FFMpeg and max bitrate

Question

For a personal project, using AV1 codec, I have a bitrate constraint for a video to 88kbps, with choosen video bitrate at 66kbps and mono audio bitrate at 22kbps.

I currently use this command:

ffmpeg -i input.mp4 -c:v libaom-av1 -strict -2 -b:v 66150 -c:a libfdk_aac -ar 22050 -b:a 22050 -ac 1 -maxrate 66150 -bufsize 66150 -vf scale=720:-1 -movflags +faststart output.mp4

However, my final video has a 95kbps bitrate:

Duration: 00:01:09.73, start: 0.000000, bitrate: 95 kb/s

Stream #0:0(und): Video: av1 (Main) (av01 / 0x31307661), yuv420p(tv, progressive), 720x302, 69 kb/s, 24 fps, 24 tbr, 12288 tbn, 12288 tbc (default)

Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 22050 Hz, mono, fltp, 22 kb/s (default)

Is there a method to validate my need? Is it normal that 66+22>88kbps?

Regards,

EDIT 1: as @Gyan suggested, I've tried to reduce -bufsize parameter, but I still have a too high bitrate. The most working way is to set video bitrate -b:v to 50kbps, but it gives poorer image...

ffmpeg -i input.mp4 -c:v libaom-av1 -strict -2 -b:v 50k -c:a libfdk_aac -ar 22050 -b:a 22050 -ac 1 -minrate 33075 -maxrate 66150 -bufsize 44100 -vf scale=-1:360 -movflags +faststart -threads 1 output.mp4

Any idea to limit overhead, if overhead is in cause?

The container has its own overhead. You have to account for that. Lower the bufsize to tighten the deviation by the video encoder. — Gyan, Sep 25 '19 at 12:50
@Gyan thanks for your tip. I make tests and will Edit my post with the result. — Aurelien Stride, Sep 26 '19 at 09:32
@AurelienStride Might be a good time to consider using libravie rather than libaom-av1... — andrew.46, Jan 24 '20 at 10:39
@andrew.46 sure, but I don't find any documentation on it. Can you share a link plz? — Aurelien Stride, Jan 24 '20 at 11:32
@AurelienStride I have done some work with this here: https://askubuntu.com/q/1189174/57576 while FFmpeg docs have this: https://askubuntu.com/q/1189174/57576 — andrew.46, Jan 24 '20 at 11:38

score 1 · Answer 1 · answered Sep 26 '19 at 20:27

After a few exchanges, here's a very suitable solution, with a 84kbps bitrate and a good image quality, thanks to this page, using 2-pass encoding:

ffmpeg -i input.mp4 -c:v libaom-av1 -strict -2 -b:v 65k -pass 1 -c:a libfdk_aac -ar 22050 -b:a 22050 -ac 1 -minrate 54k -maxrate 72k -bufsize 33075 -vf scale=-1:360 -movflags +faststart -threads 1 -an -f mp4 /dev/null && _
ffmpeg -i input.mp4 -c:v libaom-av1 -strict -2 -b:v 65k -pass 2 -c:a libfdk_aac -ar 22050 -b:a 22050 -ac 1 -minrate 54k -maxrate 72k -bufsize 33075 -vf scale=-1:360 -movflags +faststart -threads 1 output.mp4

Note: it may ask you to overwrite /dev/null.

Thanks to all of you for your help!

You can add the `-y` global option to the first command if you want to avoid it asking you to overwrite `/dev/null`. — llogan, Sep 26 '19 at 21:05

FFMpeg and max bitrate

1 Answers1