Re: [codec] draft-ietf-codec-oggopus: R128_TRACK_GAIN units

[Calvin Walton <[e-mail]>]

On Mon, 2014-09-08 at 13:11 -0700, Timothy B. Terriberry wrote:
> As an individual...
> 
> Mark Harris wrote:
> >  output gain field of the Ogg Opus header.  Is it expected that 
> > these
> >  units would be adopted by other codecs?
> 
> I can't speak for anyone else, but if _I_ were making a new codec, I
> wouldn't see a need to innovate here. This design has the lessons of
> doing this for 15+ years baked into it.
> 
> >  concerned that if Ogg Opus uses 1/256 LU units, and another 
> > format or
> >  codec uses the much more obvious LU (dB) units, then someone may 
> > be in
> 
> Then they should pick a different tag name.
> 

Just to comment on this:

The format of the "vorbiscomment" fields is very similar between at 
least Ogg Vorbis, FLAC, and Ogg Opus. As a result, the tags are often 
copied verbatim between the three formats during transcoding, and the 
interpretation of tag meaning based on name is usually shared between 
all three codecs in parsing code.


As a result, without explicit code in the parser to e.g. reject or 
interpret differently this tag between different formats, a player 
that implements the R128_*_GAIN fields will as a side-effect add 
support for the same tag in FLAC and Ogg Vorbis.

(This is the same reason that some players - at least gstreamer-based 
players, rockbox, and foobar2000 - currently accept and use 
REPLAY_GAIN tags in Ogg Opus files)

Since this tag format is defined/specified in the Ogg Opus 
specification, it might make sense to use a prefix on the tag name - 
e.g. OPUS_R128_TRACK_GAIN - if the intent is that this tag is to be 
used only in the Ogg Opus format. This would reduce the likelihood of 
conflicts in the mostly-unregulated vorbiscomment namespace.

If the intent is that the R128 tags are intended to be used also in 
FLAC and Ogg Vorbis then I think it would be best for it to be defined 
outside of the Opus specification, somewhere that developers can 
reference independently. In particular, care would be needed to 
describe interactions with the REPLAY_GAIN tags that are still used in 
these other formats.

-- 
Calvin Walton <[e-mail]>