Saturday, July 23, 2016

Ableton Live and High Resolution MIDI Control

MIDI Continuous Controller data is usually restricted to 7-bit values, thereby having a limited range of 0 - 127. This is because a MIDI CC message is structured as a command byte followed by two data bytes:

• Command byte: 0xB0 to 0xB7 for MIDI CC message using MIDI channel 1 - 16
• Data byte: 0x00 to 0x7f for Controller Number 0 - 127
• Data byte: 0x00 to 0x7f for Controller Value 0 - 127

In other words, there are sixteen MIDI channels. Each MIDI channel can carry 128 MIDI controllers (numbered 0 - 127), and each controller can has a value range of 0 - 127 (7 bits worth). This is great if we need lots of different control pathways, but not so great if we need something with a higher resolution than just 0 - 127. 

High resolution MIDI control messages offer a solution to this issue. Two standard, 7-bit MIDI CC messages can be combined to form a single, high resolution MIDI CC data point. This is only possible if the software that is receiving the data is able to interpret the two MIDI CC messages as a single 14-bit message. A 14-bit MIDI message has a range of 0 - 16383, as opposed to 0 - 127.

For each of the sixteen MIDI channels, MIDI CC 32 - 63 can be combined with MIDI CC 0 - 31 to form a high resolution message. The value from controller 0 - 31 forms the high byte (i.e. bits 8 - 14) and the value from controller 32 - 63 forms the low byte (i.e. bits 1 - 7).

For example:

• MIDI CC Channel 1; Controller 0, Value 63
• MIDI CC Channel 1, Controller 32, Value 20

These two messages, when read as a 14-bit value, give a high resolution result of 8084. The value of MIDI CC 0 is 63, and is the high byte. The value of MIDI CC 32 is 20, and is the low byte. Thus, the 14-bit combined value is 63 * 128 + 20.

This can be expressed in a Max patch such as this:

Ableton Live supports high resolution MIDI. Simply use the MIDI map function as per usual, but send a pair of high resolution MIDI bytes one after the other. The high byte is sent first, directly followed by the low byte. If the bytes are of the correct MIDI controller and channel pair, then Ableton Live will recognise this as a high resolution MIDI control message.

This option can also be manually selected by mapping a parameter to a MIDI controller in Live, and then selecting the MIDI controller in the MIDI Mapping Mode screen. In the bar on the very bottom left hand side, a menu allows the selection of a variety of MIDI mapping modes, including "Absolute 14-bit".

Each of the 16 MIDI channels supports up to 32 high resolution MIDI controllers in this way.