Lesson 15 · Interleaving channels - Build a WAV/PCM Toolkit

Interleaving channels

Before samples can be written they must go back into the interleaved layout the format stores - the reverse of lesson 12. Today you weave per-channel slices back into one stream.

The goal

Interleave per-channel sample slices into a single frame-ordered stream.

Start here - the target

TO DO

Scenario: Per-channel slices interleave into one stream

Givenchannel 0 [100, 300] and channel 1 [200, 400]

Whenthey are interleaved

Thenthe stream is [100, 200, 300, 400]

Anda single channel [100, 200, 300, 400] interleaves to itself unchanged

Background

De-interleaving split a stream into channels; interleaving puts them back. You walk frame by frame - index 0 of every channel, then index 1 of every channel, and so on - emitting one sample per channel per frame in channel order. Left [100, 300] and right [200, 400] weave back to L R L R = [100, 200, 300, 400], exactly the stream lesson 12 started from.

This is the precise inverse of de-interleaving, so the two together must round-trip: interleave(deinterleave(s, n)) returns s. A single channel interleaves to itself, since there is nothing to weave. From here the path to a file is short - turn the interleaved samples into bytes with the encoder, then wrap those bytes in the fmt and data chunks and the RIFF header, which is the next two lessons.

Make it work

// emit frame by frame: for each index, one sample from every channel in order
func interleave(channels [][]int) []int {
  var out []int
  n := len(channels[0])          // frames per channel
  for i := 0; i < n; i++ {
    for ch := range channels {
      out = append(out, channels[ch][i])
    }
  }
  return out
}

CheckpointDONE

You can interleave channels back into one stream. Commit and stop here.