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I'm using ffmpeg -i input.mp4 -c:a copy -vn -sn output.m4a
to extract audio from an mp4 file. However, when I open the mp4 in Spek, it shows 255kbps AAC and the extracted file is only 200kbps.
Also, the spectrogram isn't 1:1 identical, there are small differences.
Why is that so? Is it normal, or is there a better way to do it?
As per Spek docs, it uses ffmpeg to read and decode files, so my best guess is that Spek uses an older version when ffmpeg did not discard initial samples from MDCT-codec streams. The ffmpeg output should be taken as accurate. – Gyan – 2018-09-19T05:01:54.060