27

I converted an audio stream into 3 different settings using essentially the same format. They ended up being exactly the same size. Why is this?

ffmpeg -i "Likoonl-Q1-All.mp4" -c:v copy -c:a libmp3lame -q:a 1 -b:a 192k "Q1-All-192k.mp4"
ffmpeg -i "Likoonl-Q1-All.mp4" -c:v copy -c:a libmp3lame -q:a 1 -b:a 160k "Q1-All-160k.mp4"
ffmpeg -i "Likoonl-Q1-All.mp4" -c:v copy -c:a libmp3lame -q:a 1 -b:a 128k "Q1-All-128k.mp4"
52

Because you're setting -q:a which is LAME's VBR setting. When you use -q:a, the CBR setting (-b:a) will have no effect.

If you look into the MP3 encoding guide from the FFmpeg wiki you'll find the possible values for -q:a with their corresponding average bitrate.

For completeness' sake, here's the relevant part of libmp3lame.cqscale is the long name of q:

/* rate control */
if (avctx->flags & CODEC_FLAG_QSCALE) { // VBR
    lame_set_VBR(s->gfp, vbr_default);
    lame_set_VBR_quality(s->gfp, avctx->global_quality / (float)FF_QP2LAMBDA);
} else {
    if (avctx->bit_rate) {
        if (s->abr) {                   // ABR
            lame_set_VBR(s->gfp, vbr_abr);
            lame_set_VBR_mean_bitrate_kbps(s->gfp, avctx->bit_rate / 1000);
        } else                          // CBR
            lame_set_brate(s->gfp, avctx->bit_rate / 1000);
    }
}
  • 28
    4 minutes to find the source. +1 – Jonas Schäfer Oct 15 '14 at 19:38
  • I wondered if that wasn't the case! Thanks a lot. – Arlen Beiler Oct 15 '14 at 19:55

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