] Brown | Into the wild: challenges and opportunities for field trial methods[END_REF].

Subjective experience

Experience is by definition a subjective phenomenon and is therefore difficult to assess in a quantitative manner. Moreover, qualitative user feedback represent useful insights that should be taken into consideration. In order to keep this information and allow for comparative analysis, the protocol addresses the subjective experience in 2 ways :

• A guided rating task where the participants are asked to independently rate constituents of the performance : the music, the virtuosity of the musician, the visual aspects and their overall appreciation of the performance. This task is tested twice and constitutes Block 01 and Block 04 (Figure 1). We placed the task at the beginning and at the end to let the time to the participants to integrate the SEATs' behaviour and control for a potentially too flat learning curve. • Semi-structured interviews allow for more personal and detailed feedback and are crucial to fully understand what the participant experienced. For this purpose, with the consent of the participant, the experiment is fully audio taped. The interviews occurred after the familiarisation phase and at the end of the experiment (Figure 1).

Objective comprehension

The objective comprehension refers to the ability of the spectator to integrate factual elements of the interaction, like who from the musician or the machine is the author of a sound modulation, addressed in Block 02, or who is more contributing in a performance, addressed in Block 03 (Figure 1).

Subjective comprehension

Midway between subjective experience and objective comprehension, subjective comprehension relates to the inner feeling of being able to perceive an aspect of the interaction. This aspect of the experience is tested in Block 03 (Figure 1).

Experimental conditions

To study the impact of the two SEATs, we presented our stimuli under four experimental conditions 1 :

Control condition

In the control condition, the performance is showed as is, without any cues or explanation. (See Figure 2-A)

Preliminary explanations (Explain)

In the Explain condition, a short video demo (65 seconds) is played before the performance. This video is a close-up on the musician hands and the instrument that will be used in the subsequent performance. The musician demonstrates each mapping, from sensors (button, slider, knob) to sound parameters, with verbal explanations (See Figure 2-B). The participant can hear each sound processes played individually as well as their modulations allowed by the instrument. The performance is played right after the demo.

Visual augmentations (V-AUG)

In the V-AUG condition, visual augmentations are overlaid on the performance (See Figure 2-C). Visual augmentations are graphical representations of the behaviour of the instrument updated in real time. Similar to Berthaut et al. [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF] they represent:

• The sensors and their activity in order to make them more visible (Figure3, I) • The mappings between sensors and sound processes, which appear when the musician manipulates them and disappear after. (Figure3, II) • The three sound processes with three shapes whose appearance changes according to changes in their sound parameters (from either the musician or the computer) : colour hue represents pitch, size represents volume, rotation represents scrolling in the sound, contours represent a delay effect. (Figure3, III)

Disruptive visual augmentations (V-DIS)

In the V-DIS condition, visual augmentations are also overlaid on the performance but they are uncorrelated with the musician gestures. They were actually extracted from another performance. This condition aims at controlling that the effect of the visual augmentations on the experience are not only due to the increased richness of graphical material inclusion. The V-DIS condition is not introduced until Block 03 in order to let the participants integrate the correct behaviour of the visual augmentations. Note that the participants were not informed that incongruent visual augmentations could occur.

Controlled stimuli

The stimuli used in this study are videos of short electronic music performances (about 30s) shot from an identical point of view 1 . Each video presents a musician behind a table on which a digital musical instrument (DMI) is placed next to a computer (see Figure 2-A). The video starts by a fade in with music already playing and the musician manipulating the DMI.

The video ends with a fade out.

The control device used by the musician is a MIDI control surface (Korg NanoKontrol) composed of sensors : linear potentiometers (faders), rotating potentiometers (knobs) and buttons (see Figure 2-B). Linked to the device by USB, the computer is running a music software (Pure Data) allowing for the control of three sound processes (and their associated parameters) : a bass track (pitch, note_length, filter, volume), a beat/percussion track (trigger_note, echo, filter, volume) and a textural track (position_in_sample, filter, volume, echo). This setup (controller + computer) constitutes the common architecture of many DMIs.

The mapping

The mapping is the relation between the sensors of the control device and the sound parameters. It impacts multiple aspects of the performance [START_REF] Hunt | The importance of parameter mapping in electronic instrument design[END_REF]. In particular, the mapping influences the contribution of the musician by setting the ratio between the controls given to the musician and those given to autonomous processes. Thus the role of the mapping is crucial in the definition of an instrument [START_REF] Hunt | The importance of parameter mapping in electronic instrument design[END_REF]. In order to vary the contribution level of the musician, from very high to very low, we designed three instruments from the MIDI controller :

• In the first instrument, all sound parameters are associated with sensors, meaning that all changes in the sound of the performance originate from the sensors manipulated by the musician. The musician's contribution is very high. • In the second instrument, some of the parameters are automated while others are connected to sensors, the musician's contribution is lower. • In the third instrument, most sound parameters are automated and some are shared, i.e. the musician can take control of them temporarily. The musician's contribution is therefore very low.

To limit the complexity in gesture perception by participants, all mappings were one-to-one : one sensor controls one parameter only.

Performance masking -Learning bias

The controlled evaluation of the augmentation techniques requires to display the same performances under each augmentation condition. Because of this repetitive exposition, the participant could memorise a performance and answer the questions related to a condition having in mind the cues provided by the same performance under a different condition.

To avoid this learning bias, we created a number of sound banks, i.e. different bass, percussive and textural sounds, and used them in post production to generate multiple stimuli from original video footages. Thus, the resulting stimuli presented the very same gestures, performance structure and instrument mapping but with a slightly different sound output making it harder for a participant to notice the potential repetition of a performance. Besides this "performance masking", participants were informed before the experiment that the device used by the musician may have different settings throughout the videos and thus should be considered as a distinct instrument regardless of its identical physical aspect.

Participants 21 participants (16 males, 4 females, 1 deliberately not reported) aged of 34.9 ±4.2 were involved in the study.

The protocol follows the ethic rules of the Helsinki Declaration. All participants were voluntary and signed an informed consent before getting equipped and start the experiment. The exclusion criteria included hearing and vision impairments. After control, no subject was excluded.

Procedure

After having read and signed the consent form, the participant was equipped with two watchbands measuring cardiac and electrodermal activity, and with a light eye-tracking device.

The physiological aspects of the experiment are not mentioned in the present document and will be covered in a future study.

During the whole experiment, the participant was seating at a desk in front of a laptop equipped with a mouse (see Figure 2 -D). She or he was informed that the experiment consisted in watching videos and answering the subsequent questions via a survey form. After each block, the participant was offered to take a short break.

The experiment started with a questionnaire to control the participant's hearing and sight abilities and to evaluate their expertise in three disciplines : music as a performer, electronic music (as a spectator in concerts) and recent technologies. Data treatment of the scores of expertise led to 3 expertise profiles : musicianelectrotechno. These profiles were used to control the homogeneity of the groups in Block 02 that followed a between-groups design.

