[sdiy] Looking for cheap lowpass filter algorithm

[Seb Francis]

Antti Huovilainen wrote:
> On Wed, 16 Aug 2006, Seb Francis wrote:
>
>> I could spare enough cycles for 32 multiplies per sample per channel, 
>> but
>
> I should emphasize that those are 32 multiplies at _24 kHz_ 
> samplerate, not 48 kHz! So for each actual ADC/DAC sample you need 
> only 16 multiplies.
>

Ah, that sounds more do-able!

I will probably skip the output filter as it's more complicated than 
just a fixed filter since when the delay time is varied in realtime the 
output buffer step is adjusted so the output buffer pointer catches up 
to where it needs to be.  This has the effect of re-sampling (changing 
the pitch) all audio that is currently in the buffer between the output 
and input pointers.

Thus the output buffer pointer is not always pointing exactly at a 
sample - the actual sample to send to the DAC is linear interpolated 
from the adjacent samples.  This sounds actually very good so I don't 
think I need to do additional filtering.

Seb