I have a number of FLAC files with replaygain tags. I'm attempting to use
ffmpeg to create AAC versions (I've tried both "aac" and "libfdk_aac" in a
separate build). By default it seems to bring over much of the metadata,
but not the replaygain information. Is there something I am doing wrong
that is leaving these behind/unconverted? I see that the replaygain data
is recognized and shown as side data for the original container. But it
isn't present in the output file.
I've tried with "-map_metadata 0" as well, but that seems to have identical
output.
Thank you for assistance.
ReplayGain in FLAC is implemented in a totally different way than in an
MP4 container. Short answer, AAC itself does not support ReplayGain, It
is the container holding the AAC stream (usually MP4) which does that in
it's own proprietary way.
So the ReplayGain tags used by FLAC cannot be converted to AAC or MP4
(M4A) tags. Most reliable way is to transcode FLAC to AAC and then
recalculate ReplayGain for the M4A container.
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