Lesson 18 · Meta event framing - Build a MIDI Parser

Meta event framing

Not every event makes sound - meta events carry text, tempo, and structure. They all share one frame - 0xFF, a type byte, a variable-length length, then that many data bytes. Today you read that frame.

The goal

Read a meta event's type byte and its variable-length data payload.

Start here - the target

TO DO

Scenario: A meta event frames a type and a length-prefixed payload

Giventhe bytes 0xFF 0x03 0x04 0x53 0x6F 0x6E 0x67

Whenthey are parsed as a meta event

Thenit is a meta event of type 0x03 (track name) with data "Song"

Andthe length 0x04 was read as a VLQ, so exactly 4 payload bytes were consumed

Background

A meta event is data for the sequencer rather than a message for a synth, and they all share one envelope: the byte 0xFF, a single type byte, a variable-length payload length, and then exactly that many data bytes. 0xFF can never be confused with a channel status because it is reserved, and the VLQ length means a reader can skip any meta type it does not recognise - the same extensibility trick as chunks, one level down.

Type 0x03 is the track name, whose payload is ASCII text - 0x53 0x6F 0x6E 0x67 is "Song". Type 0x01 is a general text comment, 0x02 copyright, 0x04 instrument name; they all frame identically and differ only in meaning. Read the frame generically today - type, length, raw payload - and the next few lessons give specific types (tempo, time signature, key signature, end-of-track) their interpretation.

Make it work

// 0xFF marks a meta event; then a type byte, then a VLQ length, then data
r.NextByte()               // consume 0xFF
metaType := r.NextByte()   // e.g. 0x03 = track name
n := r.ReadVLQ()           // payload length
data := r.buf[r.pos : r.pos+int(n)]
r.pos += int(n)

CheckpointDONE

You can read a meta event's type and payload. Commit and stop here.