Optimization on should have inlined the 64-bit lib-calls. Anyway, 64-bit arithmetic may end up being slower than floating-point on IA32 (FPUs in general may be more performant than ALUs nowadays, and 64-bit int operations may be done as tw 32-bit int ops, which is slooow as well). But I don't get it why you need so much precision, though - haven't looked at the code, but is your filter a linear xform? And what is the window size?

V.

My filter is a lowpass butterworth with four poles and four zeros. And no, like every other bloody thing on the MT-32 the transform is logarithmic. As for the number of poles and zero's I've used, I found it was necessary to get the required rectangular lowpass filter window.

But how wide is your window? Ideally all the precision you need would be log2(width) [in samples] for the fractional, and 15-16bit (if not 14, since IIRC the DAC has two input pins floating) for the integral part, nay?
V.

canadacow wrote:

My filter is a lowpass butterworth with four poles and four zeros. And no, like every other bloody thing on the MT-32 the transform is logarithmic. As for the number of poles and zero's I've used, I found it was necessary to get the required rectangular lowpass filter window.