Data analysis

Data was recorded, anonymised and stored in real time during the experiment by a bespoke experiment software developed in Python. For technical reasons, one participant did not finish the second block and was excluded of the analysis for this block. Subjective reports were obtained via Likert scales and were analysed with parametric tools when the normality assumptions were met [START_REF] Gail | Analyzing and interpreting data from Likert-type scales[END_REF][START_REF] Carifio | Resolving the 50-year debate around using and misusing Likert scales[END_REF]. All the analyses were conducted under the common frequentist paradigm and were combined to Bayesian statistics.

When the assumptions for parametric tests were met, we looked for a main effect through the analysis of variance (ANOVA or repeated measures ANOVA). We used a Mauchly's test to control the sphericity (with a Greenhouse-Geisser correction when required). In case of a significant main effect post-hoc T tests were conducted. When the assumptions for parametric tests were not met, the analysis was conducted with non parametric statistical treatments. For repeated measures, we used a Friedman test. In case of a significant main effect, we conducted pairwise comparisons with a Wilcoxon Signed-Rank Test. For dependent comparison, we used a Kruskall-Wallis test. In case of a significant main effect, we conducted pairwise comparisons with a Mann-Whitney Test. All post-hoc tests significance were adjusted with Bonferroni correction for multiple tests.

Bayesian statistics offer sounder analysis when dealing with relatively small samples like ours (n=21) [START_REF] Kay | Researcher-centered design of statistics: Why Bayesian statistics better fit the culture and incentives of HCI[END_REF][START_REF] John | Why most published research findings are false[END_REF]. Besides, by clearly exposing the evidence in the data that support either the null or the alternative hypothesis, they also provide a more intuitive interpretation [START_REF] John K Kruschke | What to believe: Bayesian methods for data analysis[END_REF] and are increasingly considered as the researcher-centred design of statistics [START_REF] Kay | Researcher-centered design of statistics: Why Bayesian statistics better fit the culture and incentives of HCI[END_REF]. In this work, whenever possible, the ANOVA, the pairwise test [START_REF] Jeffrey N Rouder | Default Bayes factors for ANOVA designs[END_REF][START_REF] Morey | BayesFactor (version 0.9[END_REF][START_REF] Jeffrey N Rouder | Bayesian analysis of factorial designs[END_REF][START_REF] Jeffrey N Rouder | Model comparison in ANOVA[END_REF] and the linear regression [START_REF] Merlise A Clyde | Bayesian adaptive sampling for variable selection and model averaging[END_REF][START_REF] Consonni | Prior distributions for objective Bayesian analysis[END_REF][START_REF] Liang | Mixtures of g priors for Bayesian variable selection[END_REF] are expanded with their Bayesian version and a Bayes factor is reported as BF 01 when data better support the null hypothesis and as BF 10 when data support the alternative hypothesis (note that '01' becomes '10'). For example, the statement BF 10 = 2.4 means that the data are 2.4:1 in favour of the alternative hypothesis, so 2.4 times more likely to occur under a model including the corresponding effect. The posterior odds have been corrected for multiple testing by fixing to 0.5 the prior probability that the null hypothesis holds across all comparisons [34,[START_REF] Peter H Westfall | A Bayesian perspective on the Bonferroni adjustment[END_REF]. Analyses were performed with IBM SPSS v25, R studio 1.2 and JASP (Bayesian statistics) [34,[START_REF] Wagenmakers | Bayesian inference for psychology. Part I: Theoretical advantages and practical ramifications[END_REF].

TASKS AND RESULTS

Familiarisation

Before entering the first block of the protocol, the participants were asked to simply watch 3 short videos with no other instructions. The videos presented the same performance under three conditions : control, V-AUG, and Explain. After the trial, they were asked to freely comment on what they had just seen. Then they were informed that during the experiment they will see these three types of videos.

Subjective experience

The subjective experience is assessed in Block 01 and in Block 04 (See Figure 1) through a within subjects design (N = 21).

Protocol

In Block 01, the same short music performance is presented 3 times, once per condition, in random order : V-AUG, Explain, Control. In Block 04, another short music performance is presented 4 times, once per condition, in random order : V-AUG, Explain, Control and the disruptive condition V-DIS in which visual augmentations are not correlated with the performance. The participants were not informed that incongruent visual augmentations could occur.

Task

The task was the same for Block 01 and Block 04. The participants had to rate their global experience and three aspects that we hypothesise to support this experience, namely, the music, the visual aspect and the virtuosity of the musician. After each video, they had to answer 4 questions in randomised order on 7-point Likert scales :

• From 1 (min) to 7 (max), rate the global quality of the performance. • Considering the visual aspect only, rate the performance from 1 (min) to 7 (max). • Considering the virtuosity of the musician only, rate the performance from 1 (min) to 7 (max). • Considering the music only, rate the performance from 1 (min) to 7 (max).

Results

The hypothesised decomposition of the subjective experience following the music, visual and virtuosity subjective ratings was strongly supported (BF 10 = 479) by a linear regression model (F (3,17) = 14.7, p < 0.001, R 2 adj : 0.67). No difference (BF 01 = 1.9) with the V-DIS condition was detected. However, the data showed evidence for the absence of difference between the control condition and the V-DIS condition (BF 01 = 2) and the Explain condition (BF 01 = 3.6).

An effect of the SEATs on the visual rating was found in both Block 01 and Block 04 (F (3,60) = 14.938, p < 0.001 -BF 10 > 319), obviously triggered by the graphical material inclusion of the visual augmentations.

Discussion

The visual augmentations led to a greater subjective experience in the last block but no effect of the augmentation techniques (SEATs) was measured in the first block. This can be explained by the learning curve associated to the visual augmentations.

When we designed the experiment, we opted to deliver no prior information or comments about the conditions to the participants. So we hypothesised that the participants would require some expositions to the visual augmentations before integrating their behaviour and their purpose. Without a clear estimation of the learning curve, we placed the measurement of the subjective experience twice in the protocol, once at the beginning and once at the end. More formal information should be collected in ecological conditions, like a concert, but the visual augmentations used in the experiment are quite explicit and we advocate for a short learning curve of a few minutes. This estimation is supported by the emergence of better results in the V-AUG condition of Block 02 and a decisive contribution of the visual augmentations on the subjective comprehension assessed in Block 03.

Subjective comprehension

The subjective comprehension is assessed in Block 03 (See Figure 1) through a within-subjects design (N=21).

Protocol

Videos of short music performances were presented under four conditions : V-AUG, V-DIS, Explain and Control. Contrary to the V-AUG condition, the visual augmentations in the V-DIS condition are not correlated with the musician's gestures.

