Abstract
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.
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