All MIDI interfaces, whether for Macintosh, IBM PC, or other computers, are responsible for converting MIDI events originating in the computer software into a serial data stream on the cables connecting to various MIDI devices, or for converting from the serial data stream created by MIDI devices on the MIDI bus to parallel MIDI event transactions input by the computer.
The serial MIDI cables that connect to and from the computer's MIDI interface are shielded, twisted-pair cables, terminated at each end in a 5-pin DIN plug. Pins 4 and 5 are connected to the twisted-pair, which carry the MIDI signal. The MIDI signals are isolated from one device (e.g., computer) to another (e.g., synthesizer) through the use of "Opto-Isolators", which serve to aid in noise reduction of the MIDI signals and also provide pro- tection in the event of a shorted or damaged cable or interface. The elec- trical interface is a 5 milliampere current loop, in which a logic-0 is represented by current ON.
MIDI signals travel on the twisted pair at 31,250 bits-per-second, and are transmitted in groups consisting of a single "start-bit", 8 data bits, and a single "stop-bit". Thus, 10 bits are required to represent each 8-bit byte of data. Therefore, the effective data rate is 3,125 bytes-per-second.
Typically, a MIDI event will consist of two or three 8-bit bytes of data, although certain MIDI events such as "system exclusive" can contain a great number of bytes. A "note-on" event, for example, requires 3 bytes to rep- resent the type of event, the note number, and the attack-velocity values. Other events, such as program changes, channel pressure, and the like only require 2 bytes.
The MIDI specification allows each event to be associated with one of 16 MIDI channels (numbered 0 to 15, but sometimes referred to as 1 to 16 in some MIDI instrument documentation). The channel is transmitted as the lower 4 bits in the first byte of each "channel message" MIDI event. In addition to "channel messages", the MIDI data stream can contain "system common", "system exclusive", and "system real-time" messages.
All Macintosh MIDI interfaces, with rare exceptions (e.g., the MacProteus NuBus card) are external "boxes" that connect to the computer's modem and/or printer ports and provide one or more pairs of MIDI IN and MIDI OUT connectors. The most inexpensive MIDI interfaces connect to either the modem or printer port and provide a single pair of MIDI IN and MIDI OUT connectors. More expensive and elaborate interfaces connect to both the modem and printer ports and provide two or more pairs of MIDI IN and MIDI OUT connectors. For example, the Opcode Studio Plus Two interface connects to both the modem and printer ports and provides two pairs of MIDI IN and MIDI OUT connectors, for a total of 16 channels per pair, or 32 MIDI channels in total.
Even more expensive and elaborate MIDI interfaces, such as the Mark of the Unicorn's "MIDI Time Piece" interface connect to both the modem and printer ports and provide 8 pairs of MIDI IN and MIDI OUT ports, for a total of 128 MIDI channels. The MIDI Time Piece (MTP) can be chained to another like MTP, and it to another, up to a total of 4 MTP interfaces. Together this maximum configuration offers a total of 512 independent MIDI channels, both input and output. The top-of-the-line MIDI software (e.g., Opcode's Vision and Mark of the Unicorn's Performer) provide the ability to support this large number of channels through a feature known as "cabelization", in which you designate to which cable a particular group of channels are assigned.
Even the most inexpensive Macintosh MIDI interface (e.g., priced < $60) can accomodate the input and output of events on 16 MIDI channels. However, when an application requires the use of more than 16 channels, a more expensive interface will be needed. Generally speaking, all Macintosh MIDI software recognizes channels on both the modem and printer ports. Therefore, in most cases, channels are labeled M0-M15 and P0-P15 in the MIDI software.
Even if your MIDI interface includes connectors for both the modem and printer ports, it isn't necessary to connect to both of these if only 16 MIDI channels will suffice for your needs.
Opcode's new Studio 5 MIDI interface is a new type that incorporates a great deal of intelligence as well as the ability to handle a large number of MIDI channels. It connects to both the modem and printer ports and provides 15 independent pairs of MIDI IN and MIDI OUT ports, for a total of 240 MIDI channels. In addition, it includes provisions for real-time "processing" of the incoming or outgoing MIDI data, adding support for "filtering", "note- range splitting", "channelization", "controller mapping", and "velocity and control-value modification". These features are similar to what have only previously been available in an external "Patchbay" rack-module. In the Studio 5, they are included in the price. The interface also boasts built-in MIDI Time Code and SMPTE Syncronization capabilities. It reads and writes all formats of SMPTE and converts to MIDI Time Code or "Direct Time Lock".
Most MIDI interfaces come with cables that connect the interface to the computer. The interface will have two cables if both the modem and printer ports are used, or may just have a single cable to connect to either the modem or printer ports. In addition, the interface may have an internal power supply, or may have a "wall wort" that you plug into an electrical socket to supply either the AC or DC power required by the interface.
Standard MIDI cables are used to connect the MIDI IN and MIDI OUT ports on the interface to the synthesizer or other MIDI equipment being used. If the MIDI setup includes more than one input device (e.g., keyboard and wind controller), then these must plug into individual IN ports on the interface, or a "MIDI Merger" box may be required to "merge" the two inputs into a single input for connection to the interface. Similarly, if a cable attached to the computer's OUT port is to be connected to more than one MIDI instru- ment, either the THRU ports on some MIDI instruments must be used to daisy- chain one instrument's input to another, or a "MIDI Splitter" box must be used to "split" the single cable into two or more.
EXAMPLE MIDI INTERFACES
Following is an abbreviated list of Macintosh MIDI interfaces, along with their capabilities, requirements, and mail-order (list) prices:
Interface Name Manufacturer-Ports Used M or P-Total Channels-Street Price
* Some MIDI interfaces offer a different number of IN and OUT ports for connection to the MIDI bus. An example is the Opcode "Professional Plus" interface, which connects to either the modem or printer port, and offers three MIDI OUT ports and one MIDI IN port.