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

[H. Langos]

On Fri, May 08, 2009 at 04:16:45PM +0200, Richard van den Berg wrote:
> On 5/8/09 2:26 PM, H. Langos wrote:
>> Yeap. And if mp3gain can't be changed to do write RVA2 tags by itself, 
>> maybe there is a way to make mp3gain only do the analysis and pass its 
>> result to a programm that can write those RVA2 tags?
>
> That would be possible using:
>
> $ mp3gain -o -q -s s foo.mp3
> File    MP3 gain    dB gain    Max Amplitude    Max global_gain    Min  
> global_gain
> foo.mp3    -7    -10.500000    38780.534185    255    145
> "Album"    -7    -10.500000    38780.534185    255    145

Well, then thats the way to go. Just write some awk or perl wrapper around
it, push those values into your favorite tagger and there you go.

>> Do you know if the author of mp3gain is still active? reachable? alife?
>>   
>
> I have no clue.. in the thread from 2004 they mention he was unresponsive.
>
>> So how do we get mp3gain's analysis results into RVA2 / XRVA tags?
>>   
>
> By changing the mp3gain C code. It's open source after all. I just don't  
> have the stomach or time for it at the moment. Perhaps using the tagging  
> code from normalize will make it easier. However, there could be a good  
> reason not to attempt it. From  
> http://jwz.livejournal.com/370342.html?thread=4232358#t4232358
>
>> Anway, whats not clear to me is if the RVA2 value could be used with  
>> ReplayGain. Both ReplayGain and RVA2 are applied as a constant value  
>> across the complete track, but I'm not sure the units are the same as  
>> the RVA2 tag.

Well, the gain is given in dB relative to a reference level that most times
is 83dB (But sometimes might be 89dB. The nasty details are here:
http://www.mars.org/mailman/public/mad-dev/2004-February/000993.html )
I assume that mp3gain uses 83dB. But it wouldn't hurt to check.

The "units" are in both cases dB. The encoding is different. mp3gain writes
some prosa while the RVA2/XRVA tags are more terse encoding, but they both
deal with relative volume changes in dB.

>> Are you sure about the non desctructivnes of mp3gain?
>>   
>
> You can run mp3gain in several modes, either (often reversible) changing  
> the volume of the file or just tagging it. I use it only for tagging.  
> From the Debian man page:
>
>       mp3gain  actually changes your file’s gain only when you use one  
> of the
>       options -r,  -a, -g, or -l. If none of these options is given,   
> only  a
>       tag denoting the recommended gain change is written to the file.

Thats good to know! Thank you!
And thank you very much for testing mp3gain's undo feature.

cheers
-henrik