Downsampling and applying a lowpass filter to digital audio

Question

I\'ve got a 44Khz audio stream from a CD, represented as an array of 16 bit PCM samples. I\'d like to cut it down to an 11KHz stream. How do I do that? From my days of engine

Answer 1

You have to apply low-pass filter (removing frequencies above 5500 Hz) and then apply decimation (leave every Nth sample, every 4th in your case).

For decimation, FIR, not IIR filters are usually employed, because they don't depend on previous outputs and therefore you don't have to calculate anything for discarded samples. IIRs, generally, depends on both inputs and outputs, so, unless a specific type of IIR is used, you'd have to calculate every output sample before discarding 3/4 of them.

Just googled an intro-level article on the subject: https://www.dspguru.com/dsp/faqs/multirate/decimation