Bug#797965: bs1770gain somehow "destroys" gapless playback on (at least) lame encoded MP3s

[Christoph Anton Mitterer]

On Sat, 2015-12-19 at 23:24 +0100, Andreas Cadhalpun wrote:
> OK, so the problem is that after remuxing with ffmpeg, there is
> a barely audible ... gap ... between the two files, right?
Yes

> Now I'm a bit skeptical about "LAME adding some special tags".
IIRC, the LAME tag isn't actually an ID3 tag, but padded in some other
parts of the MP3 header which aren't used.
http://gabriel.mp3-tech.org/mp3infotag.html
http://wiki.hydrogenaud.io/index.php?title=LAME#VBR_header_and_LAME_tag
http://libzplay.sourceforge.net/LAMETAG.html

> You've used lame with '--id3v2-utf16 --add-id3v2 --id3v1-only'.
> What is this supposed to do? Add id3v1 tags, or id3v2 or both?
IIRC the problem was that either bs1770gain but more likely python-
rgain (which I had to use in the end, because of the problem with
bs1770gain) didn't set any replay gain tags (which *are* in fact ID3
tags) at all, when no ID3 was present at all.

These options basically made, that both a ID3v1 and ID3v2 "section" was
created (the later with UTF16 encoding), however with no tags.


> Additionally it seems the created files contain neither.
Hmm... maybe I did in addition set one dummy tag like --tc=' ', and
removed that afterwards.


> And leaving out these id3 options doesn't change the output
> from lame.
> 
> So I'm not sure how this is supposed to work.
What exactly now? The stuff with the tags? I guess you can ignore that,
since I've just had it in place for, IIRC, python-rgain,...

What I would assume that ffmpeg does, is, that it simply drops or
somehow mangles up the LAMEtag,... or actually modifies the audio
stream so that it doesn't fit to the LAMEtag anymore.


Cheers,
Chris.
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