MP3 to MIDI Drum Tracks: Channel 10 and the 47-Sound Map
A drum transcription is not stored as pitches. On channel 10 the note number stops meaning a pitch and starts naming an instrument, and the General MIDI specification fixes that list at 47 entries. Outside those 47 numbers there is no defined percussion sound, and the same number means something else on any other channel.
Drum tracks, answered
- The map is 47 numbers wide. Channel 10 is reserved for percussion, and note numbers 35 through 81 are interpreted as 47 named instruments.
- The other 81 numbers are undefined. A hit written outside that range has no defined percussion sound on channel 10.
- The same number means two things. Note 60 is middle C on a melodic channel and High Bongo on channel 10.
- The program number is not the kit selector. Under Level 1 the channel sounds as percussion whatever program change it receives.
Note numbers stop being pitches on channel 10
Channel 10 is the exception, and the specification says so outright: "Channel 10 is reserved for percussion under General MIDI; this channel always sounds as percussion regardless of whatever program change numbers it may be sent, and different note numbers are interpreted as different instruments." Everywhere else the note number is a pitch and the program number picks the instrument. On channel 10 those two roles swap.
Forty-seven of the 128 note numbers have a sound
The map runs from 35, Bass Drum 2, to 81, Open Triangle. Eighty-one minus thirty-five, plus one, is 47 named instruments. That leaves 81 of the 128 possible note numbers with no defined percussion meaning: the ranges 0 to 34 and 82 to 127. A hit written to 84 because the source sounded high has nothing to play.
The numbering runs backwards inside each family
Note 35 is Bass Drum 2 and note 36 is Bass Drum 1, so the "2" sits below the "1". Every tom family repeats that inversion.
| Family | Lower number | Higher number |
|---|---|---|
| Bass drums | 35 Bass Drum 2 | 36 Bass Drum 1 |
| Low toms | 41 Low Tom 2 | 43 Low Tom 1 |
| Mid toms | 45 Mid Tom 2 | 47 Mid Tom 1 |
| High toms | 48 High Tom 2 | 50 High Tom 1 |
Moving a part up by one number therefore moves it to a different drum rather than a higher one. The names are the only guide here, because the numbers do not sort the family for you.
Read as pitches, the map spans almost four octaves
Those 47 numbers are pitches as well. Middle C is 60, as the MIDI explainer on songscription's blog puts it, so 35 is B1 and 81 is A5: 46 semitones, three octaves and ten semitones, 61.74 Hz to 880 Hz. A drum part placed on channel 1 does not sound like a kit. It sounds like a melody walking from B1 to A5.
What to check on a drum transcription
Two mechanical checks settle most of it. First, the channel: if the drum part sits anywhere other than channel 10, every hit will sound as a pitch. Second, the range: list the distinct note numbers and mark any that fall outside 35 to 81, because those have no defined percussion sound. Track names are no help. jukeblocks advises users to name tracks "Kick, Snare, Clap, Hihat, Percussion" for its rearranger, but a name is free text; the channel is what makes notes sound as percussion.
That is the whole check, and it takes a few seconds. A drum part on channel 10 whose note numbers all sit between 35 and 81 will play on any General MIDI instrument. A part that fails either test will not, and no amount of editing the note lengths afterwards will change that.
Where the two-kit rule comes from
The same specification page lists General MIDI 2 with up to two simultaneous percussion kits, on channels 10 and 11, and nine GS drum kits. The single drum channel is a Level 1 rule, and kit selection arrives with a later level.