(compMus1) Intro to MIDI

To start things off, here are some useful readings:

• MIDI Manufacturer’s Association
  pay close attention to “The Technology of MIDI” sections. Look at the first table in “MIDI Messages”. The “MIDI and Music Synthesis” page is especially good too.
• Wikipedia MIDI entry

Since it is getting close to “the end,” I’ll post an outline of the lecture. It would be good for you to know these things before the last quiz.

MIDI

• MIDI stands for Musical Instrument Digital Interface.
• It is a specification for physically connecting devices, and for communicating between them.
• It was designed for real-time keyboard performance.
• It is a control language – it transmits performance instructions rather than audio.

History (the dark ages)

• No synthesizer could communicate with another synthesizer.
• Each synth would have its own keyboard, controlling its own synthesis system, with its own sequencer. (‘70s Rock – wall of keyboards)

Birth of MIDI

• Several synth makers came together to agree on MIDI standard in 1981 (after development of inexpensive microcontrollers).
• Designed to be relatively inexpensive.
• First MIDI instruments available in 1983.
• MIDI is still the main way musical hardware and software communicates today.

MIDI Transmission

• Binary (like all digital)
• Serial – one bit at a time.
• Asynchronous – devices can send messages whenever the device decides.
• One way – MIDI cables only carry messages in one direction.
• 16 logical channels.
• Transmission speed (original standard) – 31,250 bits per second (31.25 kbits).

MIDI Hardware

• Computer interfaces (with USB and Firewire, can be built into keyboards and other devices).
• Ports
• Cables

Ports

• Ports send and receive MIDI messages.
• Basic hardware ports include IN, OUT, THRU.

Computer Interfaces

• Usually serial, originally connected to modem ports of a computer.
• Modern way is usually USB (Universal Serial Bus); sometimes Firewire (IEEE 1394).
• Communication between computer and interface can be at whatever speed devices support.
• Common to build interface into keyboard or other device. Doesn’t do away with computer interface – integrates it into keyboard.

Cables

•  5-pin cable was a type of audio cable used in Europe.
• 2 of the pins are never used.
• Designed to run to at least 15 meters.

Common MIDI Messages

• Note-on/Note-off (note number, key velocity)
• Polyphonic and Channel Key Pressure (aftertouch)
• (Continuous) Control change. CC.
• Pitch Bend
• Program Change
• System Exclusive (escape hatch)

Representation of Data

• Numbers
• Most values use a range of 0 – 127 (7 bits = 128 possible values)
• Pitch: Middle C = 60

Structure of a MIDI Message

• Messages are sent as 10-bit words
• First and last bit are stripped by UART chip. 8 bits (1 byte) remain.
• Status byte/Data byte determined by first bit of byte. (1 for status, 0 for data)
• Status byte indicates a function (note on, note off, cc change, etc.)
• Data bytes contain values associated with function in status byte.

The Good

• One-to-many control
• Control is independent of synthesis.
• Digital representation of data allows for computer generation, control, and editing.
• MIDI data is “portable.”
• Data can control anything that understands MIDI.

The Bad

• Lowest common denominator approach.
• Keyboard bias/event oriented.
• Slow communication speeds.
• Poor data resolution.

The Ugly

• Not all devices support the same feature sets (you need to look at MIDI implementation charts).
• Serial communication means that it is impossible to accurately represent simultaneous events.
• No requirements for how fast a device responds to a message.