[FFmpeg-devel] AC3 samples per frame?

[Nico Sabbi]

Il Wednesday 07 November 2007 12:34:59 Nico Sabbi ha scritto:
> Il Wednesday 07 November 2007 01:29:00 Justin Ruggles ha scritto:
> > Nico Sabbi wrote:
> > > Hi,
> > > if I read the specs correctly in EAC3 the number of samples per
> > > frame should be given by 256*number of audio blocks, where the
> > > number of audio blocks should be
> > > -6 if fscod==3
> > > - {1,2,3,6}[next two bits]  otherwise
> > > correct?
> > > I have a sample where frmsiz==127  and fscod2==6,
> > > thus the corresponding frame should extend for 256 bytes
> > > (frmsz+1)*2 and include 1536 samples (6*256) that doesn't
> > > look correct to me.
> > >
> > > (the sample in mphq is
> > > samples/A-codecs/AC3/eac3/eac3mono64kbit.eac3)
> > >
> > > Can anyone explain if I misunderstood the specs?
> >
> > fscod2 is only 2 bits, so it can't be 6.  It is only used to
> > indicate reduced samplerates (24kHz, 22.5kHz, 16kHz).
>
> sorry, I meant numblkscod
>
> > frmsiz=127 does indeed mean that the frame is 256 bytes.
> >
> > what is the value of fscod and numblkscod?
> >
> > -Justin
>
> the frame header (always equal) is 0b 77 00 7f 32 86 thus
> fscod = 00 => 48 khz
> numblkscod = 11 (thus 6 blocks, right? )
>

BTW, the syncwords are actually offset by 256 bytes (thus frmsz==127
is correct) but 6 audio blocks in so little space looks suspicious to
me.
Is the number of samples per block fixed at 256 even in e-ac3 or can 
it vary?