MIDI Crash Course

MIDI Crash Course

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.

MIDI Messages

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):

Status	Function	Data bytes
0x8n	Note off	Note number	Note velocity
0x9n	Note on	Note number	Note velocity
0xAn	Polyphonic after-touch	Note number	Amount
0xBn	Control/mode change	Control number	Value
0xCn	Program change	Program number	(n/a)
0xDn	Channel after-touch	Amount	(n/a)
0xEn	Pitch wheel	LSB	MSB
0xF0	System Exclusive message	Vendor ID	(data)
0xF1	MIDI Time Code Qtr. Frame	(see spec)
0xF2	Song Position Pointer	LSB	MSB
0xF3	Song Select	Song number	(n/a)
0xF4	Undefined
0xF5	Undefined
0xF6	Tune request	(n/a)
0xF7	End of SysEx (EOX)	(n/a)
0xF8	Timing clock	(n/a)
0xF9	Undefined	(n/a)
0xFA	Start	(n/a)
0xFB	Continue	(n/a)
0xFC	Stop	(n/a)
0xFD	Undefined	(n/a)
0xFE	Active Sensing	(n/a)
0xFF	System Reset	(n/a)

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.)

Typical MIDI signals used by DJ controllers

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

Buttons

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

LEDs

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.

Knobs & sliders

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)

Jog wheels, touch strips, platters

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.