Task : Bellotti-Fyans challenges

This task is built upon 5 communication design issues _ namely Address, Alignment, Attention, Accident and Action _ introduced by Bellotti et al. [START_REF] Bellotti | Making sense of sensing systems: five questions for designers and researchers[END_REF] in a user-computer interaction context and transposed by Fyans et al. [START_REF] Cavan Fyans | Spectator understanding of error in performance[END_REF] to the perspective of spectators of interactions. In turn, we transpose these challenges from a design perspective to an evaluation perspective by assessing the perception of these design features by observers. We call them the Bellotti-Fyans challenges.

After each video, participants had to indicate on a 5-step likert scale their degree of accordance with statements built from the Bellotti-Fyans design challenges. Contrary to the objective comprehension task where participants must perform by giving good answers, in this task the participants were informed that there were no good or bad answer and they should report their inner feeling rather than a definitive answer. Only the extreme values of the scales had a label : "I totally disagree" and "I totally agree". The question asked was "To which extend do you agree with the following statement ?" :

• "In this video, I know when the musician is interacting with the instrument and when he is not."(Address) • "In this video, I can see when the instrument is responding to the musician gesture and when it is not."

• "In this video, I can see if the musician is controlling the instrument or if he is not."(Action) • "In this video, I can see when the instrument is correctly functioning and when it is not."(Alignment) • "In this video, I can see if the musician or the instrument made a mistake."(Accident)

Ratings

Results

The augmentation techniques (SEATs) often led to a distinct appreciation of a same interaction (See Figure 5). Despite rather subtle differences in the meaning of the questions, like in Alignment versus Attention, different patterns are clearly observed.

• Address : A substantial effect (BF 10 = 3.7) of the SEATs was detected on the evaluation of the Address challenge (χ 2 (3) = 10.07, p = 0.018). Bayesian post-hoc tests showed weak to substantial evidence of a better evaluation for the V-AUG and the V-DIS conditions compared to the other conditions (BF 10 = 2). • Accident : Data showed no effect (BF 01 = 2.4) of the SEATs on the evaluation of the Accident challenge (χ 2 (3) = 3.66, p = 0.301).

Discussion

The assessment of the subjective comprehension revealed interesting contrasted results. By analysing the tendency of a spectator to project their ability in a fictive task, we are obviously not measuring their objective abilities but rather the confidence they have in the mental model they built up from the interaction [START_REF] Cavan Fyans | Examining the Spectator Experience[END_REF]. The SEATs did not significantly influence the participants' subjective feeling of being able to detect errors from the musicians or the machine. From a cognitive point of view, this suggests that neither the visual augmentations nor the pre-performance explanations could reduce the spectator's difficulties in perceiving the performer's intention and predicting the result of an action [START_REF] Cavan Fyans | Spectator understanding of error in performance[END_REF].

However, visual augmentations (V-AUG) induce positive evaluations of the other Fyans-Bellotti challenges, where they overtake the Control and the Explain conditions, which suggests that they help spectators build and trust a mental model of the instrument and of the performer's interactions with it.

The difference between the congruent (V-AUG) and the disruptive augmentations (V-DIS) is not always measurable. The inclusion of incongruent visual augmentations led to the same feeling of being able to detect when the musician was interacting with the instrument (Address) and when the instrument was responding to the musician (Attention). Conversely the congruent condition V-AUG overtakes the V-DIS condition in the evaluation of the level of control of the musician (Action) and in the evaluation of the proper functioning of the instrument (Alignment). Since there is no global subjective rating of the stimuli in this task, these findings need refinements to clearly identify the link between the subjective comprehension and the global experience. Still, the design of SEATs and their evaluation can already benefit from these first insights of the approach by components.

Objective comprehension

The objective comprehension was assessed by two tasks, a modulation attribution task in Block 02 and a Global Contribution Task in Block 03 (See Figure 1).

Modulation attribution task

The task was to detect a given change of one of the processes parameters in a short music performance and correctly decide who from the musician or the computer was the author. The participants answered in real time, while the video was playing, by pressing a key associated with their answer (the musician or the computer) on the keyboard.

After each video, participants evaluated on Likert scales how difficult they found the task (from 'really easy' to 'really difficult') and what was their trust in their performance on 5-steps scale (1 : "I answered randomly", 5: "I'm very confident in my answer").

In order to limit the duration of the experiment, this task followed a between-groups design. The videos were displayed under a condition depending on the group to which the participant was randomly assigned. Groups: V-AUG (n=9), Explain (n=5 + 1 excluded), Control (n=6). The expertise of participants within each group was controlled, the 3 groups were homogeneous (χ 2 (12) = 25.48, p = 1).

Results

The score of the modulation attribution task was computed by measuring the percentage of time during which the key pressed by the participant matched the actual origin of the modulation.

Participants reported that the task was quite difficult. The data confirmed that impression with an average score of 42.3% globally (see Table 1 for details).

Score Despite concomitant results in favour of the visual augmentations (Table 1), the task did not permit to expose significant contrasts of the SEATs' influence on the participants' scores (F (2,17) = 3.215, p = 0.184). This lack of power is mainly due to the rather small size of the groups and the difficulty of the task. The Bayesian analysis showed weak evidence in favour of an effect of the SEATs (BF 10 = 1.6) and weak superiority of the scores in the V-AUG condition. No influence of the SEATs on the reported difficulty (F (2,17) = 0.257, p = 0.575, BF 01 = 3.1 ) was found, neither on the reported confidence (F (2,17) = 2.952, p = 0.220 -BF 10 = 1.3).

Global Contribution Task

This task occurred in Block 03 and followed a within-subjects design (N=21) with 2 factors : Augmentation technique : V-AUG, V-DIS, Explain and Control -Contribution level of the musician : contrib_low, contrib_med, contrib_high Contrary to the modulation attribution task where the authorship of a specific event was targeted, in this task, the participants had to determine who from the musician or the computer was globally the most contributing over a whole performance.

As explained in Section The mapping, we designed three instruments which allowed us to vary the musician's and computer's respective contributions. These mappings resulted in, respectively, a large contribution of the computer, a large contribution of the musician and a rather balanced ratio still in favour of a greater contribution of the musician. Thus, the musician contributed the most in 66% (2/3) of the stimuli. This two-factors design lead to 4 SEATs x 3 contribution levels = 12 videos. The stimuli implying the same contribution level of the musician were generated from the same original video footages. To control for a potential learning bias, these stimuli were post produced with different audio outputs following the performance masking technique detailed in Section Controlled stimuli. . Global Contribution Task : the participants largely promoted the greater contribution of the musician (90%, orange) and were more confident in their answers (black dots) when the performances were augmented with visual augmentations (V-AUG). The performances augmented with incongruent visual augmentations (V-DIS) led to an under estimation of the musician contribution.

After each video, the participants indicated who from the musician or the computer was the most contributing and reported their level of confidence in their answer on a 6-step likert scale with labelled extreme values; 0: "I answered randomly" -5: "I'm very confident".

Results

The average success score was 60.6% ± 12.5 and 65.1% of the reported answers indicated that the musician contributed the most.

