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moov.udta.meta.ilst.---- atoms in .m4a files
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moov.udta.meta.ilst.---- atoms in .m4a files



afconvert writes the iTunes-specific iTunSMPB (for gapless playback)
and iTunNORM (for Sound Check) data in Apple MPEG-4 Audio files, which
is great.

iTunes also uses another moov.udta.meta.ilst.---- atom, "Encoding
Params", which has the following structure in its data area:

For AAC:
       00000000  76 65 72 73 00 00 00 01  61 63 62 66 00 00 00 02
|vers....acbf....|
       00000010  62 72 61 74 00 03 e8 00  63 64 63 76 00 01 07 00
|brat....cdcv....|

For ALAC:
       00000000  76 65 72 73 00 00 00 01  61 63 62 66 00 00 00 03
|vers....acbf....|
       00000010  76 62 72 71 00 00 00 00                           |vbrq....|

When these atoms are set, iTunes will correctly display the target
bitrate and mode for the file.

I've deduced that for AAC, acbf is the bitrate mode: 1 = CBR, 2 = VBR,
3 =? VBR constrained. brat is the target bitrate, but I've no idea
what cdcv is.

Is there any documentation or information available on these atoms?

Thanks,
Stephen
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