# Impact of Audio Delay and Quality in Network Music Performance +
## 1 Introduction  
The social distancing requirements imposed during the SARS-CoV-2 pandemic brought to the forefront the value of real-time collaboration applications. One such application is Network Music Performance (NMP), which refers to musicians performing when connected over a network. All real-time human-to-human applications require low delays; for example, in voice telephony, the goal is for the end-to-end delay to not exceed 150 ms, otherwise the conversation becomes unnatural and the quality of their experience is significantly reduced. NMP is far more challenging: studies of human participants tryingto synchronize their hand claps over a network found that synchronization failed, with delays exceeding 25-30 ms [1]. Such requirements are so hard to meet that, to many researchers, NMP seems like an unattainable goal. However, musicians often state that when using specialized NMP tools, they can actually collaborate, at least over moderate distances. To assess the validity of such claims, we performed a small pilot study with four pairs of musicians, which indicated that higher levels of delay may be tolerable in actual musical performances [2]. Considering the higher bandwidths and lower latencies promised by 5G networks, this can bring NMP within the realm of feasibility. The evidence that NMP may be possible over larger delays, coupled with the need to save bandwidth to allow adding video over the, normally more limited, uplink, made us reconsider the question of what is the tolerance to delay for NMP for rhythmic performances and to what extent can we sacrifice audio quality to save bandwidth without increasing delay? Audio quality is relevant, since NMP-specific tools commonly rely on raw audio to avoid the delays introduced by audio coding; even Opus, which is optimized for low delay, introduces delays of at least 5 ms with reduced quality, under ideal conditions [3]. Unlike previous studies that either focused on hand clap synchronization or very small numbers of musicians, considering only delay and not quality, we designed a series of experiments where pairs of musicians played music, while we modified the delay and audio quality, using questionnaires to assess the Quality of User Experience (QoE) in a subjective manner. We also recorded audio and video of all the performances for possible future processing and analysis. In this paper, after presenting the experimental setup and the questionnaire used, we provide an analysis of the musicians' responses, including their statistical significance, as well as a discussion of the implications of the results and some guidance on implementing similar experiments. Our main research questions are:  
1. How does varying the one-way audio delay, from near zero up to around 60 ms, affect the musicians' QoE? 2. How does reducing the audio sampling rate (from 88.2 kHz down to 8 kHz) impact the QoE of NMP participants? We hypothesize that: (H1) musicians can tolerate higher one-way delays for actual performed pieces than earlier rhythmic hand-clapping studies have suggested; and (H2) reducing audio quality by sub-sampling is not detrimental to their overall QoE. The null hypotheses are that (N1) any delay above 25-30 ms is intolerable, and (N2) lowering the sampling rate will reduce the QoE. Our results support the rejection of these null hypotheses under the particular conditions described. The contributions of this paper are as follows:  
1. We show that the QoE of NMP for rhythmic performances is acceptable for delays of up to 40 ms, rather than the 25-30 ms found in previous work, which focused on hand-clap synchronization; this makes NMP borderline feasible for 5G networks [4]. 2. We show that the QoE of NMP is not significantly affected when reducing the sampling rate from studio-level quality to telephone quality (an order of magnitude). 3. We show that in NMP sessions where both audio and video channels are available, musicians rely far more on audio for synchronization, even when video has a lower delay. 4.