[Ffmpeg-devel] Summer of Code LGPL AC3 Decoder

John Kelley john
Thu Jan 25 10:09:28 CET 2007

With more E-AC3 files becoming available I thought I would try and
take a shot at implementing E-AC3 support in the LGPL AC3 Decoder.

In preparing to do this I tried to dump a normal AC3 stream to a PCM
wave file to ensure everything worked correctly and what I ended up
with was a stereo pcm wave file with pops spiking up every few
seconds. I'm not sure if this is a bug in the downmix code or if it is
a bug in the actual decoding routines. I've been trying to get ffmpeg
to output a 6 channel pcm wave file so I can eliminate downmixing as
the source of the problem but as of yet have been unsuccessful in
doing so.

Things I have tried/accomplished thus far:
  - forcing lgpl_ac3_codec to decode all 5 channels (how do I turn lfe
output on from the commandline to make this 6 channels?)
  - hacking out the force to stereo in ffmpeg.c in the following block
                if (codec->channels != icodec->channels &&
                        (icodec->codec_id == CODEC_ID_AC3 ||
                         icodec->codec_id == CODEC_ID_DTS )) {
                        /* Special case for 5:1 AC3 and DTS input */
                        /* and mono or stereo output      */
                        /* Request specific number of channels */

Does anyone know how to accomplish this? How does one debug the
development of a multichannel audio decoder in ffmpeg?


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