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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 20:34:57 +0200
User-agent: Mutt/1.5.18 (2008-05-17)

On Sat, May 09, 2009 at 06:37:19PM +0200, Richard van den Berg wrote:
> On 5/9/09 6:01 PM, H. Langos wrote:
>> 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.
>>   
>
> Very good point, I didn't think of that. Quite a shame to limit the RVA2  
> tag to 6 dB gain only.

Just to clarify: It's not the RVA2 tag that is limited .. It is the "volume"
attribute that the iPod expects to find in its iTunesDB. That attribute is
stored in one byte only. Valid values are 0-200 where 
0 is interpreted as -100%, 
100 is +/- 0% and 
200 is interpreted as +100% volume.

So RVA2 is still the way to go. Just don't use it to manupulate the "volume"
attribute.

I don't know how this came to be, but I guess it is from the fact that
iTunes (ab)uses the RVAD tag to store its -100% to +100% slider value that
gave the author of the RVA2 integration the idea.

>> So I guess we might want to abandon that in favor of manipulating the  
>> soundcheck attribute: http://www.id3.org/iTunes_Normalization_settings
>> 
>
> That should be easy to do. Just convert the Sound Check value to dB, add  
> the RVA value and convert it back. 

I guess you mean RVA2.

> I could write a patch for that, but  
> I'll need some test files as I don't use any RVA tags myself. I guess I  
> can use normalize to create some RVA2 tags.

Yeap. Thats probably the best.

> This will imply that gnupod will stop using setting Volume iTunesDB  
> attribute, except when a value is already found in the iTunesDB (by  
> tunes2pod.pl) or am I missing something?

Yes. We would leave volume alone and go for soundcheck instead.

We would stop using that crude one byte and start using that 32bit dBm
value. At least thats what it is supposed to encode.

I just greped through my mp3s and I have only found positive values ranging
from 21 to 4219 (= 4.219 dB )

I would suggest that you take some stuff that is too loud i.e.
louder than the magic 89dB, feed it to iTunes and see how iTunes encodes 
negative adjustments. 

cheers
-henrik





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