If you’ve ever had any experience with music technology, or, more specifically, sequencers, keyboards or synthesisers, you will have come across MIDI (Musical Instrument Digital Interface). It’s used to send notes and controller messages from musical devices, such as keyboards or sequencers which are used to play and record music, and devices that produce sounds, such as samplers or synthesizers. It’s pure control information, for example, “play a c# in the 3rd octave with a velocity of 85," there’s no actual audio involved. It dates back to the early 1980s, when a group of musical instrument manufacturers such as Roland, Sequential Circuits, Oberheim, Yamaha, and Korg got together to define the standard. It soon lead to a huge boom in low cost music production and the genesis of new musical styles. It’s no accident that rap and electronic dance music date from the mid to late 80s.
Web MIDI is a new W3C specification for an API to allow browser applications to access MIDI input and output devices on the host machine. You can enumerate the devices, then choose to listen for MIDI messages, or format and send your own messages. It’s designed to allow applications to consume and emit MIDI information at the protocol level, so you receive and send the actual raw message bytes rather the than API providing the means to play MIDI files using General MIDI, for example. Don’t let this put you off though, the protocol is very simple to interpret as I’ll demonstrate later.