[FEA] Add upsample/downsample operators

[cliffburdick]

Up/downsample are frequently used in signal processing for sample rate conversion. We should add upsample and downsample operators that can lazily-evaluate these to do resampling without storing data to memory.

[ecarl65]

This would be great. And to implement it in an overlap-save fashion using FFTs.

[cliffburdick]

We have a separate ticket for a polyphase resampler that's somewhat of a high priority: #377

The upsample/downsample functions would simply by identical to what matlab does, but it should be faster since if you combine it in a single line nothing is written out to memory. ie, upsample(downsample(x, 2), 3) would do a 3/2 conversion without any writes to global memory. However, this leaves out the filtering step, and users that want to have a filter after upsampling usually want polyphase.

[ecarl65]

Ah, that makes sense. I think the Matlab one is a one-shot FFT based approach? Or maybe I'm thinking of numpy.

[cliffburdick]

@ecarl65 if you're interested the polyphase resampler and this issue should be in by today

[ecarl65]

This is great, thank you! I'll start playing with it and let you know if I have any feedback.