The piano roll is the most direct view of a MIDI file. Each block is one note. Its vertical position is the pitch, read from the keyboard on the left; its left edge is when the key goes down; its length is how long the key is held. Behind it, the file stores a note-on and a note-off event, each with a pitch number from 0 to 127.
Velocity, from 1 to 127, records how hard the key was struck, and most sounds get louder and brighter as it rises. Timing is stored in ticks, fractions of a quarter note, and converted to seconds using the file’s tempo. That’s why moving a note changes when it plays, while changing the tempo moves every note together.
That split, notes in beats and tempo kept separately, explains most MIDI behavior. A part recorded without a click has notes floating between grid lines, because the performance never followed the grid in the first place.
Quantizing without killing the groove
Quantize snaps note starts to the nearest grid line. It’s the fastest way to tighten a sloppy take, and it’s essential before you turn MIDI into sheet music. It also removes the small pushes and drags that make a part feel human.
Pick the grid from the music, not from habit. A part built on 16th notes needs a 16th grid; quantize it to eighths and fast notes get dragged onto the wrong beat. Swing and shuffle parts sit on triplets, so a straight grid will flatten them. When in doubt, quantize a few bars, listen, and undo if it sounds stiff.
Quantize the parts that define the time, like drums and bass, more strictly than melodies and pads. And always delete ghost notes first: a stray blip snapped to the grid becomes a real, audible note.
Cleaning up transcribed MIDI
MIDI made from audio, whether by MP3 to MIDI, WAV to MIDI or Voice to MIDI, always benefits from a pass in an editor. Transcription makes predictable mistakes, and they’re quick to fix once you know what to look for.
Look for octave errors, where a note is detected an octave above or below where it was played, usually because of strong overtones. Look for ghost notes: very short, quiet notes from reverb, string noise or breath. Look for split notes, where one long sung or bowed note is broken into several short ones. And look for missing notes in dense chords, where quiet inner voices hide under louder ones.
Work in that order: delete ghosts, fix octaves, join split notes by stretching one and deleting the rest, add missing notes, then quantize. Export a .mid when you’re done, and the cleaned file is ready for a DAW, a notation app or MIDI to Sheet Music.