[FFmpeg-devel] [PATCH] QCELP decoder

[Michael Niedermayer]

On Thu, Nov 13, 2008 at 11:46:40AM -0800, Kenan Gillet wrote:
> Thanks for your reviews,
>
> here is round 10 of the QCELP decoder patch:
> - it simplifies and optimizes qcelp_lspf2lpc and lps2poly
> - it removes bandwith_expansion_coeff from QCELPContext, and replaces it
>   by a systematic recalculation as the benchmarks show it is faster
> - simplifies apply_gain_control,
> - adds FIXME comment an av_log_missing_features in apply_gain_control
>   if the vector to control gain of is a zero vector.
> - various cosmetics
>
> Kenan
>

[...]

> Index: libavcodec/qcelp_lsp.c
> ===================================================================
> --- libavcodec/qcelp_lsp.c	(revision 0)
> +++ libavcodec/qcelp_lsp.c	(revision 0)
[...]

> +
> +/**
> + * Computes the Pa or Qa coefficients needed for LSP to LPC conversion.
> + * We only need to calculate the 6 first elements of the polynomial.
> + *
> + * @param lspf line spectral pair frequencies
> + * @param v_poly polynomial input/output as a vector
> + *
> + * TIA/EIA/IS-733 2.4.3.3.5-1/2
> + */
> +static void lsp2poly(const float *lspf,
> +                     float *v_poly) {
> +    float val, *v;
> +    int   i;
> +
> +    // optimization to simplify calculation in loop
> +    v_poly++;
> +
> +    for (i = 0; i < 10; i += 2) {
> +        val = -2 * cos(M_PI * *lspf);
> +        lspf += 2;
> +        v = v_poly + FFMIN(4, i);
> +
> +        if (i < 4) {
> +            v[2] = v[0];
> +            v[1] = v[0] * val + v[-1];
> +        }
> +        for ( ; v > v_poly; v--)
> +            v[0] = v[0]
> +                 + v[-1] * val
> +                 + v[-2];
> +        v[0] += v[-1] * val;
> +    }
> +}

i thnink this can be optimized further see lsp2poly & ff_acelp_lsp2lpc


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

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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