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


From: H. Langos
Subject: Re: [Bug-gnupod] Patch to support ReplayGain / mp3gain
Date: Sat, 9 May 2009 18:01:59 +0200
User-agent: Mutt/1.5.18 (2008-05-17)

Hi Frank,

On Fri, May 08, 2009 at 12:14:53PM +0200, Frank Blendinger wrote:
> 
> On Thu 2009-05-07 21:16, Richard van den Berg <address@hidden>
> proclaimed:
> > On 5/7/09 5:06 PM, H. Langos wrote:
> >> What's important is the fact that there seems to be a standard that defines
> >> how volume adjustment data can be stored in id3 tags.
> >
> > Nice research. RVA2 is definitely the standard to use, and gnupod  
> > already has support for it. The problem is that there is not a lot of  
> > software that writes RVA2 tags. (The only ones I know about are iTunes  
> > and normalize). mp3gain should really be changed to write RVA2 tags  
> > instead of APE ones.
> 
> 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.

Oh, this is a new one .. it takes the prose that mp3gain and vorbisgain
generate and stores it in a prose id3 tag ... 

> 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?

You missed the id3v2.3 RVAD tags.

The definition in the standard is IMHO a horrible read. Little wonder the
apple guys used it in a way that certainly was not intended by the
standard's authors... https://savannah.nongnu.org/support/?105294
The intended use stays unclear. But maybe my hifi karma is not good
enough to grasp it ;-)

> All of those could potential be present at once in a file. There can
> only be either an ID3v2.3 or an ID3v.4 as far as I can tell. But there
> can always be that iTunNORM comment in addition to the RG tags, and also
> additional APE tags. I have actually seen files in the wild that have
> all of this present. [1]
> 
> 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?

Currently gnupod writes the "soundcheck" attribute it gets from iTunNorm 
comments as well as the "volume" attribute computed from the RVA2/XRVA tag. 

The volume attribute however is limited in its scale. it can only go from
-100% to +100%. The lower bound is ok as it represents silence, but the
upper bound is BS as it only allows gains of 6dB.

So I guess we might want to abandon that in favor of manipulating the 
soundcheck attribute: http://www.id3.org/iTunes_Normalization_settings


As I see it now. I am wasting a lot of time on this. I'll think about 
it but I guess I will not put too much work into it myself.


cheers
-henrik







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