# Impact of Audio Delay and Quality in Network Music Performance +
## 3 Experimental Setup
MM2 ME incorporates the possible asymmetry between the two directions; if the two directions are symmetric, which is always the case in our experiments, M2E delay is exactly half of MM2 ME delay. For simplicity and easier comparison with previous work, in the rest of the paper, we only talk about and show (one-way) M2E delays, even though we measured (two-way) MM2 ME delays. A total of 22 musicians participated in the study, performing in pairs (11 pairs in total), with each pair consisting of musicians playing different instruments. Each duo performed a one-minute musical piece of their own choosing, following their natural tempo, and repeated it ten (10) times, each repetition corresponding to a distinct M2E audio delay setting. The specific delay values used are listed in Table 1.  
Subsequently, the same musical piece was performed another ten (10) times by each pair--this time under varying audio sampling rates, as shown in Table 2. No metronome or external synchronization aids were employed during the performances, allowing musicians to rely solely on the audio-visual feedback provided by the system. Following each repetition, participants completed an electronic questionnaire using a tablet's touchscreen. The content of the questionnaire remained consistent across repetitions but was tailored based on whether the experiment varied audio delay or sampling rate (see Section 4 for details). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline
**Repetition** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline M2E delay (ms) & 5 & 12.5 & 17.5 & 15 & 10 & 0 & 20 & 30 & 40 & 60 \\ \hline \end{tabular}
\end{table} 
Table 1: Scenario A: M2E delays. Figure 3: My Mouth to My Ear delay. Figure 2: Scenario B experimental setup (variable quality). The musicians were not aware of the goals of the experiment or the variable manipulated in each experiment. We deliberately chose the sequence of audio delays and sampling rates so that it would appear to be random, rather than increasing or decreasing. By using a short musical piece (less than one minute) and minimal questionnaires that could be completed from where the musicians were sitting, we tried to minimize the time the musicians spent in the experiments. Our intention was to avoid tiring the musicians and prevent the introduction of any biases. For audio delay, we used values between 0 and 60 ms. It is important to note here that musicians are used to non-zero delays due to relatively low propagation of sound compared to light: the speed of sound is 343 m/s, so in 10 ms sound travels 3.43 m. As a result, two musicians located in the same room at a reasonable distance from each other could very well experience M2E delays of 10 ms, a band on a small stage could expect M2E delays of maybe 20 ms, while a symphonic orchestra on a large stage faces delays so high that it needs a conductor to provide visual synchronization cues. The range of delays tested focused mostly on low delays to assess up to which point the QoE remains acceptable, as well as a few higher delays to see how musicians would cope with them. On the other hand, for audio quality we used a range of standard sampling rates, plus some intermediate values. At the low end, we have voice telephony quality at 8 kHz, while at the high end, we have CD Audio quality at 44.1 kHz and Digital Audio Tape quality at 48 kHz, commonly used in motion pictures, plus 88.2 kHz, which is common in studio recording and high-end digital audio. Linear sampling with 16-bit samples was used throughout. For single channel audio, these sampling rates lead to bitrates from 128 kbps to 1.411 Mbps, which can really make a difference for the limited uplink speeds of residential DSL lines.