Considering the score of the participants to this task, the data showed a strong effect (F (2,40) = 7.18, p = 0.002 -BF 10 = 600) of the contribution level taken alone and a very strong effect (F (6,120) = 5.98, p < 0.001 -BF 10 > 100000) of the interaction between the contribution levels and the augmentation techniques. A second analysis was run to understand the details of the interaction. It exposed a very strong link (BF 10 > 7000) between the choice of the participants for the most contributing and the SEATs (χ 2 (4) = 28.432, p < 0.001). As illustrated in Figure 6, post-hoc tests showed that 90% of the stimuli presented in the V-AUG condition were associated to a greater reported contribution of the musician, 67% in the Explain condition, 60% for the control condition. Interestingly, the lowest count of reports for the contribution of the musician was measured in the V-DIS condition where 56% of the stimuli were associated with a greater contribution of the computer.

A moderate (BF 10 = 2.9) effect of the SEATs was found on the confidence participants had in their answer (F (3,60) = 3.683, p = 0.017). Following the overestimation of the musician contribution in the V-AUG condition, post-hoc tests showed that the confidence of the participants in their answer was significantly higher (p = 0.049, BF 10 = 4.19) in the V-AUG condition than the control condition and (BF 10 = 2) the Explain condition.

Discussion

The impact of augmentation techniques (SEATs) on the objective tasks is contrasted. The lack of power did not permit to clearly expose their role with weak evidence in favour of the visual augmentations in the Modulation Attribution Task. Conversely, the Global Contribution Task led to significant results and revealed a misleading effect of the visual augmentations that conducted the participants to over estimate the musician contribution. These findings are discussed in the general discussion alongside with the insights of the other blocks.

DISCUSSION

In this section, we combine the findings discussed in the previous sections and provide insights and perspectives on Spectator Experience Augmentation Techniques (SEATs), and in particular the use of visual augmentations. We also propose to add the Association as a new item in the Bellotti-Fyans challenges.

Visual augmentations offer a better experience

The findings of this study expose the positive role of the visual augmentations over multiple aspects of the experience of spectators of performances with digital instruments. They also confirm the weak contribution of the preliminary explanations reported in the literature [START_REF] Sma Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF].

The low efficiency of preliminary explanations can be interpreted as a difficulty for the audience to memorise and manipulate the large amount of information necessary to integrate complex interactions. The real time support of visual augmentations as well as their ability to abstract interactions mechanisms probably made a strong difference. Yet, with this sole first study, we can not strengthen these interpretations because we are missing formal descriptions of the two techniques. A taxonomy of the SEATs could help to refine the study of their impact by allowing for an approach by components rather than the holistic comparison we present here.

The contribution of the visual augmentations was not measurable on the ratings of the music, or the virtuosity of the musician, but the data do support a significant improvement of the overall experience of the spectator. Importantly, the effective impact of the visual augmentations is not due to the only effect of the additional graphic material of this technique. When the augmentations were not correlated to the interactions, participants gave a better rating to the visual aspect but no difference with the control condition was detected in terms of global experience.

The interest for the visual augmentations was also reported during short interviews when we asked the participants what augmentation technique they would choose if they were to attend an electronic music performance. A large majority (80%) opted for the visual augmentations but we also had some participants advocating for no augmentations at all, neither explanations nor visual augmentations, to preserve "their natural experience of music". Depending on the expertise of the participants and their tolerance to graphical inclusions in terms of cognitive interference, the SEATs should permit to vary the level of details and even let the user turn it off.

Restoring the trust in musicians

Even if the visual augmentations did not increase the objective comprehension of the spectator as we hypothesised, they led to an interesting side effect. When the participants had to distinguish the contribution of the musician from the one of the computer, they over estimated the contribution of the musician and at the same time were more confident in their evaluation when the performances were displayed with visual augmentations (V-AUG). From an objective perspective, this observation lowers the efficiency of the visual augmentations used in the study to clearly expose the reality. But it also represents a positive bias that can compensate the doubt of uncomprehending spectators and restore their trust in the musician. Conversely, the disruptive augmentations (V-DIS), extracted from another performance, led to a sub estimation of the contribution of the musician, a reduced confidence of the participants in their answers, and the misperception of a greater contribution of the computer.

These observations could highlight the presence in the audience experience of a subtle balance between the continuous objective cues emitted by the performer and an inner, potentially discreet, validation by the observer that everything is going as expected, with a clear contribution of the musician and a moderate support of the machine. As a design guideline, this suggests that the more a performance is demanding in term of artistic virtuosity, the more the audience needs to be able to perceive the ratio of the human/machine contribution, in order to preserve their trust.

The unexpected benefits of disruptive cues

The insertion of uncorrelated visual augmentations led the participants to evaluate differently some aspects of the interactions but not all. The confusion is more visible in the results of the Bellotti-Fyans challenges (See the discussion of the Subjective comprehension section). These contrasts in the results reflect the composite nature of the mental models at the origin of the spectator's experience and expectations [START_REF] Brian | Sweet anticipation: Music and the psychology of expectation[END_REF][START_REF] James | Mental models concepts for system dynamics research[END_REF]. They also highlight the inconsistent effect of the augmentations used in the study. Now that the potential efficiency of the visual augmentations is formally supported, design research should compare different types of visual augmentations, graphical material or levels of detail and explore their influence on identified observer's mental models like the one from the Bellotti-Fyans challenges or the components of the experience we addressed in this work.

Controlling the level of magic

We saw that disruptive visual augmentations can lead to the perception of a lower contribution of the musician. They also tend to blur the mental models of the observers. This property could be manipulated by the musicians. By designing the SEATs to increase the amount of disruptive cues in the augmentations, musicians could decide to veil their manipulations at the same time of a music climax and let the audience think that something "magical" is happening. In a sense, if too much doubt about the musician's contribution on the music can reduce the interest of the audience, a parsimonious approach can lead to interesting strategies in the artistic performance, like a navigation in the 2-dimensional space proposed by Reeves et al. [START_REF] Reeves | Designing the spectator experience[END_REF] (See Figure 7).

Call for a new design challenge : Association

The Bellotti-Fyans challenges provided insightful data for the evaluation of the subjective experience of the spectators. However, it is not clear if these challenges are sufficient to characterise the ability of a device to expose the contributions of both the user and the system to the audience. To this end, we propose to add another "A" to the five already present in the Bellotti-Fyans challenges with the "Association" challenge. The term Association relates to the design decisions that enable a device to expose to the spectators the respective and shared contributions of its user and the system itself. This challenge also relates to the exclusivity dimension of agency defined by Wegner et al. [START_REF] Daniel | Apparent mental causation: Sources of the experience of will[END_REF], transposed to the audience.

Interactions with simple devices do not need such augmentations as the integration by an observer of the inherent cues is obvious to evaluate the contribution of the user, like when observing a seller with a cash machine. But when the interactions become more complex, when the system they address is capable of producing actions that mimic the one a user could make, like a musician with a digital instrument, or a worker with an exoskeleton, the contributions of the user and the machine are mixed from a spectator perspective. An observer of such interaction implying a device with a high Association score should have a clear appreciation of the actual part of the user's actions in the intended production, without the difficulty to integrate the multiple cues perceived from the interaction. The shared contribution of both the system and the user should also be appreciable. The decisions that lead to a good Association score should be part of the design process or ensured by dedicated augmentations.

