I'd like to generate a silence audio file using ffmpeg with specific bitrate and the duration is less than one second. I found that the generated audio file does not have the desired bitrate. But if I expand the duration to 2 seconds, the bitrate seems to be correct.
For example:
ffmpeg -f lavfi -i anullsrc=r=22050:cl=mono -t 0.3 -acodec mp3 -ab 48k -y silence.mp3
The generated silence.mp3's bitrate seems not 48kb/s:
$ ffprobe silence.mp3
ffprobe version 3.4.2 Copyright (c) 2007-2018 the FFmpeg developers
built with Apple LLVM version 9.1.0 (clang-902.0.39.1)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.4.2 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-jack --enable-gpl --enable-libmp3lame --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxvid --enable-opencl --enable-videotoolbox --disable-lzma
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
Input #0, mp3, from 'silence.mp3':
Metadata:
encoder : Lavf57.83.100
Duration: 00:00:00.37, start: 0.050113, bitrate: 52 kb/s
Stream #0:0: Audio: mp3, 22050 Hz, mono, s16p, 48 kb/s
If the duration is 2 seconds, bitrate seems OK:
$ ffmpeg -f lavfi -i anullsrc=r=22050:cl=mono -t 2 -acodec mp3 -ab 48k -y silence.mp3
$ ffprobe silence.mp3
ffprobe version 3.4.2 Copyright (c) 2007-2018 the FFmpeg developers
built with Apple LLVM version 9.1.0 (clang-902.0.39.1)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.4.2 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-jack --enable-gpl --enable-libmp3lame --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxvid --enable-opencl --enable-videotoolbox --disable-lzma
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
Input #0, mp3, from 'silence.mp3':
Metadata:
encoder : Lavf57.83.100
Duration: 00:00:02.06, start: 0.050113, bitrate: 48 kb/s
Stream #0:0: Audio: mp3, 22050 Hz, mono, s16p, 48 kb/s
Is there any way to keep the bitrate as desired?
