LIBFAAC cannot resample channels

Question

Im using avconv to convert video files into MP4, but whenever I try to encode files with 6 or 8 channels into 2 channels using libfaac I get these error messages:

Can not resample 8 channels @ 44100 Hz to 2 channels @ 44100 Hz

(EDIT: the full output of messages can be found here: http://pastebin.com/UXGrBy1D)

These are the codecs we have installed for avconv: http://pastebin.com/hyQrqXqW

And this is the command I am running:

avconv -y -i input.mp4 -vcodec libx264 -bufsize 20M -maxrate 4000k -threads 12 -same_quant -acodec libfaac -ac 2 -ar 44100 -ab 128k output.mp4

Am I doing something wrong?

Or, is there any alternatives to libfaac which will resample 8 channels into 2 channels?

I need to be able to do this from a command line on my ubuntu 12.04 server.

Answer 1

First of all, if you haven't already done so, check out this ffmpeg h.264 encoding guide - it applies to avconv as well, just change every instance of ffmpeg to avconv. Also, consider upgrading to a more recent version of avconv or ffmpeg - since you're on Ubuntu, you could use this PPA, or compile it yourself (this last option will give you access to fdk_aac, which is a much better AAC encoder than FAAC).

Unless you know exactly what you're doing & have a specific reason, you probably shouldn't use -bufsize 20M -maxrate 4000k -threads 12 -same_quant. In fact, going by the input in that pastebin, you should probably just use -codec:v copy, which won't touch the video stream.

As for your stated problem... I don't think libfaac is your problem. Look here (from your pastebin):

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from \'/var/www/up/up50eefce404e4f.mp4\':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isomavc1mp42
    creation_time   : 2007-12-08 19:28:08
  Duration: 00:46:47.64, start: 0.000000, bitrate: 308 kb/s
    Stream #0.0(und): Audio: aac, 44100 Hz, stereo, s16, 111 kb/s
    Metadata:
      creation_time   : 2007-12-08 19:28:08
    Stream #0.1(und): Video: h264 (Baseline), yuv420p, 320x240 [PAR 1:1 DAR 4:3], 195 kb/s, 11.99 fps, 11.99 tbr, 11988 tbn, 23976 tbc
    Metadata:
      creation_time   : 2007-12-08 19:28:11

FFmpeg thinks your input audio is stereo. Much later on:

Input stream #0:0 frame changed from rate:44100 fmt:s16 ch:2 to rate:44100 fmt:s16 ch:8
Resampling output channel count must be 1 or 2 for mono input; 1, 2 or 6 for stereo input; or N for N channel input.
Can not resample 8 channels @ 44100 Hz to 2 channels @ 44100 Hz

The video stream is also throwing up a hell of a lot of errors. I suspect that your input may be corrupted.

It's possible that the following command will work:

avconv -ac 8 -i input.mp4 -c:v copy -c:a libfaac -b:a 128k -ac 2 output.mp4

-ac sets the number of audio channels: if the AAC stream isn't corrupted, it's possible that the container format is just providing incorrect data to avconv, and putting -ac 8 before the input overrides the setting provided by the MP4 container (and putting -ac 2 before the output tells ffmpeg to output to 2 audio channels).