1.3 IN, OUT, THRU

Most MIDI-capable instruments feature a MIDI-IN, MIDI-OUT, and occasionally a MIDI-THRU connection in the form of five-pin DIN jacks. 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.

Although nowadays most PC soundcards have the ability to terminate a MIDI connection (usually through a converter on the game port), the 1985 Atari ST was the first home computer to sport the original five-pin format — which made the ST a very popular platform for running MIDI sequencer software such as Steinberg's Cubase, Digidesign 's Pro Tools, Jazzware 's JAZZ++ , Cakewalk's Sonar and Mark of the Unicorn's Digital Performer.

1.4 General MIDI

In MIDI, instruments are selected by number (0-127), using the Program Change message. However, the basic MIDI 1.0 specification did not specify what instrument sound (piano, tuba, etc.) corresponds to each number. This was intentional, as MIDI originated as a professional music protocol, and in that context it is typical for a performer to assemble a custom palette of instruments appropriate for their particular repertoire, rather than taking a least-common-denominator approach.

Eventually interest developed in adapting MIDI as a consumer format, and for computer multimedia applications. In this context, in order for MIDI file content to be portable, the instrument program numbers used must call up the same instrument sound on every player. General MIDI (GM) was an attempt by the MIDI Manufacturer's Association (MMA) to resolve this problem by standardizing an instrument program number map, so that for example Program Change 1 always results in a piano sound on all GM-compliant players. GM also specified the response to certain other MIDI messages in a more controlled manner than the MIDI 1.0 specification. The GM spec is maintained and published by the MIDI Manufacturer's Association (MMA).

From a musical perspective, GM has a mixed reputation, mainly because of small or large audible differences in corresponding instrument sounds across player implementations, the limited size of the instrument palette (128 instruments) and its least-common denominator character, and the inability to add customized instruments to suit the needs of the particular piece. Yet the GM instrument set is still included in most MIDI instruments, and from a standardization perspective GM has proven durable.

Later, companies in Japan's Association for Musical Electronic Industry (sic) (AMEI) developed General MIDI Level 2 (GM2), incorporating aspects of the Yamaha XG and Roland GS formats, extending the instrument palette, specifying more message responses in detail, and defining new messages for custom tuning scales and more. The GM2 specs are maintained and published by the MMA and AMEI.

Later still, GM2 became the basis of Scalable Polyphony MIDI (SP-MIDI), a MIDI variant for mobile applications where different players may have different numbers of musical voices. SP-MIDI is a component of the 3GPP mobile phone terminal multimedia architecture, starting from release 5.

GM, GM2, and SP-MIDI are also the basis for selecting player-provided instruments in several of the MMA/AMEI XMF file formats (XMF Type 0, Type 1, and Mobile XMF), which allow extending the instrument palette with custom instruments in the Downloadable Sound (DLS) formats, addressing another major GM shortcoming.

1.5 Low bandwidth

MIDI messages are extremely compact, due to the low bandwidth of the connection, and the need for near real-time accuracy. Most messages consists of a status byte (channel number in the low 4 bits, and an opcode in the high 4 bits), followed by one or two data bytes. However, the serial nature of MIDI messages means that long strings of MIDI messages take an appreciable time to send, and many people can hear those delays, especially when dealing with dense musical information or when many channels are particularly active. "Running status" is a convention that allows the status byte to be omitted if it would be the same as that of the previous message, helping to mitigate bandwidth issues somewhat.

2 MIDI file formats

MIDI messages (along with timing information) can be collected and stored in a computer file system, in what is commonly called a MIDI file, or more formally, a Standard MIDI File (SMF). The SMF specification was developed by, and is maintained by, the MIDI Manufacturer's Association (MMA). MIDI files are typically created using desktop/laptop computer-based sequencing software (or sometimes a hardware-based MIDI instrument or workstation) that organizes MIDI messages into one or more parallel "tracks" for independent recording and editing. In most but not all sequencers, each track is assigned to a specific MIDI channel and/or a specific General MIDI instrument patch. Although most current MIDI sequencer software uses proprietary "session file" formats rather than SMF, almost all sequencers provide export or "Save As..." support for the SMF format.

An SMF consists of one header chunk and one or more track chunks. There are three SMF formats; the format is encoded in the file header. Format 0 contains a single track and represents a single song performance. Format 1 may contain any number of tracks, enabling preservation of the sequencer track structure, and also represents a single song performance. Format 2 may have any number of tracks, each representing a separate song performance. Format 2 is not commonly supported by sequencers nor commonly found in the wild.

Large collections of SMFs can be found on the web, most commonly with the extension .mid. These files are most frequently authored with the assumption that they will be played on General MIDI players, but not always. Occasional unintended bad-sounding playback is the result.

MIDI-Karaoke (which uses the ".kar" file extension) files are an "unofficial" extension of MIDI files, used to add synchronized lyrics to standard MIDI files. Most SMF players do not display these lyrics, however numerous .kar-specific players are available. These often display the lyrics synchronized with the music in "follow-the-bouncing-ball" fashion, essentially turning any PC into a Karaoke machine.

Note: ".kar" files can often be played by SMF players if the filename extension is changed to ".mid".

The MIDI Manufacturer's Association has now defined a new family of file formats, XMF (eXtensible Music File), some of which package SMF chunks with instrument data in DLS format (Downloadable Sounds, also an MMA specification), to much the same effect as MOD files. The XMF container is a binary format (not XML-based).

3 MIDI usage and applications

3.1 Extensions of the MIDI standard

Although traditional MIDI connections work well for most purposes, in 1994 a new high-bandwidth standard, named ZIPI, was proposed to replace MIDI for professional purposes. ZIPI failed due primarily to lack of demand.

3.2 Beyond MIDI

The Open Sound Control or OSC protocol, devised by CNMAT , transcends some of MIDI's musical coding limitations, and is considered by some to be technically superior. OSC has been implemented in software like SuperCollider, Max/MSP and Csound, however at present few mainstream musical applications and no standalone instruments support OSC, making whole-studio interoperability problematic. It can run over ethernet connections. OSC is not owned by any private company, however it is also not maintained by any standards organization.

3.3 Other applications of MIDI

MIDI can also be used for applications other than music:

Any device built with a standard MIDI-OUT port should (in theory) be able to control any other device with a MIDI-IN port, providing that developers of both devices have the same understanding about the semantic meaning of all the transmitted MIDI messages. This agreement can come either because both follow the published MIDI specifications, or else because for non-standard functionality the message meanings are agreed upon by both manufacturers.

See also: mobile phone ringtone.

4 MIDI editors/sequencers

• Rosegarden for Linux. GPL open source.
• MusE for Linux
• Cakewalk [2] for Windows
• Powertracks [3] for Windows
• Cubase [4] for Windows
• Noteworthy Composer [5] for Windows
• GNMIDI [6] MIDI utilities for Windows

5 External links

• MIDI Manufacturers Association
• MIDI Technical Fanatic's Brainwashing Center contains extensive technical information
• XMF FAQ
• MIDIPlugins list contains links to VST, OPT and MFX MIDI plugins for software applications
• VanBasco MID-Karaoke Player
• MP3/WAV to Midi Converter
• A good introduction to MIDI