[sdiy] Looking for cheap lowpass filter algorithm

[Martin Naef]

Hi Seb

Seb Francis wrote:
> It works fairly well just storing every other input sample, then 
> interpolating between samples to generate the missing output samples.  
> However high frequencies (e.g. 'S' sounds in speech) come out quite 
> distorted.
> 
> So I'm looking for a cheap way in software to filter the high 
> frequencies to make the distortion less noticeable.  Any pointers?

Have a look here (it's called "fast downsampling with anti-aliasing):
http://www.musicdsp.org/archive.php?classid=3#214

It's essentially a gauss-type decimator for /2 downsampling, which 
should be very simple and efficient to implement on a fixed point DSP as 
well.

If you want better (higher order) filtering, check out the examples 
dealing with sinc interpolation.

On the same page there's also a 5 and 7 tap downsampler (titled "Hiqh 
quality /2 decimators").

Bye
Martin


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