In an ideal world people interacting using real-time multimedia links experience perfectly synchronized media, and there is no latency of transmission: the interlocutors would hear and see each other with no delay. Methods to achieve the former are discussed in other chapters in this book, but for a variety of practical and physical reasons, delay-free communication will never be possible. In some cases, the delay will be very obvious since it will be possible to observe the reaction time of the listeners modified by the delay, or there may be some acoustic echo from the listeners' audio equipment. However, in the absence of echo, the users themselves do not always explicitly notice the presence of delay, even for quite large values. Typically, they notice something is wrong (for example "we kept interrupting each other!"), but are unable to define what it is. Some useful insights into the impact of delay on a conversation can be obtained from the linguistic discipline of Conversation Analysis, and especially the analysis of "turn-taking" in a conversation. This chapter gives an overview of the challenges in evaluating media synchronicity in real-time communications, outlining appropriate tasks and methods for subjective testing and how in-depth analysis of such tests can be performed to gain a deep understanding of the effects of delay. The insights are based on recent studies of audio and audiovisual communication, but also show examples from other media synchronization applications like networked music interaction.

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Digital Representation and Interaction, Ericsson Research , British Telecom Research and Innovation
Centrum Wiskunde & Informatica, Amsterdam (CWI), The Netherlands

Berndtsson, G, Schmitt, M.R, Hughes, P, Skowronek, J, Schoenenberg, K, & Raake, A. (2018). Methods for human-centered evaluation of MediaSync in real-time communication. In MediaSync: Handbook on Multimedia Synchronization (pp. 229–270). doi:10.1007/978-3-319-65840-7_9