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Topic: aacgain - Tags and Values (Read 2995 times) previous topic - next topic
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aacgain - Tags and Values

Hi,

I would like to have a "Apply ReplayGain to MP4 Data" option in foobar, analogue to "Apply ReplayGain to MP3 Data" which is already there. As this option does not seem to exist at the moment, I am trying to create it by myself, using aacgain as an external application. However, I have two question regarding aacgain's operation. Let's start with an example:

AAC-File, ReplayGain info added by foobar:
Code: [Select]
Atom "----" [replaygain_album_gain] contains: -7.32 dB
Atom "----" [replaygain_album_peak] contains: 1.124630
Atom "----" [replaygain_track_gain] contains: -6.76 dB
Atom "----" [replaygain_track_peak] contains: 1.075577


After running aacgain for an analysis:
Code: [Select]
C:\aacgain\test\test.m4a
Recommended "Track" dB change: -6.760000
Recommended "Track" mp3 gain change: -4
Max PCM sample at current gain: 35244.492188
Max mp3 global gain field: 182
Min mp3 global gain field: 99


Recommended "Album" dB change for all files: -7.320000
Recommended "Album" mp3 gain change for all files: -5


The relevant tags look like this:
Code: [Select]
Atom "----" [replaygain_album_gain] contains: -7.32
Atom "----" [replaygain_album_peak] contains: 1.12
Atom "----" [replaygain_track_gain] contains: -6.76
Atom "----" [replaygain_track_peak] contains: 1.08
Atom "----" [replaygain_track_minmax] contains: 99,182


And after applying a gain change of -7 to the file:
Code: [Select]
Atom "----" [replaygain_album_gain] contains: 3.21
Atom "----" [replaygain_album_peak] contains: 0.33
Atom "----" [replaygain_track_gain] contains: 3.78
Atom "----" [replaygain_track_peak] contains: 0.32
Atom "----" [replaygain_track_minmax] contains: 92,175
Atom "----" [replaygain_undo] contains: 7,7


So my questions are:

1) Is the only difference between foobar's and aacgain's tags that foobar puts "dB" into the tag and uses more digits? Could I therefore just strip the "dB" and some digits from foobar's tags to make aacgain use them? "com.apple.iTunes" as mentioned in this thread (http://www.hydrogenaudio.org/forums/index.php?showtopic=55934) does not seem to be added by aacgain anymore.

2) What does the gain value stand for? I cannot find the value of 7, neither directly nor in a dB-ized form in either gain or peak tag changes.gain went from -7.32 to +3.21 (+10.53), peak went from 1.08 to 0.32 (1/3.38). 10^(10.53/20)=3.36. So this fits, but where is the 7?

Thanks for your help.

rumpel

aacgain - Tags and Values

Reply #1 –
Quote
So this fits, but where is the 7?

7 * 1.50515 dB = 10.53605 dB.

1.50515 = 1/4 * 20*lg(2) - the smallest possible step of volume adjustment for mp3 files (and, probably, AAC - I just don't know).
 

aacgain - Tags and Values

Reply #2 –
Quote from: lvqcl on
1.50515 = 1/4 * 20*lg(2) - the smallest possible step of volume adjustment for mp3 files (and, probably, AAC - I just don't know).

Thanks, that answers question 2 and also explains the difference between "Recommended dB change" and "Recommended mp3 gain" in aacgain's output.