[codec] OggOpus: Rational for excluding replaygain tags?

[Calvin Walton <[e-mail]>]

Hi everyone,

I've been going through draft-terriberry-oggopus-01, and I'm wondering
why this section is present:

   To avoid confusion with multiple normalization schemes, an Opus
   comment header SHOULD NOT contain any of the REPLAYGAIN_TRACK_GAIN,
   REPLAYGAIN_TRACK_PEAK, REPLAYGAIN_ALBUM_GAIN, or
   REPLAYGAIN_ALBUM_PEAK tags.

As an end-user, I'm considering using Opus for part of my music
collection. The rest of my collection (a mix of file formats including
MP3, Vorbis, FLAC, AAC) is all normalized using the replaygain standard.

While I don't see any technical problems with R128 - it is a good volume
normalization scheme - the issue is that it is a *different*
normalization scheme from the replaygain tags that are supported in
other audio formats - it gives different results, and isn't directly
interoperable, and isn't supported by many existing players.

I've done a survey of existing player applications with Ogg Opus
support. All of these players:
* Rockbox (development version with Opus support)
* foobar2000
* GStreamer-based (e.g. Rhythmbox)
will use the REPLAYGAIN_TRACK_GAIN and REPLAYGAIN_ALBUM_GAIN tags (as
specified in Vorbis) to apply replaygain to Opus tracks. Both Rockbox
and Gstreamer-based players will first apply the stream header's output
gain field prior to applying the replaygain correction; I haven't
checked foobar2000 on that point.

Of the players in that list, only foobar2000 supports the R128 gain tag.
I am not sure what happens if both tags are present.

I would suggest that instead of simply saying that the OggOpus file
SHOULD NOT contain the replaygain tags, it should say that they MAY be
present, and should specify how they are interpreted and applied (This
mainly would concern their relation to the stream "output gain").

I would however recommend that the files still SHOULD NOT have the
REPLAYGAIN_{TRACK,ALBUM}_PEAK tags, as Opus does not have a bit-accurate
decoder specified, and the peak values will also vary based on the
sample rate selected for decoding.

I'd appreciate any comments you have, I'm really curious as to why the
draft is recommending against the use of replaygain tags.

-- 
Calvin Walton <[e-mail]>