Re: [codec] draft-ietf-codec-oggopus-01

[Calvin Walton <[e-mail]>]

On Fri, 2013-05-24 at 23:45 +0800, Ralph Giles wrote:
> I've updated the Ogg Opus encapsulation draft.
> 
> https://tools.ietf.org/html/draft-ietf-codec-oggopus-01

Hi,

I note that the specification still has some wording regarding the gain
header fields that I hope could be clarified.

>    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.
> 
>    There is no Opus comment tag corresponding to REPLAYGAIN_ALBUM_GAIN.
>    That information should instead be stored in the ID header's 'output
>    gain' field.

My concern is regarding the note about the album gain.

>From previous discussion, my impression is that the ID header's "output
gain" is supposed to have no semantic meaning, other than that it is the
preferred playback volume offset as set by the last person to edit the
file metadata.

Unfortunately, existing players (some versions of foobar2000, although I
haven't tested the latest) have taken the wording of this section to
mean that the value in the ID header field can be interpreted as per the
EBU-R128 specification, and be used for volume normalization relative to
the R128 reference level.

This means that if I e.g. put a value such that a track will be the same
volume level as my (89 dB SPL-reference) ReplayGained vorbis files, then
foobar2000 will reinterpret it, and play the file at a different volume
than I desired! (Roughly ~3dB offset.)

Other players (e.g. rockbox and gstreamer-based players) will use the ID
header value as-is, and apply no other correction.

At this point, I'm not sure what my recommendation would be, but I hope
the wording in this section can be clarified somewhat to indicate that
the player cannot assume that the value in the ID header gain field
cannot be interpreted as having any particular semantic meaning - and
should be simply applied as-is.

-- 
Calvin Walton <[e-mail]>