Limitations

In this work, we aimed at investigating refined aspects of the experience and minimising the biases inherent to the study of human behaviour by conducting a controlled study. Although the results of the experiment did provide genuine insights on the effect of SEATs, a number of limitations remain. A first improvement would be a validation of the questionnaires, in particular regarding subjective comprehension.

As other "in the lab" studies, our work relies on the data analysis methods for controlled experimentation that can be considered as a risk for misleading or overly narrow results [START_REF] Nielsen | Risks of quantitative studies[END_REF]. In consequence, we chose to include interviews and to mix objective and subjective trials to limit the bias often met in field studies [START_REF] Brown | Into the wild: challenges and opportunities for field trial methods[END_REF]. Besides, we believe that the extension of the data analysis with Bayesian statistics is particularly helpful in this context of medium sample size (21 participants).

Finally, another limitation of our work naturally lies in the evaluation of a live phenomena in a controlled environment. In this context, important aspects of the experience of live music are missing, from the social environment to the ceremonial of the concert hall. Aware of these limitations, we do not consider this work as an exhaustive approach, much to the contrary as we are very open to the contributions of other disciplines in the study of the relations between humans and artificial systems. Joining the "movement" of mixed-methods [START_REF] Creswell | The "movement" of mixed methods research and the role of educators[END_REF], we believe that there should be more dialog between the "in the lab" and "in the wild" paradigms. This enthusiasm is shared by recent discussions in the HCI community [START_REF] Liu | Rigor, Relevance and Impact: The Tensions and Trade-Offs Between Research in the Lab and in the Wild[END_REF], which highlights the importance of identifying insights and biases of each paradigm.

CONCLUSION

This work presents a controlled study to evaluate the effect of Spectator Experience Augmentation Techniques on the audience experience. We showed that, contrary to pre-concert explanations, visual augmentations increase the global subjective experience, support the observer's confidence in their mental representations of the interactions, and induce an over estimation of the musician's contribution compared to the machine's, thus restoring spectators' trust in electronic musicians. We proposed to extend the Bellotti-Fyans challenges with the Association challenge, as the ability of a device to expose the contributions of both the user and the system to the audience.

We showed strong evidence of differences in the impact of the preliminary explanations and the visual augmentations. However, the explanation of these contrasts would benefit from formal descriptions of the two techniques. To this end, future work could propose a taxonomy of SEATs to facilitate their formal comparisons and allow for grounded design guidelines. We believe that the evaluation of Spectator Experience Augmentation Techniques on specific components of the experience can provide robust insights for applications in many other domains where the collaboration of humans and machines occur in public, like performance interfaces or pedagogical devices.

Figure 1 .

 1 Figure 1. After a familiarisation phase, participants followed 4 successive blocks targeting the components of the experience : subjective experience in Blocks 01 and 04, objective comprehension in Blocks 02 and 03 and subjective comprehension in Block 03.

Figure 2 .

 2 Figure 2. Conditions and Spectator Experience Augmentation Techniques evaluated in our study : A) Control, B) Explain (pre-performance explanations), C) V-AUG (visual augmentations), D) The experimental setup

Figure 3 .

 3 Figure 3. Visual augmentations are graphical representations updated in real time of the behaviour of the instrument. I) Sensors II) Mapping III) Processes. See Section Visual augmentations (V-AUG) for details.

Figure 4 .

 4 Figure 4. Bayesian posterior distributions of the global experience ratings in Block 04. Participants reported a better experience when the performance was augmented with visual augmentations. The V-DIS (uncongruent visual augmentations) and Explain conditions did not differ from the control condition.A strong effect (BF 10 = 19.5) of the SEATs was found on global experience (χ 2 (3) = 17.40, p < 0.001) only in Block 04. As illustrated on Figure4, post-hoc tests showed that ratings in the V-AUG condition presented very strong evidence (BF 10 = 29.7) of a superiority with the control condition and a substantial superiority (BF 10 = 3.3) with the Explain condition. No difference (BF 01 = 1.9) with the V-DIS condition was detected. However, the data showed evidence for the absence of difference between the control condition and the V-DIS condition (BF 01 = 2) and the Explain condition (BF 01 = 3.6).

5 Figure 5 .

 55 Figure 5. The effect of the Spectator Experience Augmentation Techniques (SEATs) on the Bellotti-Fyans challenges.

•

  Attention : A very strong effect (BF 10 = 160) of the SEATs was detected on the evaluation of the Attention challenge (χ 2 (3) = 12.95, p = 0.005). Post-hoc tests showed a better evaluation in the V-AUG condition compared to the Control condition (p = 0.02, BF 10 = 99) and to the Explain condition (p = 0.051, BF 10 = 13.8) and weak evidence (BF 10 = 2.5) for a better evaluation in the V-DIS condition compared to control.• Action : A very strong effect (BF 10 = 161) of the SEATs was detected on the evaluation of the Action challenge (χ 2 (3) = 18.75, p < 0.001). Post-hoc tests showed a better evaluation in the V-AUG condition compared to the control condition (p = 0.03, BF 10 = 112), to the Explain condition (p = 0.02, BF 10 = 104) and, with weak evidence, to the V-DIS condition (BF 10 = 2.7). • Alignment : A substantial effect (BF 10 = 5.12) of SEATs was detected on the evaluation of the Alignment challenge (χ 2 (3) = 10.99, p = 0.012). Post-hoc tests showed a better evaluation in the V-AUG condition compared to the control condition (p = 0.051, BF 10 = 9), the Explain condition (BF 10 = 2.8) and quite interestingly a substantial difference (BF 10 = 4.4) with the V-DIS condition.

Figure 6

 6 Figure 6. Global Contribution Task : the participants largely promoted the greater contribution of the musician (90%, orange) and were more confident in their answers (black dots) when the performances were augmented with visual augmentations (V-AUG). The performances augmented with incongruent visual augmentations (V-DIS) led to an under estimation of the musician contribution.

Figure 7 .

 7 Figure 7. When controlled by the musician, the nature and the amount of visual augmentations allow to navigate in the 2-dimensional space proposed by Reeves et al.[START_REF] Reeves | Designing the spectator experience[END_REF] (Illustration adapted from[START_REF] Reeves | Designing the spectator experience[END_REF])

Table 1 .

