Lesson 05 · The fmt chunk fields - Build a WAV/PCM Toolkit

The fmt chunk fields

The fmt chunk holds everything you need to interpret the audio - format, channels, sample rate, and bit depth. Today you unpack its six fields and confirm the two that are derived agree with the rest.

The goal

Parse the fmt payload into its six fields and check the derived fields are consistent.

Start here - the target

TO DO

Scenario: The fmt payload unpacks into six fields

Givena 16-byte fmt payload 01 00 02 00 44 AC 00 00 10 B1 02 00 04 00 10 00

Whenit is parsed

ThenaudioFormat 1, numChannels 2, sampleRate 44100, byteRate 176400, blockAlign 4, bitsPerSample 16

Andthe consistency checks hold: blockAlign == numChannels*bitsPerSample/8 and byteRate == sampleRate*blockAlign

Background

The fmt chunk is the audio’s data sheet. Its 16-byte payload holds six fields in a fixed order: audioFormat (1 means uncompressed PCM), numChannels (1 mono, 2 stereo), sampleRate (samples per second per channel), byteRate, blockAlign, and bitsPerSample. Read them with the u16/u32 helpers you already have - two u16, two u32, two u16.

Two of these fields are derived and must agree with the others, which makes them a free integrity check. blockAlign is the size of one sample frame across all channels: numChannels * bitsPerSample/8 (here 2 * 16/8 = 4). byteRate is how many bytes one second of audio takes: sampleRate * blockAlign (here 44100 * 4 = 176400). A file whose stored values disagree with these formulas is malformed; computing them yourself both validates the header and gives you the numbers you will regenerate when you write files.

Make it work

type Format struct {
  AudioFormat, NumChannels, BitsPerSample int
  SampleRate, ByteRate, BlockAlign        int
}
// fields in order: u16 u16 u32 u32 u16 u16
func parseFmt(p []byte) Format {
  // AudioFormat = u16(p[0:2]); NumChannels = u16(p[2:4]); ...
}

CheckpointDONE

You can parse the fmt chunk into a Format struct. Commit and stop here.