>We've tried to reach both David Robinson and the author of Vorbis Gain,
>but to no avail.
I received no email from you.
All practical implementations of ReplayGain use 89dB as the reference level (83 6db).
The 6dB was added, as far as I understand, to bridge the gap between non-ReplayGained and ReplayGained files a little more.
If you play a ReplayGained file alongside a non-ReplayGained file, the difference will currently be between 5 and 10dB, depending on how badly the CD was mastered. Thats a big difference, but I'll do no harm. If the 6 dB wouldn't have been added, the difference would be between 11 and 16dB.
A difference of 16dB is potentially dangerous. If you forget to turn down the volume, it could have some nasty consequences.
Bridging the gap a little also solves (somewhat) the problem of users moaning that the ReplayGained files are 'too silent'.
89dB is still low enough the vast majority of CD's will not clip at this level, or only very (inaudibly) little. Or alternatively, if Clipping Prevention is in use, the changes in volume can be kept to a minimum. You certainly don't want to go over 89dB, though.
So, in short, 89dB is a practical compromise that seems to work very well. The VorbisGain spec uses a 89dB reference level, as does MP3Gain and MusePack.
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GCP