Audio Contribution over IP
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Contents |
[edit] Introduction
- Streaming of high quality audio over IP networks is being increasingly used by broadcasters and others to provide high-quality audio contribution feeds (for example to provide audio links between a sports venue or concert hall and the broadcaster's studios)
- Audio quality and delay (on duplex transmissions) are key issues for contribution links.
- In the past these links have made widespread use of ISDN services but these are becoming increasingly difficult or expensive to obtain in some parts of Europe and are being phased out in others
[edit] Systems
Many proprietary systems exist for transporting high quality audio over IP based on TCP or UDP for contribution purposes.
[edit] Example of use
The British Broadcasting Corporation has begun using Audio Contribution over IP in Scotland as part of the Pacific Quay development in Glasgow. The aptX® algorithm is used with 24 bit depth and 48 kHz sampling. This is a higher specification than normal CDs, and is used to avoid concatenation effects. There have been problems found in the past when NICAM and MPEG audio paths have been used in sequence in the broadcast chain. The audio heard in the presentation studio has been acceptable, but listeners at home using Digital Audio Broadcasting (DAB) have complained about objectionable artefacts. To avoid this aptX® is used at high bit rates. This is a differential PCM system that does not use psycho -acoustic processing. It is able to resynchronise after momentary loss of the IP path without noticeable clicking, and is licensed to most Codec manufactures A similar system is being installed in the English Regions and will be installed in Wales and Northern Ireland. The audio packets are sent using User Datagram Protocol (UDP) over the BBC’s Layer3 network. To ensure the audio is not corrupted Quality of Service (QoS) set to ensure that the packets are give priority over other network traffic.
[edit] EBU N/ACIP
The European Broadcasting Union together with many equipment manufacturers has defined a common framework for Audio Contribution over IP in order to achieve interoperoperability between products.
The frameworks defines RTP as a common protocol and media payload type formats according to IETF definitions. SIP is used as signalling for call setup and control. The recommendation is currently published as an EBU Tech3326 document
