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Do all mp3 decoders take gain data into account when decoding?
If you are talking about "gain data" as applied by MP3Gain, then the answer is yes. foobar's implementation (storing the gain values in APE tags) is technically superior, but currently not used anywhere but in foobar itself.
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BTW, could u recommend a decoder for mp3?
Use Winamp's decoder (FhG) or LAME - both are very accurate. AFAIK, Winamp's is
slightly better, but I highly doubt the difference is audible.
MAD is also very good, but don't believe all the hype that it's supposed to make such a huge (audible) difference.