 1 Modulation attribution task -Descriptive statistics

	[% of success] V-AUG Explain Control
	Mean	44.9	42.3	38.4
	Stdev	4.0	4.3	6.3
	Min	39.8	35.6	28.0
	Max	53.7	47.0	45.0
	Difficulty [-3,3]
	Median	0.000	1.000	0.500
	Min	-0.667	-0.667	-0.667
	Max	2.000	1.500	2.000
	Confidence [-3,3]
	Mean	0.213	-0.700	-0.111
	Stdev.	0.498	1.083	0.455
	Min	-0.667	-2.000	-0.667
	Max	0.667	0.667	0.667

All stimuli, illustration videos of the conditions, anonymised raw results and statistical analyses can be found here :

http://o0c.eu/0DA


Olivier Capra
email: olivier.capra@univ-lille.fr

Florent Berthaut
email: florent.berthaut@univ-lille.fr

Laurent Grisoni
email: laurent.grisoni@univ-lille.fr

A Taxonomy of Spectator Experience Augmentation Techniques

Keywords: audience experience, augmentation techniques, taxonomy, performance CCS Concepts, Applied computing → Sound and music computing, Performing arts, •Human-centered computing

In the context of artistic performances, the complexity and diversity of digital interfaces may impair the spectator experience, in particular hiding the engagement and virtuosity of the performers. Artists and researchers have made attempts at solving this by augmenting performances with additional information provided through visual, haptic or sonic modalities. However, the proposed techniques have not yet been formalized and we believe a clarification of their many aspects is necessary for future research. In this paper, we propose a taxonomy for what we define as Spectator Experience Augmentation Techniques (SEATs). We use it to analyse existing techniques and we demonstrate how it can serve as a basis for the exploration of novel ones.

INTRODUCTION

While digital interfaces open numerous possibilities for artistic expression, they may have a negative impact on the experience of spectators. In particular, previous research has shown that digital interfaces, in particular Digital Musical Instruments (DMIs), may diminish the perceived liveness [START_REF] Schloss | Using contemporary technology in live performance: The dilemma of the performer[END_REF], the attributed agency [START_REF] Berthaut | Liveness through the lens of agency and causality[END_REF], the perception of error [START_REF] Bin | In-the-moment and beyond: Combining post-hoc and real-time data for the study of audience perception of electronic music performance[END_REF] and the perception of the performer's skill [START_REF] Fyans | Examining the spectator experience[END_REF]. A first solution to this issued is to shift the audience focus towards the aural aspect of electronic performances [START_REF] Stuart | The object of performance: Aural performativity in contemporary laptop music[END_REF]. A second solution is to take into account the audience experience in the design and evaluation of DMIs [START_REF] Barbosa | Considering audience's view towards an evaluation methodology for digital musical instruments[END_REF][START_REF] Hsu | Evaluating interactive music systems: An hci approach[END_REF] and to ensure that instruments are sufficiently transparent [START_REF] Fels | Mapping transparency through metaphor: towards more expressive musical instruments[END_REF]. As an alternative solution, artists and researchers have designed techniques which augment performances with additional information for the audience without requiring changes in the design of DMIs, therefore preserving their expressive power. The goal of these Spectator Experience Augmentation Techniques (SEATs) [START_REF] Capra | Have a seat on stage : Restoring trust with spectator experience augmentation techniques[END_REF] is not necessarily to "explain" the instru-ments and "demistify the apparatus" [START_REF] Auslander | Liveness: Performance in a mediatized culture[END_REF] but to enrich the performance for spectators who wish it. They range from textual information [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF] and pre-performance demos [START_REF] Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF], to augmented reality displays [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF][START_REF] Perrotin | Visualizing gestures in the control of a digital musical instrument[END_REF] and haptic feedback devices [START_REF] Armitage | Revealing timelines: Live coding and its gestures[END_REF][START_REF] Turchet | Haptification of performer's control gestures in live electronic music performance[END_REF]. These techniques may be essential in order to ensure the accessibility and enjoyment of performances with existing and future interactive systems. However, to our knowledge no formal analysis has been conducted yet, restraining the development of such techniques.

In this paper we propose a taxonomy for Spectator Experience Augmentation Techniques (SEATs). This taxonomy is aimed at artists and researchers as it can help inform the design of performances with the experience of spectators in mind.

Related work

Most classifications related to DMIs are defined from the musician or instrument designer point of view, such as the dimension space proposed by Birnbaum et al. [START_REF] Birnbaum | Towards a dimension space for musical devices[END_REF].

Previous work on audience experience has led to classifications such as challenges for the design of DMIs from an audience perspective [START_REF] Gurevich | Digital musical interactions: Performer-system relationships and their perception by spectators[END_REF] and to a 2D space for the analysis of action and effect perception [START_REF] Reeves | Designing interfaces in public settings: Understanding the role of the spectator in Human-Computer Interaction[END_REF]. In non-performance contexts, previous work has explored the use of visual and haptic displays to enhance the listeners experience. In particular Nanayakkara et al. [START_REF] Nanayakkara | Enhancing musical experience for the hearing-impaired using visual and haptic displays[END_REF] compare haptic and visual representations of music. More specifically on techniques for augmenting the audience experience, Capra et al. describe a number of strategies for augmentation (visual, haptic) but do not provide a formal analysis [START_REF] Capra | Toward augmented familiarity of the audience with digital musical instruments[END_REF]. Bin et al. provide the evaluation of a technique which uses a pre-concert demo to increase the level of comprehension [START_REF] Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF] . Benford et al. discuss the effect of diverse techniques (before, during and after the performance) on the audience journey [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF].

However to our knowledge, no formal classification of existing techniques for the augmentation of spectator experience, nor analysis of their components has been performed.

Contribution

In this paper, we describe a taxonomy of Spectator Experience Augmentation Techniques, which constitutes the first formal analysis of the strategies that researchers and artists use to compensate for the alteration of audience experience in performances with digital interfaces. We demonstrate how it can be used to analyse, extend or create techniques, and how it can be useful for artists and researchers.

SPECTATOR EXPERIENCE AUGMEN-TATION TECHNIQUES

It is essential to point out that, by using the term augmentations, we focus on techniques which have no impact on the design of the performer's interface, i.e. they do not change the way performers interact. Rather, they provide addi- tional information to the audience on various components of the performance, such as performer's equipment, gestures or intentions. They are also not imposed on spectators, who should be able to attend the performance without the augmentations. SEATs may be designed with various goals in mind : pedagogical, with a focus on helping spectators understand the performers' interactions; aesthetic or engaging, with a focus on increasing the implication of the audience; or even disruptive, so that performances appear as magical or secretive [START_REF] Reeves | Designing the spectator experience[END_REF]. Although SEATs may be combined with an interface that allows for audience participation in the performance, i.e. the music produced is altered by spectators, we chose to keep this aspect outside our taxonomy to focus on transmission of information to the audience. Thus, the values chosen for each dimension of our taxonomy may have an impact on multiple aspects of the audience experience. For genericity reasons, our taxonomy does not take the display technology into account. For instance, visual augmentations with the same dimensions could be achieved through projection mapping, with video AR on a smartphone or with an optical see-through head-mounted display. Our aim is that this taxonomy will remain usable by designers of performances with future technologies.

