Lesson 12 · De-interleaving channels - Build a WAV/PCM Toolkit

De-interleaving channels

Stereo samples are stored interleaved - left, right, left, right - but to work on one channel at a time you need them split apart. Today you de-interleave a flat sample stream into per-channel slices.

The goal

Split an interleaved sample stream into one slice per channel.

Start here - the target

TO DO

Scenario: Interleaved samples split into per-channel slices

Giventhe interleaved samples [100, 200, 300, 400] with numChannels 2

Whenthey are de-interleaved

Thenchannel 0 (left) is [100, 300] and channel 1 (right) is [200, 400]

Andthe same samples with numChannels 1 give a single channel [100, 200, 300, 400]

Background

In a multi-channel WAV the samples are interleaved: one sample per channel for the first instant (a frame), then one per channel for the next instant, and so on. Stereo is L R L R L R.... That layout is right for streaming to hardware, but wrong for processing - to fade the left channel or average two channels you want each channel as its own contiguous slice.

De-interleaving walks the flat stream and routes sample index i to channel i mod numChannels. So [100, 200, 300, 400] at 2 channels becomes left [100, 300] and right [200, 400]. With one channel it is a no-op - a single slice equal to the input. This per-channel form is what the sample-math chapter operates on, and interleaving (the reverse) is how you will lay samples back down before writing. Together with decoding, this is the second half of turning raw bytes into usable audio.

Make it work

// sample i belongs to channel (i mod numChannels), frame (i div numChannels)
func deinterleave(samples []int, channels int) [][]int {
  out := make([][]int, channels)
  for i, s := range samples {
    ch := i % channels
    out[ch] = append(out[ch], s)
  }
  return out
}

CheckpointDONE

You can de-interleave a sample stream into channels. Commit and stop here.