On Tue, 8 May 2007 01:22:45 +0200
"Miguel Lobo" <[EMAIL PROTECTED]> wrote:
> Hi list,
>
> We have this code in fluid_synth.c:
>
> #define DITHER_SIZE 48000
> #define DITHER_CHANNELS 2
>
> static float rand_table[DITHER_CHANNELS][DITHER_SIZE];
>
> static void init_dither(void)
> {
> float d, dp;
> int c, i;
>
> for (c = 0; c < DITHER_CHANNELS; c++) {
> dp = 0;
> for (i = 0; i < DITHER_SIZE-1; i++) {
> d = rand() / (float)RAND_MAX - 0.5f;
> rand_table[c][i] = d - dp;
> dp = d;
> }
> rand_table[c][DITHER_SIZE-1] = 0 - dp;
> }
> }
>
> This creates two tables of length 48000 with random floating point numbers
> between -1.0 and 1.0 (the probability distribution is not uniform).
>
> And in the same file, in function fluid_synth_write_s16 (which converts the
> result of synthesis from floating point to signed 16-bit format):
>
> left_sample = roundi (left_in[cur] * 32766.0f + rand_table[0][di]);
> right_sample = roundi (right_in[cur] * 32766.0f + rand_table[1][di]);
>
> di++;
> if (di >= DITHER_SIZE) di = 0;
>
> For each output sample, one of the previously generated random values is
> added to the result, which has now been scaled and varies between -32766.0and
> 32766.0.
>
> I don't understand the purpose of all this. The effect of adding the random
> values surely is increasing the noise in the output, and that by such a
> small amount that seems sure to be inaudible.
>
> I'm no audio expert, however, so does anyone think that this code serves any
> useful purpose?
>
> Regards,
> Miguel
>
http://en.wikipedia.org/wiki/Dither
In our case we have TPDF.
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