[sdiy] Looking for cheap lowpass filter algorithm

[Seb Francis]

I'm looking for a very simple algorithm to achieve the following ...

With a digital delay running at 48kHz sampling rate I want to have the 
option to double the delay time by discarding half the data (i.e. have a 
24kHz sampling rate)

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?

Thanks,
Seb