What is USB MIDI and how does it work?

MIDI and USB are terms that you’ll see frequently on the internet, particularly when reading about electronic music and music technology. But have you ever wondered what they mean?

Since the main subject of this website is USB MIDI controllers, we would like to explore some of the underlying technologies involved and some of their history.

USB and MIDI are two independent systems which are capable of functioning both in isolation, and together. Obvious everyday examples of USB devices are mice, flash drives, USB microphones and headsets. USB ports have been added to computers since the end of the 1990s. MIDI has existed on electronic music equipment since the 1980s.

Since historically, it was the earliest to be used, let’s first take a look at what MIDI is. Then we’ll describe how MIDI works within USB.

What is MIDI?

MIDI 5-Pin connector usbmidicontrollers.com

The standard 5-pin MIDI connector

An abbreviation of “Musical Instrument Digital Interface” – MIDI is a technical standard that describes a method of communication between electronic musical instruments. It also describes the round 5-pin digital connector and socket type used for the communication.

MIDI sockets, plugs and cables usbmidicontrollers.com

5-pin MIDI sockets, plugs and cables

MIDI has been a standard since 1983, and is managed by the MIDI Manufacturers Association. It has become the most widespread protocol for connecting music equipment together.

MIDI was invented so that musical instruments could communicate with each other and so that one instrument could control another. It allows synthesizers, drum machines, sequencers, samplers, computers and other related devices to connect and send music information. MIDI does not transfer sounds between devices, instead it communicates note information; such as pitch, duration, and changes to the sound over time – information about what notes you play and how you play them is transmitted, it instructs the receiving electronic instruments to play their on-board sounds.

MIDI isn’t music, it doesn’t contain any sounds; it isn’t a digital audio file format like MP3 or WAV.

Instruments are connected together using MIDI cables. MIDI allows bulky full-sized instruments to be replaced with small devices which don’t need keyboards.
It becomes possible for just one or two people to make compositions, which without MIDI would require a large number of people contributing. MIDI allows a person to play, for example, acoustic piano sounds, drums and string intruments simultaneously. You can play all of these different instruments using a single keyboard.

Arturia Keystep with MIDI usbmidicontrollers.com

The Arturia Keystep with MIDI sockets, released in 2016

MIDI data contains a list of events or messages that tell a musical device how a particular sound should be generated. Messages such as “note on,” “note off,” pitchbend, velocity, control signals for volume, vibrato, audio panning, cues, and clock signals to synchronise tempo, can be sent to a number of devices at once. Sixteen channels of information can be sent on each MIDI link, effectively giving the ability to trigger sixteen instruments per channel. For drum instruments or samplers, each drum sound or sample is given its own note value, (the Bassdrum is often on C3, Snaredrum = D3) so a drum machine will only need one of the sixteen MIDI channels.


Sequencing MIDI

MIDI events can be recorded, and replayed exactly by a MIDI sequencer. A sequencer will either be a hardware device or a piece of software – today that usually involves a Digital Audio Workstation (DAW) running on a computer: such as Cubase, ProTools, Logic Pro X, Ableton Live, Reason, Renoise, or FL Studio. MIDI as an integral component of modern Digital Audio Workstations.

Sequencers provide a number of benefits to a composer or arranger. They have a graphical overview of the arrangement, allowing recorded MIDI notations to be manipulated, using standard computer editing features such as cut, copy and paste, and drag and drop. The recorded MIDI messages can be modified extensively, re-ordered and manipulated in many ways that pre-recorded audio cannot. The ability to compose ideas and quickly hear them played back, enables composers to experiment, and tools such as looping, arpeggiators, randomization, and transposition expand the arranging process.

One melody can be played and recorded on the sequencer, then replayed whilst another melody is played alongside it which is also recorded. Both of those melodies can be played again together, and another melody or a drum beat played and added to the sequence, and so on. This can continue until either all of the tracks on the sequencer are used up – hardware sequencers are often capable of recording 16, 32 or 64 tracks. Or in the case of the professional versions of Digital Audio Workstation software, you can keep adding new tracks until you either run out of computer memory or until there is too much information for the computer to process.
It is often not desirable to have too many tracks in a song, because as complexity increases it becomes more difficult to manage, also the computer no longer be able to keep up with the tempo of the music. But a typical composition could easily contain between 30 and 50 lines of melodies, drums, vocals and effects.

Additional expression can be added after the notes have been played, and manipulated in realtime using controllers. MIDI can be synchronized with recorded vocals and audio samples. MIDI makes it easy to get your ideas down quickly and save them.

When you come back to your saved ideas at any time in the future, the sequence will be exactly the same as what you created. You can then perform further manipulations, processing, and mixing to it; spending as much time as you like to adjust your compositions to meet your creative desires.

The data captured in the sequenced MIDI recordings can be saved as a Standard MIDI File (SMF), easily distributed online, and opened in other systems.
The sound will be reproduced by any computer software or electronic instrument that adheres to and can interpret the MIDI specification. MIDI data file sizes are typically thousands of times smaller than recorded audio files.

The Atari 520 ST with MIDI ports. What is MIDI? usbmidicontrollers.com

The Atari 520 ST with MIDI ports (1985). Click image to zoom

In the mid 1980s, a number of relatively cheap computers, such as the Atari ST and Apple II plus, came with MIDI ports fitted. In many ways, this brought MIDI to the masses.



Currently the most common method of connecting computers, tablets, smartphones, and other digital equipment, is by USB.

You can buy a USB to MIDI convertor, a little box which plugs into one of your computer’s USB outputs and has MIDI sockets on it, you can then plug in MIDI cables to attach to music hardware devices.USB to MIDI interface usbmidicontrollers.com

Essentially this is a translating device – USB and MIDI use different communication protocols to send instructions. In many ways these protocols are like different languages.

Until recent years, using such a device was the only way for a computer to talk to musical instruments. But it was realised that it would be more convenient and flexible to send MIDI data directly via the computer’s existing USB outputs, without a translator.



Before the late 1990s, there were many types of connectors on computers: serial ports, parallel ports, PCMCIA ports, joystick ports, keyboard and mouse connectors. Even the shapes of serial connectors and parallel connectors varied between different manufactures. A printer, for example, which you had bought to use with one type of computer, may not work with a different computer, or you may have buy a new connecting cable for it.
A standard connection port was desirable. The USB Implementers Forum founded in 1996 by MicroSoft, Intel, NEC and Compaq, established the USB (Universal Serial Bus) standard.

USB port usbmidicontrollers.com

The Standard USB 1.0 connector socket

The first computer to eliminate most ports in favour of USB was the 1st generation Apple iMac, released in 1998.
Although it did retain for Ethernet and a telephone (MODEM) connection ports, the keyboard and mouse were USB only. There was no floppy drive (popular on computers at the time) and the first iMac only had a CD reader. Therefore the way to copy data from the internal hard drive to an offline disk was by USB. Any printers, scanners, webcam, or other devices that you wanted to connect, also had to be USB versions. This may sound routine now but it was a radical departure when the computer was first released.

Many types of data can be sent over USB – digital audio, digital images and video (cameras, webcams, etc), keyboard, mice, storage data for backups and transfer (hard drives, flash drives, to smartphones and tablets, CD and DVD drives, etc), network traffic data (USB network cards), scanners and printers, and more. Another advantage of USB is that it can provide power to these devices.

MIDI messages are carried within the USB stream, USB treats it as just another form of data. The recieving instrument will then react to the instructions being sent.


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