Lesson 20 · Time signature - Build a MIDI Parser

Time signature

The time signature meta event stores four bytes, and the denominator is the tricky one - it is a power of two, not the number itself. Today you decode all four and get the denominator right.

The goal

Decode a time-signature meta event, expanding the denominator as a power of two.

Start here - the target

TO DO

Scenario: Time-signature bytes decode with a power-of-two denominator

Giventhe bytes 0xFF 0x58 0x04 0x06 0x03 0x24 0x08

Whenthey are parsed as a time-signature meta event

Thenthe time signature is 6/8 with 36 MIDI clocks per metronome click and 8 thirty-second notes per quarter

And0xFF 0x58 0x04 0x04 0x02 0x18 0x08 is 4/4 (denominator 2 to the power 2)

Background

The time signature meta event (type 0x58, length 4) packs four values. The first is the numerator as written - 6 for 6/8. The second is the surprise: the denominator is stored as the power of two, not the value, so 3 means 2^3 = 8, and 2 means 2^2 = 4. Expand it with a shift: 1 << d[1].

The last two bytes describe the metronome and notation: clocks per metronome click (there are 24 MIDI clocks per quarter note, so 0x24 = 36 clocks makes the metronome tick every dotted quarter) and the number of thirty-second notes per quarter note (8, the normal value). Decode all four, but the one that bites is the denominator - miss the power-of-two and 6/8 reads as 6/3. Pin both a compound 6/8 and a plain 4/4.

Make it work

// meta type 0x58, length 4: numerator, denominator-power, clocks, 32nds
numerator := d[0]
denominator := 1 << d[1] // 3 -> 8, 2 -> 4
clocksPerClick := d[2]
thirtySecondsPerQuarter := d[3]

CheckpointDONE

You can decode a time-signature meta event. Commit and stop here.