Hardware Transport (electrical and Mechanical Connections)
The MIDI standard consists of a communications messaging protocol designed for use with musical instruments, as well as a physical interface standard. It consists physically of a one-way (simplex) digital current loop electrical connection sending asynchronous serial communication data at 31,250 bits per second. 8-N-1 format, i.e. one start bit (must be 0), eight data bits, no parity bit and one stop bit (must be 1), is used, so up to 3,125 bytes per second can be sent.
Only one end of the loop is referenced to ground, with the other end "floating", to prevent ground loops which may otherwise cause interference and hum in analog audio signals. The current loop on the transmitter side drives the LED of an opto-isolator on the receiver side. The current loop is specified as 5 mA. The opto-isolator must be a high-speed type, with less than 2 μs risetime. As most opto-isolators have asymmetrical positive-going and negative-going slew rates, they slightly alter the signal's duty cycle. If several MIDI devices are connected in series by daisy-chaining the MIDI THRU to the next device's MIDI-IN, the signal gets more and more distorted, until receive errors occur due to pulse narrowing.
At the physical layer (MIDI cable), a pair of wires carry the MIDI signal. The voltage difference is normally 0 volts (both at positive potential referenced to ground) in the idle state, which is seen as a '1' at the MIDI receiver due to logic inversion by the Opto-isolator. A MIDI message start bit (0) causes a voltage differential on the wire pair (current loop) which is seen at the MIDI receiver as a '0'. The 8 data bits can be either '0' (low) or '1' (high) with the stop bit (1) seen at the MIDI receiver as a '1'. To summarize:
- Logic 1 → High → no current flow → Opto-isolator LED off → MIDI receiver sees High, logic '1' (data bits, stop bit or idle)
- Logic 0 → Low → current loop flow → Opto-isolator LED on → MIDI receiver sees Low, logic '0' (data bits, start bit)
MIDI connectors are standard 5-pin 180° DIN connectors which at one time were a de facto European standard for audio interconnection. Over time the simpler American RCA phono jack has left MIDI as the only place where DIN is commonly encountered in modern equipment. Only two of the five pins (pins 4 and 5) are used for MIDI signal transmission.
Some computers or their sound cards have 15-pin D-subminiature connectors, called game ports, that can be used for MIDI IN/MIDI OUT. The connector supports both MIDI and analog joystick functions. Access to the MIDI signals is provided by a short adapter cable that converts the D-subminiature pinout into DIN connectors. The recommended method of connecting two 5-pin DIN cables to a 15-pin D-subminiature computer port can be found at the MIDI.org web site. The MIDI specification very conservatively states that the maximum distance MIDI can be transmitted is 15 meters (50 feet), but it can normally go much farther.
There exists a USB connection standard and a standard for MIDI over Ethernet and Internet called RTP MIDI being developed by the IETF, available from standard RFC sites.
Most MIDI capable instruments feature a MIDI IN, MIDI OUT, and occasionally a MIDI THRU connection in the form of five-pin DIN connectors. In order to build a two-way physical connection between two devices, a pair of cables must be used. The MIDI THRU jack simply echoes the signal entering the device at MIDI-IN. This makes it possible to control several devices from a single source.
The 1985 Atari ST was the first home computer to sport the original five-pin DIN format, making it a very popular platform for running MIDI sequencer software. Most PC soundcards from the late 1990s had the ability to terminate a MIDI connection, usually through a MIDI IN/MIDI OUT converter on the game port. The game port has been supplanted in the modern PC by USB devices, and so typically a PC owner will need to purchase a MIDI interface that attaches to the USB or FireWire port of their machine to use MIDI. Most current digital audio interfaces are equipped with MIDI ports.
Read more about this topic: MIDI 1.0
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