Lesson 16 · Building the fmt chunk - Build a WAV/PCM Toolkit

Building the fmt chunk

Now you generate a header instead of reading one. Today you build the 16-byte fmt payload, deriving byteRate and blockAlign from the basic parameters so they are always consistent.

The goal

Build a 16-byte fmt chunk payload from channels, sample rate, and bit depth.

Start here - the target

TO DO

Scenario: The fmt payload is built with derived fields

GivennumChannels 1, sampleRate 8000, bitsPerSample 16, audioFormat 1 (PCM)

Whenthe fmt payload is built

ThenblockAlign is 2 and byteRate is 16000 (derived, not passed in)

Andthe 16 payload bytes are 01 00 01 00 40 1F 00 00 80 3E 00 00 02 00 10 00

Background

Building the fmt chunk is the mirror of parsing it, with one improvement: you do not accept byteRate and blockAlign as inputs, you compute them. That is the right design because they are fully determined by the other fields, and computing them means a file you write can never have the inconsistent header a careless writer might produce. blockAlign = numChannels * bitsPerSample/8, and byteRate = sampleRate * blockAlign.

You will need little-endian writers to match the readers from lesson 2 - a putU16 and putU32 that lay an integer down low byte first. With those, encode the six fields in order into 16 bytes: format 1, channels, sample rate, the derived byte rate, the derived block align, and bits per sample. For mono 8000 Hz 16-bit that is blockAlign 2, byteRate 16000, and the exact bytes above - the same 16-byte payload your parser would read straight back. Next you wrap this in the full file.

Make it work

func buildFmt(channels, sampleRate, bits int) []byte {
  blockAlign := channels * bits / 8
  byteRate := sampleRate * blockAlign
  p := make([]byte, 16)
  putU16(p[0:], 1)              // audioFormat = PCM
  putU16(p[2:], channels)
  putU32(p[4:], sampleRate)
  putU32(p[8:], byteRate)       // derived
  // putU16 blockAlign at p[12:], bits at p[14:]
  return p
}

CheckpointDONE

You can build a consistent fmt chunk payload. Commit and stop here.