Our taxonomy is composed of eleven dimensions, described below. For each of them, we provide the range of possible values and discuss their potential impact.

Spatial Alignment

This dimension describes how augmentations are spatially aligned with the performance components. Possible values include : aligned, when the augmentations are perceived as coming from the component itself; co-located when they can be perceived while maintaining the focus on the performance; distant when there is a shift in focus required to access them. For instance, visual augmentations of a music performance can be aligned if using a Pepper's ghost augmented-reality display, co-located if projected on a screen behind the musician, or distant if displayed on the spectators' mobile devices. While aligning augmentations seem ideal because it preserves the focus on the performance, the presentation of complex information might cause sensory or cognitive overload for the audience, in which case a more distant presentation is preferable.

Temporal Alignment

This dimension pertains to when spectators can access to the augmentations: before, during or after the performance. For instance, in the case of verbal explanations, they may be provided as a pre-performance demo, as comments during the performance, or as a post-performance discussion. If presented before the performance, augmentations may provide cues for the audience without any alteration to the performance. However if they are too complex or given too long before, some of the information may not be remembered when needed. If presented during, the information is given when needed but it might distract or overwhelm the audience. If presented after, the information might be lacking to understand events when they happen, but the magical aspect [START_REF] Reeves | Designing the spectator experience[END_REF] of the performance is preserved.

Temporal Density

This dimension relates to the temporal range that the augmentations cover. It can be broadly defined as a low, medium and high. For example, augmentations of a music performance can range from displaying only the last played note to displaying the whole score. While a high density will help the audience forge a stronger sense of performer's intentions or actions over time, and may help them in perceiving virtuosity or errors, providing too much information might also distract them from the current actions.

Temporal Control

This dimension describes the possibility for the audience to control the temporal range covered by the augmentations, i.e. on which part of the performance they will access information. It ranges from none to full. An example would be textual explanations provided one by one either synchronously with events of the performance or on request, with spectators being able to freely scroll through them. From a design perspective, allowing for temporal control may help spectators build a better understanding of the performance, but it might also lead them to miss augmentations that should be perceived at a specific time.

Semantic Density

This dimension relates to how much information is provided by a SEAT. It can also be seen as the information level-ofdetail. Like temporal density, it can be broadly classified low, medium and high. For instance, augmentations on a digital musical instrument may range from only showing which synthesizer is being played, to showing the detailed audio graph including the activity of all synthesis parameters. This dimension is essential as one needs to ensure that the density is sufficient to provide useful information but not too high so that it does not result in sensory or cognitive overload for the audience.

Semantic Control

Symmetrically to temporal control, this dimension indicates if the level of semantic detail can be chosen by the audience, allowing for a personalized access to narrative elements [START_REF] Benford | Designing the audience journey through repeated experiences[END_REF] or for information that matches the expertise of the spectator [START_REF] Capra | All you need is lod : Levels of detail in visual augmentations for the audience[END_REF]. It ranges from none to full. In the case of textual explanations, it can for example go from a single level of detail given to all spectators to multiple versions targeted at different levels of expertise, e.g. children / adults or novices / experts. This level may also be automatically selected according to emotional or cognitive states measured through wearable physiological sensors [START_REF] Capra | Toward augmented familiarity of the audience with digital musical instruments[END_REF]. Regarding the accessibility of the augmentations, increasing the control helps 1: Analysis of Spectator Experience Augmentation Techniques along the dimensions of our taxonomy avoiding too simple or too complex information depending on spectator's expertise, but it also increases the complexity of implementation for the performance designers. One also needs to ensure that the control interface is not too difficult or that it does not distract spectators from the performance.

Presentation nature

This dimension relates to the form given to the information provided to the audience. Possible values include figurative, abstract, conceptual, and linguistic. For instance, to indicate that a certain key was played by a musician the augmentation can display respectively a close-up of their hands and the keyboard, a colour changing shape, the note on a scale or the note name. This dimension involves a tradeoff between the information explicitness, maximised with linguistic or figurative augmentations, and its compactness which can be optimised with abstract or conceptual augmentations showing only essential information. It also has implications on the aesthetic integration of the augmentations in the performance.

Presentation modality

This dimension describes how the augmentations are displayed. The modality can be visual, auditory and haptic.

For example, the subtle gestures of a performer on a sensor can be amplified using changes in a visual shape that represents the sensor or through vibrotactile feedback reproducing the gestures. The choice of modality depends on the type of performance and its scalability. While visual and auditory displays can be generalised, haptic ones imply more restrictions as individual devices need to be designed.

Content nature

This dimension pertains to the nature of the content displayed by the augmentations, i.e. on which aspects of the performance the SEAT provides information. We identified four possible aspects: technical to reveal the mechanisms of the interface [START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF]; gestural, to amplify subtle/hidden movements [START_REF] Perrotin | Visualizing gestures in the control of a digital musical instrument[END_REF]; intentional, so that the audience understand what performers are trying to accomplish [START_REF] Fyans | Spectator understanding of error in performance[END_REF]; causal, so that spectators have a clear perception of who from the performer or autonomous processes is responsible for variations in the sound. The choice of content nature depends strongly on the aim of the augmentations. Showing the intention can highlight the performer's virtuosity or affect the audience emotional response, while technical augmentations may increase the level of comprehension of the audience.

Agents

This dimension describes the representation a SEAT can give of the agents in a performance and of their interactions.

Agents are entities that can interact with instruments and interact together, like 3 musicians on stage or a composite crew including virtual agents. We envision 4 values for this dimension. Origin, so that spectators perceive who is the source of events in the performance, for example amongst members of an orchestra [START_REF] Perrotin | Visualizing gestures in the control of a digital musical instrument[END_REF] or between the performer and automated processes. Avatar indicates the use of representations of the agents to provide additional information. This modality can be used in performances where the musicians are not physically located in the same place or when an agent is virtual. Communication when musicians exchange information not directly linked to sound production, such as synchronisation or support signals. Interactions indicates when the SEAT augments the interactions between agents. This can be useful for instruments with shared controls and rich musician-to-musician interfaces such as bf-pd [START_REF] Dahl | Enabling mediated communication and cooperation in improvised digital orchestras[END_REF].

Content reactivity

This dimension describes the relation between the augmentations and the performance. It can be fixed when the augmentations are pre-defined and do not change with the performance, semi-fixed when some elements of the augmentations change according to the performance, and reactive when augmentations are generated from information extracted in realtime during the performance. For instance, visual augmentations can be pre-recorded videos provided at pre-defined moments or synthetic graphics triggered and adapted dynamically based on the performers actions. While a reactive content guarantees that the augmentations will adapt to changes in the performances, it involves a technical complexity not always achievable. For example performers intentions can not be extracted dynamically during the performance. It may also lead to information being provided in a less accessible way than fixed content if it is not designed carefully, e.g. with multiple visual indications overlapping.

