[Speex-dev] Speex inner_prod()
Jean-Marc.Valin at USherbrooke.ca
Fri Feb 3 18:54:54 PST 2006
Basically, inner_prod() can and should be adapted to the architecture it
will run on. It is not really sensitive to noise, so it's possible to
tweak it a lot. Also, in the current code, I saturate it to +-16384,
which is OK to prevent overflows. I'm not concerned with the case of a
constant -16384 value because it can't really happen in practice
(especially after filtering). BTW, on platforms that have a 40-bit
accumulator, it's possible to even remove the shift from the loop and
apply it only at the end.
Le vendredi 03 février 2006 à 11:27 -0600, Jerry Trantow a écrit :
> I am overriding the inner product routine in ltp.c. To test my replacement,
> I threw some test vectors at it. I understand the loss of resolution caused
> by the shift. I also see a FIXED_POINT danger with the summation of four
> mults overflowing the 32 bit before the shift.
> I can fix this by accumulating each term into a long, but if the code scales
> the x,y vectors to avoid this problem I could use parallel 16x16
What do you mean here?
> You can see this problem with the following test case.
> for (i=0;i<40;i++)
The value -32768 is not supposed to happen in vectors sent to
> sum0=inner_prod(x, y, 40);
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