[FFmpeg-devel] Audio conversion and floating-point codecs

[Michael Niedermayer]

On Sat, Jul 10, 2010 at 10:09:12AM +0100, M?ns Rullg?rd wrote:
> Peter Ross <pross at xvid.org> writes:
> 
> > On Tue, Jul 06, 2010 at 03:13:26PM +0100, M?ns Rullg?rd wrote:
> >> Peter Ross <pross at xvid.org> writes:
> >> 
> >> > On Sat, May 15, 2010 at 08:17:51PM +0100, M?ns Rullg?rd wrote:
> >> >> There is a long-standing desire from some to make the floating-point
> >> >> decoders output float samples instead of converting to int16
> >> >> internally, and I agree with the reasons for this.  However, making
> >> >> this change hastily will make decoding orders of magnitude slower on
> >> >> many CPUs.  The reason is that when a decoder outputs float samples,
> >> >> the fast asm code for float-to-int conversion is not used.
> >> >> 
> >> >> In order to change the output format of these decoders without
> >> >> impacting performance, we must first make a few improvements to the
> >> >> avcodec API and to the generic audio format conversion code.
> >> > [...]
> >> >
> >> >> - The decoders should output planar audio instead of interleaved for
> >> >>   multichannel streams.  This probably means introducing
> >> >>   avcodec_decode_audio4() with an AVFrame output.
> >> >
> >> > Q: does it make sense to expand the existing AVFrame structure, or
> >> > define a new struct specific to audio?
> >> >
> >> > #define FF_MAX_CHANNELS  8
> >> > struct AVAudioFrame {
> >> >     uint16_t *data[FF_MAX_CHANNELS];
> >> > };
> >> 
> >> I've posed the same question myself, without finding a good answer.
> >> Some codecs support a huge number of channels.  I can say for sure,
> >
> > Second contenious point:
> > At present, the user allocates the samples buffer that is handed
> > of to avcodec_decode_audioN().
> >
> > IMHO this is sloppy. Just look at how ffmpeg.c guesses the buffer size.
> > The alternative is to have the decoder do it, e.g. by calling
> > avctx->get_buffer() with the number the samples/channels to be output.
> > Thoughts?
> 
> Makes sense, although get_buffer() is presently rather image-centric.

yes, 
AVFrame could be extended and used for audio frames for example

[...]

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