Control of playback

Clearly, a multimedia recording and playback server is a small part of a much larger architecture. It is possible to design and build a large set of applications on top of real- time multimedia indexes. For future reference, there are at least two major efforts underway to define protocols for retrieving multimedia from web servers, and even organising collections of web servers to function as a type of application level multicast delivery network. These are the RealAudio effort from Progressive Networks, and the RTSP (Real Time Stream Protocol) currently udner discussion in the IETF Working Group on multimedia. More details are available from http://cgi.realaudio.com/, and the relevant workingdraft standards.

RTSP is designed to operate over TCP, and over RTP/UDP, and is largely aimed at controlling the playout of media, much i nthe way that a (typical Infra Red based digital wireless transmission) remote control is used to control a domestic VCR. The actual delivery of the media itself, if VoD, near VoD, or realtime, would be accomplished typically using techniques described in the rest of this book.

One interesting exception to this design approach is one of the proposals in DAVIC, designed for delivery of digital TV over Cable TV networks. Here audiovisual information is the only first class citizen, and other data types are layered on top of the media stream. which, it has been proposed, would be carried i nan MPEG systems stream over an ATM layer modulated on the raw cable signal.

Whether this takes off in any way remains to be seen, but it seems more likely that a pervasive IP based approach offers more flexibility and management, as well as immediate availability of a range of other Internet based applications.