[FFmpeg-devel] [PATCH] Common ACELP code & G.729 [1/7] - filters

Vladimir Voroshilov voroshil
Fri May 9 08:35:31 CEST 2008


Michael Niedermayer wrote: 
> On Wed, May 07, 2008 at 10:23:50AM +0700, Vladimir Voroshilov wrote:
> [...]

[...]

> > >  > >    + MULL(hpf_f[1], -7667);                     /* (13.13) = (13.13) * (0.13) */
> > >  > >    + 7699 * (in[i] - 2*in[i-1] + in[i-2]);      /* (14.13) =  (0.13) * (14.0) */
> > >  > >
> > >  > >  out[i] = av_clip_int16((t + 0x800) >> 12);      /* (15.0) = 2 * (13.13) = (14.13) */
> > >  > >
> > >  > >  hpf_f[1]= hpf_f[0];
> > >  > >  hpf_f[0]= t;
> > >  >
> > >  > This will require either two samples before start in "in" buffer, copied from
> > >  > the previous subframe or
> > >  
> > >  yes, i thought that would be just something like
> > >  memcpy(in-2, in+len-2, 2*sizeof(*in))
> > >  
> > >  ?
> > >  
> > >  If so that should be simpler than it is currently
> > >  
> > Current code work as following:
> > 
> > context
> > {
> >   syn_filter_data
> >   post_filter_data
> >   hpf_z
> >   hpf_z
> > }
> > 
> > mainloop
> >   {
> >    ...
> > 1  syn_filter(syn_filter_data+10)
> > 2  memcpy(syn_filter_dta+10, syn_filter_data+subframe_size,10)
> > 3  postfilter(syn_filter_data+10, postfilter_data) 
> > 6  high_pass_filter(out_data,syn_filter_data+10,hpf_z,hpf_f)
> > 7 }
> > 
> > syn_filter requires first 10 items of previous data saved just after call to it.
> > 
> > postfilter does not requires previous data (it uses post_filter_data as such),
> > but it changes contents of syn_filter data (from 10 to the end of buffer).
> > 
> > high_pass_filter (in your suggestion) requires previous data in buffer,
> > but buffer does not contains it already (were overwritten in line 2)
> > 
> > The problem is in fact that syn_filter requires unfiltered past data, while
> > high_pass_filter requires them filtered with postfilter.
> > 
> > Moreover, since top of buffer already contains data saved for future
> > syn_filter call, i cant save there
> > copy of hpf_z (can't do memcpy(in,hpf_z) inside filter).
> 
> If postfilter would be writing in a "new" buffer that problem should disapear.
> 
> also one can easily chain filters so each has previous samples with few
> memcpy() and little buffer space:
> 
> caAAAA
> db....
> 
> c....A
> dbBBBB
> 
> cCCCCA
> d....B
> 
> ....CA
> dDDDDB
> 
> ....CA
> ....DB
> 
> CA....
> DB....
> 
> 
> [...]
> 

Done. One temp buffer in main loop and two small buffers for
keeping data between subframes are added.
Postfilter is not yet touched.

-- 
Regards,
Vladimir Voroshilov mailto:voroshil at gmail.com
Omsk State University
JID: voroshil at jabber.ru
ICQ: 95587719
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