[sdiy] Digital filtering (Oversampling and downsampling)

[Eric Brombaugh]

Tom Wiltshire wrote:

<snip>

> This downsampling filter seems to be where most of the problems are.

<snip>

> If instead I choose a higher output sample rate, I can relax the 
> downsampling filter specification.

<snip>

> This means that sometimes it seems to actually be *easier* to run with a 
> faster sample rate.
> 
> Is this really true? How far does this go? Would I do better still 
> trying to run at 192KHz?

Yes and no. Yes it's true that the demands on the filter become less 
difficult to meet, but now you have to compute the filter output more 
often which means you spend more time filtering and less time doing the 
things you really want to do and pushing closer to the envelope of 
what's possible with the processor you're using. Like all engineering 
it's a tradeoff. You pick what's important and choose the solution that 
meets your needs.

Actually what you'll find is that there's a 'knee of the curve' in the 
trade-offs. Beyond this there are diminishing returns for higher sample 
rates. Finding that exact point is probably not necessary, but getting 
close optimizes your performance/cost.

> Balancing these competing demands without a lot of previous experience 
> seems to be very difficult and very much an art not a science. If anyone 
> has any useful experience or pointers to offer, I'd be grateful for any 
> help I can get.

And there you have the root of the nub. Experience is only gained by... 
experience. Build systems, trade off the requirements. Bang your head on 
the wall, piss into the wind. Get a gut feel so you don't waste time 
going down a fruitless path. Only one way to do it.

Eric