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

[Richard van den Berg]

On 5/9/09 9:14 AM, Frank Blendinger wrote:
> I'll talk to the mpd developers to see if they are willing to support
> RVA2/RGAD and/or APE tags.

I used mpd before switching to a Squeezebox. IIRC it is written in perl. 
If they are using MP3::Info adding APE support is easy; just add an 
extra option to the get_mp3tag() call.

> Do I understand you correctly that this could lead to have two gain
> values added on top of each other? I don't think you'd want that...
>   

That depends. It gives the user some manual control over the desired 
volume to use. If someone feels that applying the automatic algorithm 
makes a track too loud or too quiet, he can manually set an RVA value to 
compensate for it. This could be preferred over adjusting the normalized 
value directly because when the files are rescanned by the normalization 
algorithm, the RVA tag will not be reset. This is how iTunes does it 
(iTunNORM is set by the Sound Check algorithm, while RVAD can be set 
manually with a volume slider, they are both combined at playback). I'm 
not saying I prefer combining these tags, but it's how the iPod works: 
the Sound Check and Volume values are combined at playback (which Sound 
Check is enabled on the iPod). With my patch the Replay Gain data is 
converted to the iTunesDB Sound Check value while gnupod already 
translates RVA2 values the iTunesDB Volume value.

So far we've only established that the "normalize" software writes RVA2 
tags (iTunes writes RVAD which gnupod currently ignores). Using both 
normalize and mp3gain is silly and makes no sense, but would indeed 
result in these two algorithms being used on top of each other unless 
you disable Sound Check on the iPod.

> I think this would destroy the whole "have all songs at the same average
> volume level" concept, when you apply something else on top of the
> ReplayGain data.
>   

I agree, and I think Apple implemented it this way because Sound Check 
uses a crappy algorithm. When you use Replay Gain there is really no 
need for manual adjustments or combining the values.

> To have only ReplayGain and nothing else applied in the iPod, I will
> have to not use the --max-vol-adj parameter and not use Sound Check,
> right?
>   

ReplayGain is stored as a Sound Check value, so you will have to enable 
Sound Check on your iPod. Leave the --max-vol-adj alone so gnupod 
ignores the RVA tags (0 is the default).

> At least according to the mpd guys, those TXXX tags are quite common, so
> it might help some people if those are supported in gnupod. Personally,
> I'm going to keep the APE tags on my files, so I don't care that much.
>   

I just checked and MP3::Info returns each TXXX tag as an individual 
value (not a hash). So TXXX:replaygain_track_gain="-2 dB" becomes 
$hs{REPLAYGAIN_TRACK_GAIN}="-2 dB". This means the TXXX tags overwrite 
the APE ReplayGain tags by default. I just tested this, and it works; 
when ReplayGain data is set differently in the APE tag and TXXX tag, the 
TXXX is used by gnupod. Sweet.

Cheers,

Richard