MIDI is short for "Musical Instrument Digital Interface", a serial interface created by the synthesizer industry in the early 1980s in an attempt to modernize how synthesizers, drum machines and computers talked to each other. In gaming and on PCs, however, it refers specifically to General MIDI, a specification championed by synth maker Roland to provide a standard base of sounds and commands for an entry-level synthesizer. The first General MIDI-compliant synthesizer was Roland's own SC-55 Sound Canvas, released in 1991.
General MIDI was quite attractive in the early days of computing, because it was (and is) very portable. It only provided notation for a song, not the sounds themselves; if WAV files were recordingsnote and synthesizers were orchestras and bands, then MIDI files were merely the sheet music, an object that is smaller (often only a few KB) and rather more portable than an orchestra. The problem came with the lack of orchestra. Sheet music needs to be performed, after all, and if the entity performing it (like a Pac-Man cabinet) can't produce very good noises, you're stuck, and most computerized entities of the day could not produce very good noises.
To get something that wasn't just beeps and bloops, you needed a General MIDI module using PCM samples, which were very expensive at the time; the cheaper FM synthesizers used on PC sound cards weren't up to the task of simulating an entire orchestra well.note Instead, MIDI music on PCs sounded like music on the Sega Genesis, another platform with an FM synthesizer. This started to change when Creative and Gravis introduced sound cards with samplers on-boardthe Sound Blaster AWE32 and the Ultrasound, repectivelynote .
As PCs got more powerful, sound card makers (and, eventually, the operating systems themselves via QuickTime on Macs and Windows machines, and the Microsoft GS Synthesizer module on newer versions of Windows, both of which use licensed Sound Canvas ROMs) added software-based PCM synthesizers, making General MIDI a much more palatable solution comparable to MOD. General MIDI reached its height around the days of the first PlayStation, with its 24-voice ADPCM sampler. In Japan, the Sharp X68000 played music with its eight-channel FM synthesizer and one ADPCM channel. However, MIDI data could be routed to an external Roland MT-32 or SC-55 synthesizer to produce superior audio quality, and it was pretty awesome (albeit expensive) for its time.
The increase in cheap computing power also had the effect of making General MIDI itself obsolete on consumer devices. As hard disk and memory capacity increased, it became easier to include pre-recorded music and elaborate sound-processing engines directly in games for PCs and stationary consoles, making the size advantages of General MIDI moot. Today, some see MIDI as a Scrappy in digital audio. Poor or unconvincing emulations of real instruments with synthesizers (and by extension the music they produce) are labelled as "MIDI-ish", even though technically, as described above, the format itself has nothing to do with the quality of the samples/sound synthesis used. However, MIDI is still used today by composers who work with music/audio workstations to create and perform music on computers and synthesizers. In terms of consumer applications, MIDI (along with MOD) are not used for much besides games that need precise tempo control, music in Nintendo DS, Nintendo 3DS and a handful of freeware doujin games, and ringtones on low-end cellphones. Even smartphones have switched to MP3 or M4A ringtones.