[Speex-dev] Fwd: Re: Fixed Point on wideband-mode: Single Frame loss on 2000 Hz sine causes "freak off"

[Jean-Marc Valin]

Basically, if you have coefficients [a, b, c, d], then it means that your filter
is: 1 + a*z^-1 + b*z^-2 + c*z^-3 + d*z^-4
In fixed-point mode, the scaling factor is 8192 (i.e. Q13).

   Jean-Marc

Quoting Frank Lorenz <Frank_wtal at web.de>:

> Hi Jean-Marc,
> I'm currently trying to analyze the LPC synthesis filter. For this, can you
> please tell me the exact format of the LPC coeffs (in variable interp_qlpc),
> please?
>
> cheers,
> Frank
>
> -----Ursprüngliche Nachricht-----
> Von: "Frank Lorenz" <Frank_wtal at web.de>
> Gesendet: 02.02.10 11:00:16
> An: jean-marc.valin at usherbrooke.ca
> CC: speex-dev at xiph.org
> Betreff: Re: [Speex-dev]Fwd: Re: Fixed Point on wideband-mode: Single Frame
> loss on 2000 Hz sine causes "freak off"
>
> Jean-Marc Valin <Jean-Marc.Valin at USherbrooke.ca> hat am 1. Februar 2010 um
> 17:29 geschrieben:
>
> > Can you look at the residual signal? i.e. what it looks like after the
> pitch is
> > removed. Any significant difference there? Also, the pitch gain can be
> > informative.
> >
>
> I catched the "orignal" residual (st->exc from line 400 inside nb_celp.c)
> plus the "synthesized" one (exc on line 880 inside nb_celp.c) both for fixed
> and floating point.
>
> There is a significant difference in the residual signal -- the fixed point
> signal shows some kind of "glitches" (see attachments) while the floating
> point version looks more or less smooth. But it is interesting that the huge
> overdrive you can observe in the re-synthesized output signal isn't present
> in the synthesized residual. This leads to an instable LPC synthesis filter,
> or am I wrong?
>
> cheers,
> Frank
>
>
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