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

[Michael Niedermayer]

On Sat, Jul 10, 2010 at 04:23:12PM +1000, Peter Ross wrote:
> 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?

yes

also get_format() may be interresting

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
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