EBU TECHNICAL has published an EBU – TECH 3339 document entitled “EBU Evaluations of Multichannel Audio Codecs – Phase 3”. This report represents a follow-up of an earlier Phase 1 and 2 report given in EBU – TECH 3324 published in September 2007. Both reports are the result of the extensive studies performed by the Project Group D/MAE (Multichannel Audio Evaluations) chaired by Gerhard Stoll (IRT).
This EBU – TECH document addresses a potential practical issue of cascading a variety of different multichannel audio (MCA) codecs used in the studio, contribution, distribution, emission and in the home. For example, in a typical TV chain, up to five Dolby E or E-aptX MCA codecs may be used using different bitrates. Such a cascade of multiple codecs may result in some unexpected behaviour and may cause significant impairments of end-to-end audio quality.
The D/MAE Group selected as many as 14 MCA codec cascades and performed extensive subjective evaluations involving three EBU laboratories: IRT, WDR and SR. The tests were large-scale indeed – each cascade has been tested by about 20 assessors each evaluating 10 different test sequences!
The report shows that significant impairments are mainly due to the emission codecs, particularly when they are used at low bit rates (e.g. 256 kbit/s for Dolby Digital and 160 kbit/s for HE-AAC). Most importantly, the level of impairments depends on the audio item – on average, “applause” and “harpsichord” test items are most critical and give the worst overall impressions. Our results also highlight an interesting positive effect of Dolby E codec which may, due to a band-limiting effect, reduce some unwanted artefacts.
These tests show that the EBU Members are uniquely positioned to undertake very complex measuring campaigns such as this one. The D/MAE work will be continued, if required, by the new Expert Community on Audio (ECA).
Read the details
For more information, see the full EBU – TECH 3339. If you are interested to join this type of work or learn more about the audio-related topics the EBU is addressing, see the new Expert Community on Audio (ECA).