MIDI is a way for instruments, computers and software to send performance data to each other. It carries no sound at all. It carries small messages that say which note to play, when, how hard, and how to shape it, and the instrument on the receiving end turns those messages into audio. Because the message and the sound are separate, you can change either one after you have played the part.
MIDI is a shared standard that works across almost every DAW, keyboard and synth, hardware or software. Nothing here is tied to one product or one brand.
MIDI is a standard way for musical instruments, computers and software to talk to each other. The name stands for Musical Instrument Digital Interface, and it has been in wide use since the early 1980s. The one idea to hold onto is that MIDI is a control protocol, not audio. It carries instructions about a performance, not the sound of one.
When you press a key on a MIDI keyboard, it does not send any sound down the wire. It sends a short message that says, in effect, this note just started, and this hard. Release the key and it sends another that says that note stopped. Turn a knob, push a wheel, press a pedal, and each of those sends its own message too. A performance in MIDI is just a stream of these small messages arriving in time.
The sound you actually hear is made entirely by the instrument that receives the messages, whether that is a hardware synth, a drum machine or a software instrument in your DAW. The same MIDI part can come out as a piano, a bass or a drum kit depending only on what you point it at, because the notes and the sound are two separate things.
Think of MIDI as sheet music or an old player piano roll rather than a recording. It is a set of instructions for a performance. Hand the same instructions to a different player and you get a different performance of the same notes. What MIDI never contains is the sound itself.
Audio is a recording of actual sound, a waveform you play back but cannot easily change. MIDI is a set of instructions that an instrument reads to make sound, so you can change the notes, the timing or even the whole instrument after the fact, without recording anything again. That difference is the reason both exist side by side in a DAW.
Picture a wrong note in a guitar take. On an audio track that note is baked into the recording, and fixing it means editing the waveform carefully or playing the part again. On a MIDI track the same note is just a message that says play this pitch here, so you drag it to the right pitch and it is fixed. Nothing was re-recorded, because there was never any sound to re-record, only the instruction.
This is why producers lean on MIDI so heavily. You can move a note, fix a wrong pitch, change how hard each note hits, and quantize the timing so it sits on the grid. You can nudge a whole part earlier or later, or copy it and try it an octave up. And because the sound lives in the instrument rather than the notes, you can swap that instrument at any point and hear the same part played through something else entirely. Audio gives you a fixed clip. MIDI gives you a part you can keep editing.
MIDI carries short messages about a performance. The main pair is note on and note off, which start and stop a note. Each note message includes a note number that says which note, and a velocity that says how hard it was played. Around those sit controller messages for things like the pitch bend, the mod wheel and the sustain pedal, along with patch changes and a clock that keeps devices in time.
A note number runs from 0 to 127, and each number is one note on the keyboard. Under the most common convention note 60 is middle C, though makers disagree on whether to label that octave C3 or C4, so the same number can carry a different name from one program to the next. Velocity also runs from 0 to 127 and usually sets how loud the note is, but it is up to the instrument, and some map it to brightness or attack as well as level.
MIDI also splits into channels. A single MIDI connection carries 16 independent channels, so one cable or one port can address up to 16 separate instruments, each listening on its own channel. By a long standing convention channel 10 is used for drums, though nothing forces that. On top of the notes, control change messages, usually shortened to CC, carry continuous movement: there are 128 controller numbers, and common ones are the mod wheel on CC 1, channel volume on CC 7, pan on CC 10 and the sustain pedal on CC 64. Pitch bend has its own message for smooth slides, program change swaps to a different sound or patch, and the clock sends timing so a drum machine, arpeggiator or sequencer stays locked to your tempo.
| Message | What it does | What it carries |
|---|---|---|
| Note on | Starts a note | A note number (which note) and a velocity (how hard) |
| Note off | Ends that note | The note number, and sometimes a release velocity |
| Control change (CC) | Moves a controller such as the mod wheel, volume, pan or sustain pedal | A controller number and a value from 0 to 127 |
| Pitch bend | Bends the pitch smoothly up or down | A bend amount around a centre resting point |
| Program change | Swaps to a different sound or patch | A program number |
| Aftertouch | Adds pressure while a key is held down | A pressure value |
| Clock and transport | Keeps devices in time and starts or stops them | Timing pulses plus start, stop and continue |
A MIDI file, usually ending in .mid, is just this stream of messages saved to disk with its timing. It stores the performance, not the sound, so opening the same file on different instruments plays it back through whatever sounds you have loaded. That is why a MIDI file is tiny next to an audio file of the same music, and why it can sound completely different from one machine to the next.
In a DAW you record or draw MIDI onto a MIDI track, and that track plays a software instrument. The notes appear in a piano roll where you can move them, fix wrong pitches, change how hard each one hits and quantize the timing, and you can send the same notes to a different instrument whenever you want. Most of the MIDI you meet today never touches a cable at all.
That is what people mean by virtual MIDI. Inside the computer, MIDI connects your controller keyboard to a software instrument, and connects one track to a plugin, with no physical link in between. The piano roll is the grid where you see and edit the notes. Quantize snaps their timing to the beat. Swapping instruments is a matter of pointing the part at a different plugin and pressing play. This is where the editable nature of MIDI does its real work.
There is still a hardware side. A physical keyboard, pad or synth connects to your computer over USB, and older or studio gear uses the original round 5 pin DIN sockets, with adapters and interfaces to bridge the two. The message is the same either way, USB MIDI and 5 pin MIDI carry exactly the same kinds of note and controller data. There is also a newer version, MIDI 2.0, agreed in 2020, that adds higher resolution and two way communication while staying compatible with the MIDI most gear still uses.
None of this writes the part for you. If you want a line to put onto a MIDI track in the first place, our guide to how to write a melody walks through building one you can then edit and re-voice as much as you like.
Because MIDI is separate from sound, the notes survive every later decision about the instrument. You can rewrite the whole timbre of a track a month later and the performance is untouched, still sitting there as messages you can edit. The part is the thing you keep. The sound is a choice you can remake at any time.
Everything above comes down to one habit: keep your ideas as MIDI for as long as you can, so the notes stay editable and the sound stays your choice. The slow part is coming up with a line in the first place and getting it into the track as notes you can still shape.
REHEAT is a writing tool. It generates acid and melodic lines as MIDI you can drag straight into any DAW, or play live into your own synth through the REHEAT MIDI DRIVER. Either way you end up with editable MIDI, notes you can move, quantize and point at any instrument, rather than a fixed audio clip. It runs as an Audio Unit, a VST3 and an AAX.
That is the whole point of this page. The MIDI is the part, and the sound is whatever you decide to play it through, which stays your call long after the notes are down.
Free to hear in the browser, no signup. Everything above still works by hand in any DAW too, which is rather the point of writing it down.
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