MIDI

Press middle C (C4) on a MIDI keyboard and it sends a short message: note on, note number 60, and a velocity from 1 to 127 for how hard you pressed, on one of 16 channels. Let go and it sends note off. A4, the 440 Hz A, is note 69. Pedals and wheels send their own messages: the sustain pedal and the mod wheel send control changes, and the pitch wheel sends pitch bend. Whatever receives the messages makes the sound, so the same key press can play a grand piano in one app and a synthesizer bass in another.

Because MIDI is a list of instructions, a recorded MIDI part stays editable. In a DAW you can move one wrong note, tighten the timing, or swap the instrument without playing the part again. An audio recording keeps exactly the sound you captured, mistakes included. Some songwriting apps export a song as a MIDI file so you can keep editing it in a DAW, and our list of songwriting apps for iPhone notes which ones do.

A MIDI controller is a keyboard, pad grid or box of knobs with no sounds inside. It plays whatever app or synth it is connected to. A USB MIDI keyboard plugs into the USB-C port on iPhone 15 and later, or into Apple’s Lightning to USB Camera Adapter on older iPhones. Keyboards that work on a Mac without installing a driver (called class compliant) usually work on iPhone too, and some controllers connect over Bluetooth. The app has to respond to MIDI for any of this to work, so check its listing before you buy a keyboard for it.

MIDI dates from 1983. Dave Smith of Sequential Circuits and Ikutaro Kakehashi of Roland led its development, and at the 1983 Winter NAMM Show, Smith connected a Sequential Prophet-600 to a Roland Jupiter-6 so one brand’s synth played the other’s. The MIDI 1.0 specification was published that August, and MIDI 2.0 was introduced at the 2020 Winter NAMM Show.