ANALYSIS OF EXISTING SEATS

USING THE TAXONOMY FOR DESIGN

When creating SEATs for a performance, our taxonomy can be used to explore designs with various effects on the audience experience. Starting from a common visual display of information behind the performer, the following directions could be chosen, depending on the desired effect: In order to improve the perception of the performer's control, i.e. the audience's attributed agency [START_REF] Berthaut | Liveness through the lens of agency and causality[END_REF], value of the Spatial Alignment dimension can be increased, for example using augmented reality displays which would place the information directly on top of the performer's gesture, therefore improving the causal link between actions and effects. However, attention must then be paid to the quantity of displayed information, so as not to overload visually the performance and reduce the visibility of the performer's gestures. In order to increase the audience engagement in the performance, the Presentation Modality dimension can combine visual and haptic feedback, through the use of belts or wristbands providing vibrotactile feedback. This direction however poses issues for implementation, as it requires to equip each spectator with a device or constrains them to fixed positions (e.g. seats) with built-in feedback. Finally, one can choose to adapt the augmentations to spectators with levels of expertise. In that case, the value of the Semantic Control dimension can be increased. This would however require an individual or grouped device for the selection of the level of information provided by the augmentations.

CONCLUSION

Spectator Experience Augmentation Techniques (SEATs) are techniques that add information to digital performance interfaces in order to enrich the audience experience. In this paper, we proposed a taxonomy for these techniques, we detailed its dimensions and showed how it can be used for the analysis and design of SEATs. As future work, we believe a formal evaluation should be conducted which would precisely determine the impact of each of these dimensions on the audience experience, and how they would be appropriated by artists and researchers.

Figure 1 :

 1 Figure 1: Spectator Experience Augmentation Techniques: a) pre-performance explanation, b) textual information on mobile, c) haptic wearables, d) visual augmentations

Table

	[20]	co-located	during	medium	none	medium	none	abstract / conceptual	visual	technical / gestural /	reactive	avatar / origin
	Benford [4]	co-located / distant	before / during / after	high	full	medium	limited	abstract / linguistic	visual	technical / intentional	semi-fixed	-
	Armitage [1]	distant	during	low	none	low	none	figurative	haptic	gestural	reactive	-
	Turchet [25]	distant	during	low	none	low	none	figurative	haptic	gestural	reactive	-
	Bin [7]	aligned	before	medium	none	low	none	linguistic	auditory / visual	technical / gestural	fixed	-
	Berthaut [6]	aligned	during	low	none	medium	none	abstract	visual	technical / causal gestural /	reactive	--

Table 1

 1 to the audience with an augmented reality display. Bin et al.[START_REF] Bin | Skip the pre-concert demo: How technical familiarity and musical style affect audience response[END_REF] uses pre-concert explanations of the instrument to increase the spectators' level of enjoyment and comprehension. Turchet et al.[START_REF] Turchet | Haptification of performer's control gestures in live electronic music performance[END_REF] use a wearable haptic device to provide vibrotactile feedback to the audience to amplify the musician's gestures.

	provides a decomposition of the following SEATs
	from the literature. Perrotin et al. [20] use visual augmen-
	tations projected behind a digital orchestra to display the
	musical parameters (e.g. notes) played by each musician.
	Benford et al. [4] use visual augmentations, both conceptual
	and textual, projected on stage and available on spectators'
	mobile devices to provide an overview of the structure and

intentions in a musical performance. Armitage

[START_REF] Armitage | Revealing timelines: Live coding and its gestures[END_REF]

uses haptic augmentations to provide the audience with vibrotactile feedback that amplifies the performer's key presses in a live coding performance. Berthaut et al.

[START_REF] Berthaut | Rouages: Revealing the mechanisms of digital musical instruments to the audience[END_REF]

use visual augmentations to reveal the mechanisms of a DMI (including sensor values, sound processes activity and sensor to pro-cesses mappings)


domain: phys spi

text:
Takanori Matsumoto

Benjamin Ducharne
email: benjamin.ducharne@insa-lyon.fr

Tetsuya Uchimoto

Numerical scheme for the simulation of the Eddy Current Magnetic Signature (ECMS) non-destructive micro-magnetic technique

Keywords: Eddy current magnetic signature (ECMS), hysteresis model, mechanical stress

published or not. The documents may come

Numerical scheme for the simulation of the Eddy Current Magnetic Signature (ECMS) non-destructive micro-magnetic technique

Residual stresses inevitably occur in metallic components due to industrial machining or heat treatment processes. These local micro-residual stresses affect real-life performance of industrial parts. Measurements and analysis of residual stresses are necessary for quality assurance and maintenance anticipations.

Local magnetization processes are highly dependent on the distribution of the residual stresses. Consequently, the use of micro-magnetic techniques such as the Magnetic Barkhausen Noise (MBN) [START_REF] Ducharne | Characterization and modeling of magnetic domain wall dynamics using reconstituted hysteresis loops from Barkhausen noise[END_REF][2] or the Magnetic Incremental Permeability (MIP) [3][4] has increased exponentially in the industrial field. In [START_REF] Matsumoto | Investigation of magnetization methods to evaluate residual strain in carbon steel by eddy current magnetic signature method[END_REF], authors propose a new micro-magnetic technique called Eddy Current Magnetic Signature (ECMS) particularly effective for the mapping of these residual stresses. The ECMS experimental setup is similar to the MIP's one

The ECMS method consist on plotting the evolution of the real versus imaginary part of the EC probe impedance during minor loop situation.

The industrial use of the micro-magnetic characterization techniques is very empirical: based on experimental data obtained from well-known samples, operators set thresholds of validation. Once rejected, the targeted samples are destroyed and no extra investigations are done to determine the origin of the defects.

In the extended version of this abstract, a precise description (experimental setup, data treatment …) of the ECMS method will be done first.

Then, a numerical method relying on physical properties will be proposed for the simulation of the ECMS technique. As ECMS is a particularly sensitive experimental method, the accuracy of the model is a key property. A Time domain, 1 dimension space discretized resolution is proposed. The Jiles-Atherton (J-A) scalar hysteresis model is used for the quasi-static contribution (hysteresis losses) [START_REF] Jiles | Theory of ferromagnetic hysteresis[END_REF]. The dynamic behavior is simulated by the simultaneous resolution of both the diffusion equation (classical losses) and a material law (excess losses) [START_REF] Raulet | The magnetic field diffusion equation including dynamic hysteresis: a linear formulation of the problem[END_REF], [START_REF] Ducharne | Fractional model of magnetic field penetration into a toroidal soft ferromagnetic sample[END_REF].

For this study, a carbon steel SB410 (JIS G3103) has been investigated (Tab. 1) Tab. 1 -Carbon steel SB410, manufacturer properties

Figure 1 ,

 1 Figure 1, shows as a first validation of the model, a comparison measure/simulation for a virgin sample of carbon steel SB410.

Fig. 1 -

 1 Fig. 1 -Comparison measure (left) / simulation (right) for ECMS on a virgin carbon steel SB410.