Lesson 17 · The media header and packed language - Build a Video Container Parser

The media header and packed language

Each track's mdia has an mdhd box with its own timescale and duration, plus a compactly packed 3-letter language code. Today you parse all three, including the bit-unpacking that recovers the language.

The goal

Parse an mdhd box to its timescale, duration, and ISO-639-2 language string.

Start here - the target

TO DO

Scenario: An mdhd yields per-track timing and language

Givena version-0 mdhd with timescale 0x00 0x00 0xAC 0x44, duration 0x00 0x02 0x04 0xCC, and language bytes 0x15 0xC7

Whenthe mdhd is parsed

Thenthe timescale is 44100 and the duration is 132300

Andthe language decodes to "eng"

Background

mdhd, the media header, gives a track its own clock, independent of the movie timescale: an audio track might use a 44100 timescale (its sample rate) while the movie uses 1000. In version 0 the timescale and duration are the 32-bit fields at offsets 12 and 16, exactly like mvhd. Here 0xAC44 is 44100 and 0x000204CC is 132300 - three seconds at that timescale.

The quirk is the language, packed into a single 16-bit word: three 5-bit letters, each stored as its ASCII code minus 0x60. So eng becomes (5,14,7), which packs to 0x15C7. Unpacking masks off 5 bits at a time and adds 0x60 back. This ISO-639-2 code is how a player labels subtitle and audio track languages.

Make it work

// language is 3 letters packed into 15 bits, 5 bits each,
// each letter stored as (ascii - 0x60)
func unpackLang(v uint16) string {
  a := byte((v>>10)&0x1F) + 0x60
  b := byte((v>>5)&0x1F) + 0x60
  c := byte(v&0x1F) + 0x60
  return string([]byte{a, b, c})
}

CheckpointDONE

You can parse an mdhd. Commit and stop here.