Re: [Bug-gnupod] Patch to support ReplayGain / mp3gain

[Richard van den Berg]

On 5/8/09 12:14 PM, Frank Blendinger wrote:
> You might want to take a look at this python script:
> http://mpd.wikia.com/wiki/Hack:ape2id3.py
>
> It will read the ReplayGain settings from APE tags and add it to ID3v2
> tags. It has worked without any problems for me so far.
>   

That script seems to create custom TXXX tags. This is not a standard and 
it surprises me that players support it. I know rockbox (alternative 
iPod firmware) does as well. Besides, I don't need to store the data I 
already have in APE tags in yet another format. This would require me to 
always run a conversion script after I run mp3gain, and I would have to 
add the script to abcde (the ripper script I use) as well.

> So to sum this up, we could have to deal with:
>    * APE tags
>    * RVA2 in ID3v2.4 or XRV/XRVA in ID3v2.3 or RGAD in ID3v2.3
>    * iTunNORM as Comment in either ID3v2.3 or v2.4
>
> Did I miss anything?
>   

Yes, the following TXXX tags:

TXXX:replaygain_album_gain
TXXX:replaygain_album_peak
TXXX:replaygain_track_gain
TXXX:replaygain_track_peak

> So how will gnupod handle this? I suppose the iPod will just use
> iTunNORM and ignore anything else. Will gnupod use a present iTunNORM
> comment or will it always create one from ID3v2/APE information?
>   

The patch I created will favor ReplayGain info found in an APE tag over 
iTunNORM since RG is a superior algorithm. RVA2 tags are converted to 
the iTunesDB volume value when "--max-vol-adj" is used with 
gnupod_addsong.pl. It is combined with normalization info (if also 
present) when Sound Check is enabled on the iPod.

> What will be used if both ID3v2 and APE is present? I read about a
> (possible?) --ignore-ape option, but I didn't really get what the
> default behaviour will be. 

The new default behavior will be to read APE tags, including ReplayGain 
information. ID3 tags with the same name of APE tags will overwrite the 
APE tags. ID3v2 tags are favored over ID3v1 tags.  Perhaps we should 
also read TXXX:replaygain_album_gain and TXXX:replaygain_track_gain from 
ID3v2 tags to replace the respective APE tags.

> I, for one, would propose to use RVA2 if
> present and APE only as a fallback, as it is more standardize and also
> more often present in actual files, APE does not seem to be supported by
> many (software) players (and I guess not by a single hardware device),
> while some do support ReplayGain tags in ID3v2.
>   

My Neo 35 running OpenNeo reads RG info in APE tags just fine. As do my 
various Squeezeboxes (powered by SqueezeCenter). One of the problems is 
that normalization data like RG APE tags and iTunNORM comments is 
sometimes combined with RVA2 volume information. iTunes is one of the 
software that does this, and SqueezeCenter also combines the tags (but 
there is some debate to change that). So favoring RVA2 over RG info 
should really be selectable by the user. As explained above, iPods will 
combine the two when both are present in the iTunesDB and Sound Check is 
enabled.

> And one last thing: what about support for the music formats covered by
> the gnupod_convert_* scripts? Those can carry ReplayGain tags, too, so
> it would be nice to keep/convert them when the audio gets converted.
>   

The tags of non-native file formats are spit out by the GET_META part of 
the gnupod_convert_* scripts. If they spit out the correct soundcheck 
value, that would work. So if you can show us how to get ReplayGain 
information from those non-native files, we can probably make it work.

> So far I only have experience with OGG Vorbis files. There is tool
> similar to mp3gain called vorbisgain to calculate the gain data and
> write the tags, and they can be read with the vorbiscomment tool. So I
> guess it should not be that hard to read those tags as it is done with
> the common artist/title/... tags and add them to the converted mp3
> files.
>   

gnupod_convert_OGG.pl uses Ogg::Vorbis::Header which may or may not 
support the RG tags. If you send me an ogg file (privately) that 
contains RG info I can check (and patch gnupod_convert_OGG.pl).

> I have read that there is also support for ReplayGain tags in FLAC
> files, but I have no experience with that.
>   

We'll leave that for someone that actually uses FLAC (I don't).

Cheers,

Richard