[sdiy] Looking for cheap lowpass filter algorithm

[Seb Francis]

Hi Magnus,

Magnus Danielson wrote:
> Look at it this way: You could do 32 multiplies at 48 kHz, it is just that your
> sampling of every other sample on the output of that FIR filter will actually
> waste half of the processed samples and thus half of your total amount of
> filtering computation. By letting two samples pass between each computation
> rather than one, you have integrated the sampling somewhat with the filtering.
> This would be equalent but only half the processing. Neat huh?
>
>   

Sounds good.  I just wondered if there was anything to be gained (other 
than wasted CPU cycles) in using all the 48kHz input data in the filter, 
but I guess not.

>> And the other thing I'm still getting my head around is how to calculate 
>> the optimal coefficients for my application - which requires a very 
>> steep response filter with the stopband at 0.25*Fs (or possibly at 
>> 0.5*Fs depending on the answer to the above question).
>>     
>
> Fs/4 is your answer regardless.
>   

I was referring to the fact that by feeding only every other sample into 
the filter, then the Fs is effectively 24kHz which means that I need to 
design the filter coefficients with stopband at Fs/2.  Right??

>
>
> The Parks-McClellan/Remez-exchange should do what you want, you just need to
> learn to twist the knobs a bit.

Thanks.  The Java applet for calculating this one was a bit buggy and 
didn't always give a result for all parameter settings (so I couldn't 
quite get it how I wanted it).  I've now found a better applet here:
http://www.nauticom.net/www/jdtaft/lopassE.htm

Seb