[FFmpeg-devel] [PATCH] Common fixed-point ACELP routines (1/3) - math

[Michael Niedermayer]

On Tue, Apr 22, 2008 at 09:12:16AM +0700, Vladimir Voroshilov wrote:
[...]
> +/**
> + * Table used to compute log2(x)
> + *
> + * ff_acelp_tab_log2[i] = (1<<15) * log2(1 + i/32), i=0..32
> + */
> +static const uint16_t ff_acelp_tab_log2[33] =
> +{
> +     0,  1455,  2866,  4236,  5568,  6863,  8124,  9352, 10549, 11716,
> + 12855, 13967, 15054, 16117, 17156, 18172, 19167, 20142, 21097, 22033,
> + 22951, 23852, 24735, 25603, 26455, 27291, 28113, 28922, 29716, 30497,
> + 31266, 32023, 32767
> +};

with following table:
      4,   1459,   2870,   4240,   5572,   6867,   8127,   9355,  10552,  11719,
  12858,  13971,  15057,  16120,  17158,  18175,  19170,  20145,  21100,  22036,
  22954,  23854,  24738,  25605,  26457,  27294,  28116,  28924,  29719,  30500,
  31269,  32025,  32769,

the log2 code has a 10 times smaller squared difference to the real log2(), its
worst case error is also slightly less.

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

Old school: Use the lowest level language in which you can solve the problem
            conveniently.
New school: Use the highest level language in which the latest supercomputer
            can solve the problem without the user falling asleep waiting.
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