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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.