[FFmpeg-user] Varying audio bitrate when transcoding from AC3 5.1 to AAC Stereo

Thu May 23 16:42:01 EEST 2019

Hello,

I am seeing a fluctuating bitrate when I attempt to convert an AC3 audio stream that is 5.1 Dolby to AAC in stereo. I thought that to do the proper downmux you simply need to add the -ac 2 option to your audio output command but I must be missing something. The target bitrate is 128 but I see it dipping as low as 114 and peak as high as 140. Am I missing something or is this expected behavior for what I am doing?

Here is the command and output:

user at gpu-transcoder:~/scripts$ ffmpeg -i 'udp://@239.1.1.92:59092?fifo_size=1000000&overrun_nonfatal=1' -vcodec h264_nvenc -bf:v 2 -g 60 -rc cbr -b:v 6000K -minrate 6000k -maxrate 6000k -bufsize 12000k -profile:v high -level 4.0 -acodec libfdk_aac -ac 2 -b:a 128k -f mpegts  'udp://@233.1.1.100:59100?pkt_size=1316'
ffmpeg version N-93005-gd92f06e Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 7 (Ubuntu 7.3.0-27ubuntu1~18.04)
  configuration: --prefix=/home/circle/ffmpeg_build --pkg-config-flags=--static --extra-cflags=-I/home/circle/ffmpeg_build/include --extra-ldflags=-L/home/circle/ffmpeg_build/lib --extra-libs='-lpthread -lm' --bindir=/home/circle/bin --enable-gpl --enable-libaom --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-nonfree --enable-nvenc
  libavutil      56. 26.100 / 56. 26.100
  libavcodec     58. 44.100 / 58. 44.100
  libavformat    58. 26.100 / 58. 26.100
  libavdevice    58.  6.101 / 58.  6.101
  libavfilter     7. 48.100 /  7. 48.100
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
Input #0, mpegts, from 'udp://@239.1.1.92:59092?fifo_size=1000000&overrun_nonfatal=1':
  Duration: N/A, start: 58320.721844, bitrate: N/A
  Program 3
    Stream #0:0[0x31]: Video: mpeg2video (Main) ([2][0][0][0] / 0x0002), yuv420p(tv, top first), 1920x1080 [SAR 1:1 DAR 16:9], Closed Captions, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
    Stream #0:1[0x34](eng): Audio: ac3 ([129][0][0][0] / 0x0081), 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:2[0x35](spa): Audio: ac3 ([129][0][0][0] / 0x0081), 48000 Hz, stereo, fltp, 96 kb/s (visual impaired) (descriptions)
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg2video (native) -> h264 (h264_nvenc))
  Stream #0:1 -> #0:1 (ac3 (native) -> aac (libfdk_aac))
Press [q] to stop, [?] for help
Output #0, mpegts, to 'udp://@239.129.2.220:59220?pkt_size=1316':
  Metadata:
    encoder         : Lavf58.26.100
    Stream #0:0: Video: h264 (h264_nvenc) (High), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 6000 kb/s, 29.97 fps, 90k tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc58.44.100 h264_nvenc
    Side data:
      cpb: bitrate max/min/avg: 6000000/0/6000000 buffer size: 12000000 vbv_delay: -1
    Stream #0:1(eng): Audio: aac (libfdk_aac), 48000 Hz, stereo, s16, 128 kb/s
    Metadata:
      encoder         : Lavc58.44.100 libfdk_aac
frame= 1622 fps= 33 q=34.0 size=   43862kB time=00:00:54.42 bitrate=6602.5kbits/s speed=1.11x

Here are the FFPROBE outputs to show the audio bitrate changes:

Input #0, mpegts, from 'udp://@233.1.1.100:59100':
  Duration: N/A, start: 47.096589, bitrate: N/A
  Program 1
    Metadata:
      service_name    : Service01
      service_provider: FFmpeg
    Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], Closed Captions, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
    Stream #0:1[0x101](eng): Audio: aac (LC) ([15][0][0][0] / 0x000F), 48000 Hz, stereo, fltp, 127 kb/s

And another:

Input #0, mpegts, from 'udp://@239.129.2.100:59100':
  Duration: N/A, start: 206.629456, bitrate: N/A
  Program 1
    Metadata:
      service_name    : Service01
      service_provider: FFmpeg
    Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], Closed Captions, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
    Stream #0:1[0x101](eng): Audio: aac (LC) ([15][0][0][0] / 0x000F), 48000 Hz, stereo, fltp, 120 kb/s

Let me know what else you may need.

Thanks,
Jimbo
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