Lesson 19 · Set-tempo - Build a MIDI Parser

Set-tempo

Tempo lives in a meta event as microseconds per quarter note - a three-byte number that later turns ticks into seconds. Today you decode it and derive the familiar beats-per-minute.

The goal

Decode a set-tempo meta event into microseconds per quarter note and BPM.

Start here - the target

TO DO

Scenario: Set-tempo bytes decode to microseconds per quarter and BPM

Giventhe bytes 0xFF 0x51 0x03 0x07 0xA1 0x20

Whenthey are parsed as a set-tempo meta event

Thenthe tempo is 500000 microseconds per quarter note, which is 120 BPM

And0xFF 0x51 0x03 0x0F 0x42 0x40 is 1000000 microseconds per quarter note, which is 60 BPM

Background

MIDI does not store tempo as beats per minute; it stores microseconds per quarter note, a meta event of type 0x51 with a length of 3 and a big-endian 24-bit value. 0x07 0xA1 0x20 is 500000 - half a million microseconds, half a second, per quarter note. That is the default every file assumes when no tempo is given.

Beats per minute is just the reciprocal in friendlier units: a quarter note is a beat, there are 60 million microseconds in a minute, so BPM = 60000000 / usPerQuarter. 500000 gives 120, and doubling the microseconds to 1000000 halves the tempo to 60. Keep the microsecond value as the source of truth - it is what the tick-to-seconds math will use directly - and treat BPM as the derived, human-readable view.

Make it work

// meta type 0x51, always length 3: a big-endian 24-bit microseconds-per-quarter
usPerQuarter := uint32(d[0])<<16 | uint32(d[1])<<8 | uint32(d[2])
bpm := 60000000 / usPerQuarter // 500000 -> 120

CheckpointDONE

You can decode a set-tempo event into microseconds per quarter and BPM. Commit and stop here.