Synchronisation of Multimedia Streams: Benefits, Challenges and Solutions
Hugh T. Melvin and Liam Murphy
Affiliation: College of Engineering&Informatics, National University of Ireland, Galway, Ireland.
Keywords: Media synchronisation, clock skew, Public Switched Telephone Network (PSTN), synchronised clocks, Next Generation Networks and Applications (NGNA), Quality of Service, Quality of Experience, Voice over IP, Network Time Protocol (NTP), Real-time Transport Protocol Control Protocol (RTCP), Multimedia terminals, PC-based soft-phones, mouth-to-ear (M2E), Massively Multiplayer Online Gaming (MMOG), scenarios, anti-jitter buffer, Media Mixers, Conferencing Services, relative skew, Voltage Controlled Oscillator, time-of-arrival, Timestamp mode, Round Trip Time, Internet Engineering Taskforce, Set Top Box, wireless sensor networks
In the traditional Public Switched Telephone Network (PSTN), synchronised clocks and circuit switching constitute basic building blocks and these coupled with dumb voice terminals ensure that network delays are deterministic, network devices are tightly synchronised and media synchronisation is largely unsupported. The same cannot be said of evolving IP-based networks that are slowly replacing legacy systems and end-user devices that host a multitude of voice/video/conferencing/gaming/messaging applications. In this new Next Generation Networks and Applications (NGNA) world, network delays are generally non-deterministic and end-user terminals are much more complex. This presents a whole range of challenges that can impact significantly on the final Quality of Service (QoS) as perceived by user (also termed Quality of Experience QoE). Previous work by the authors has shown that by incorporating synchronised time into Voice over IP (VoIP) terminals, significant gains in voice quality can be achieved. Related research by the authors has examined the extent to which a lack of synchronisation (or skew) between end-user terminal clocks can affect VoIP quality and has proposed and tested a high-level solution for skew detection/compensation. In this paper, we summarise the problem that skew presents and how it manifests itself in a number of evolving application scenarios. These include PSTN/VoIP gateways, the use of media mixers for combining media streams, conferencing services, MMOGs and IPTV. We then review the main body of research in this area and identify the key patents that have emerged in this field.
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