Digital Broadcasting - Cascaded Audio Coding
David Marston and Andrew Mason
Broadcasters have experienced significant problems with cascaded audio
coding in the broadcast chain following the introduction of digital transmission.
It has been found that cascading different codecs can result in an overall
degradation in sound that many listeners find objectionable. A comprehensive
investigation of this problem has been conducted by members of the EBU
project group B/AIM.
This paper describes typical cascades of codecs found in radio broadcast
chains, and aims to identify the most critical combinations. The intent
is to guide broadcasters in deciding which codec combinations should be
avoided to maximise sound quality.
The process initially involved deciding which were the most commonly
used codec combinations used in digital radio. The next stage was to use
objective assessment software, which gave an initial guide to the expected
quality scores for each cascade. Finally, subjective tests involving trained
listeners were performed to ensure more accurate and
reliable results.
The resulting quality results were then analysed and conclusions drawn
up to which cascades are best avoided.
This document was originally published in the Proceedings of the International
Broadcasting Convention, September 2005.
http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP118.pdf
Thanks to Mike Barraclough for the link.
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