[FFmpeg-devel] Fixpoint FFT optimization, with MDCT and IMDCT wrappers for audio optimization

[Michael Niedermayer]

Hi

On Mon, Jul 30, 2007 at 09:11:34AM -0400, Marc Hoffman wrote:
> On 7/30/07, Michael Niedermayer <michaelni at gmx.at> wrote:
> >
> > Hi
> >
> > On Sun, Jul 29, 2007 at 08:13:38PM -0400, Marc Hoffman wrote:
> > > On 7/29/07, Diego Biurrun <diego at biurrun.de> wrote:
> > > > On Sun, Jul 29, 2007 at 07:20:59PM -0400, Marc Hoffman wrote:
> > > > >
> > > > > sorry about the mime type gmail doesn't allow me to mark it as
> > > > > text/x-patch.  This makes config changes.
> > > >
> > > > > --- configure (revision 9807)
> > > > > +++ configure (working copy)
> > > > > @@ -573,6 +574,7 @@
> > > > >      bktr
> > > > >      dc1394
> > > > >      dv1394
> > > > > +    fixedpoint
> > > > >      ffmpeg
> > > > >      ffplay
> > > > >      ffserver
> > > > > @@ -665,6 +667,7 @@
> > > > >      fast_64bit
> > > > >      fast_cmov
> > > > >      fast_unaligned
> > > > > +    fixedpoint
> > > > >      fork
> > > > >      freetype2
> > > > >      GetProcessTimes
> > > >
> > > > Just CONFIG_LIST is enough.
> > > >
> > > > > --- libavcodec/Makefile       (revision 9807)
> > > > > +++ libavcodec/Makefile       (working copy)
> > > > > @@ -358,6 +358,10 @@
> > > > >  OBJS-$(CONFIG_VP6F_DECODER)            += i386/vp3dsp_mmx.o
> > i386/vp3dsp_sse2.o
> > > > >  endif
> > > > >
> > > > > +ifeq ($(HAVE_FIXEDPOINT),yes)
> > > > > +OBJS += fft_fixedpoint.o
> > > > > +endif
> > > >
> > > > Do this in one line, like for all the other files.
> > > >
> > >
> > > Ok guys, I removed myself from the have list...  And correct the
> > > makefile like you asked before.  Much simpiler.  Again sorry about the
> > > mime attachment....
> > [...]
> > > +/*
> > > +  This is a fixpoint inplace 16bit FFT which accepts 3 arguments:
> > > +
> > > +  @param X   - input signal in format 1.15
> > > +  @param W   - phase factors in 1.15 format
> > > +  @param lgN - log_2(N) where N is the size of the input data set.
> > > +
> > > +  X is the output and its adjusted format is S(1+lgN.15-lgN) i.e.
> > > +    if we are talking about a 256 point fft then the output format is
> > 9.6.
> > > +*/
> >
> > not doxygen compatible
> >
> >
> > [...]
> > > +                tr        = (X[k2].re*wwr + 0x4000)>>15;
> > > +                tr       -= (X[k2].im*wwi + 0x4000)>>15;
> > > +                ti        = (X[k2].re*wwi + 0x4000)>>15;
> > > +                ti       += (X[k2].im*wwr + 0x4000)>>15;
> > > +
> > > +                X[k2].re  = (X[k].re - tr)>>1;
> > > +                X[k2].im  = (X[k].im - ti)>>1;
> > > +
> > > +                X[k].re   = (X[k].re + tr)>>1;
> > > +                X[k].im   = (X[k].im + ti)>>1;
> >
> > why not >>16 ? that way you would have 4 shifts less
> 
> 
> At a greater loss of precision we could do that.  You realize those shifts
> are guards against overflow, and they cause precision to be dynamic based on
> the number of stages.  

if you have a signal of constant -32768 then the constant term of a
512 point FFT will be -512*32768/sqrt(512) this is 741455 which doesnt
fit in 16bit

if we now assume that you rescale this so it does not overflow, that is the
largest term becomes -32768 and you do a IFFT with a /2 downscale in every
step then you get a constant signal of 64
thats a huge 7bits of precission left per sample (if we assume that the intermediate
calculations dont ntroduce any inaccuracies), a 8bit ISA soundcard from
15 years ago should sound better ...

if you want to optimize the code please first do highlevel optimizations
that is change the code to a split radix fft
then _while_ very carefully testing the accuracy on _real_ sound perform
low level optimizations

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you really think that XML is the answer, then you definitly missunderstood
the question -- Attila Kinali
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20070730/d2b78266/attachment.pgp>