MIDI (Musical Instrument Digital Interface) is a standard for electronic musical devices to communicate. This page is a brief introduction to MIDI that should explain what you need to know to map MIDI controllers to Mixxx. Refer to Reverse Engineering for tips to intercept MIDI data from a device.

MIDI is a widely used standard that a lot of hardware and software support. It dates back to the 1980s when it was used to make synthesizers, samplers, and sequencers communicate. These older devices used cables with 5-pin DIN connectors to carry MIDI signals. Most MIDI devices today send the MIDI signals over a USB cable. Some modern devices can also can use cables with the 5-pin DIN connectors. DJ controllers with these 5-pin DIN connectors do not ordinarily use them to send signals to the computer; they are used to communicate via MIDI with other gear and the ability to use MIDI without being plugged into a computer (although they need to be plugged into another power source without a USB cable supplying power).

Controllers that comply with the USB MIDI class standard (also called "class compliant" devices) do not require any special drivers. Most controllers are USB MIDI class compliant, but not all. See the Mixxx DJ Hardware Guide for information about particular controllers.

Mixxx displays the numbers in MIDI signals in hexadecimal. If you are unfamiliar with hexadecimal numbers, read this tutorial.

An explanation of the MIDI signals that your controller sends to computers and how it reacts to MIDI signals that computers send to it should be available from the controller manufacturer. This is likely in a document on the product page for your controller on the manufacturer's website or in the support section of the website. If it is not in a separate document, it is likely at the end of the manual. Unfortunately, not every manufacturer provides this information.

Most MIDI messages are three bytes long. The first byte of any MIDI message is called the Status byte. The first nybble (hex digit) is the op-code and the second is the MIDI channel number. So if you have 0x90 the op-code is 0x9 and the channel number is 0x0 (Ch 1.) The full list of MIDI messages is below, where n represents the channel number (0..F inclusive):

The boldface entries in the table above are the messages we are most concerned with since most DJ controllers use only these for all functions. You'll need to consult the MIDI spec for the DJ controller you're working with to determine which messages and note/control numbers correspond to the DJ controller functions & LEDs. If your controller's MIDI spec gives only note names and not numbers, use this table to convert them. To convert from decimal to hex, use this.

(Note that in order to use System Exclusive messages, you will need MIDI Scripting.)

Every controller works differently, but these are some typical patterns.

• When pressed: Op-code 0x9, value of 0x7F
• When released: Op-code 0x9, value of 0x00
• or Op-code 0x8, value of 0x00

If there's a LED behind the button, usually it can be controlled by sending a message to the controller using the same status and note number that the controller sends when the button is pressed. As for the value field, for LEDs that can only be turned on or off, typically 0x00 turns it off and 0x01 or 0x7F turns it on. For multi-color LEDs, the color is typically controlled by sending different values. Which ones correspond to which colors should be in the MIDI specification document for your controller. If they are not, you will have to look at mappings for other DJ software, or just try a few different values.

These usually send messages with an op-code of 0xB and a value corresponding to the absolute position of the knob or slider (between 0x00 and 0x7F

Endless knobs/encoders (that you can turn continuously) typically send messages with an op-code of 0xB and the value only indicates whether the encoder is being turned left or right (0x01 & 0x7F or 0x41 & 0x3F)

These usually operate exactly like endless knobs/encoders above, and they usually also send messages just like buttons above when they're touched or released which is intended to mark when scratching begins and ends respectively.