linuxaudio.org

On Tue, Jul 31, 2012 at 05:06:17PM -0400, Paul Davis wrote:
> it is NOT a reasonable attempt. its an effort by someone who doesn't really

quoted text
> understand much about audio, and from some of the available video evidence,

> doesn't understand a lot about linux either, to reinvent what doesn't need

> reinventing and to avoid doing the actual hard work that would genuinely

> improve things.

Yep, it looks like a big waste of time and effort.
There *is* a grain of truth in the argument pro fixed point,

but it matters only in extreme cases and I really wonder if

the Klang author really understands anything of this.
If you compute a very long FIR filter in the 'obvious'

way using 32-bit floating point:
s = 0;

for (i = 0; i < N; i++) s += c [i] * x [i];

out = s;
with N = 100000 or so, you will be adding a nontrivial

amount of noise. The solution is to split up the sum,

first make sums of say 1000 samples then add these:
s1 = s2 = 0;

for (i = j = 0; i < N; i++, j++)

{

    if (j == 1000)

    {

        s2 += s1;

        s1 = 0;

        j = 0;

    }

    s1 += c [i] * x [i];

}

out = s1 + s2;
The extreme case of this happens within the FFT: each of the

N outputs is a sum of N terms, but they are added in binary

tree fashion, first sums of two, then sums of four, etc.

It's the reason why computing a long FIR using an FFT-based

convolution engine is not only much more efficient, but also

more accurate than the direct form.
BTW, how long it is since TI introduced their first 40-bit

floating point DSP chip (the C30) ? 20 years or so ...
Ciao,
--

FA
A world of exhaustive, reliable metadata would be an utopia.

It's also a pipe-dream, founded on self-delusion, nerd hubris

and hysterically inflated market opportunities. (Cory Doctorow)
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