MIDI is a term which you'll see a lot in music, and we use the term in reference to some of the keyboards we use when we put on music programs The word is pronounced like "middy" and it is actually an acronym for the term Musical Instrument Digital Interface. There's a lot we could write about MIDI, but fortunately there's a lot on the Internet as well about it so if you really want to pursue MIDI in depth, do a Google search on it. Although it is a complicated technology that is evolving into new forms, essentially it is a protocol or a means by which various electronic musical instruments can communicate with each other and also with a computer.
MIDI is in a sense a special language or file system (although the two terms are not really synonymous) by which information gets transferred between and among various electronic musical instruments, signal processors, sound modules, drum machines, sequencers and computers. Songs can be stored in sequencers and computers as MIDI files and then played back on any other MIDI compatible equipment that is capable of synthesizing MIDI sounds. MIDI is also standardized, so that all MIDI equipment should operate and sound the same way regardless of who manufactured it. There are considerable differences in imitational accuracy between high- and low-end equipment, however, ranging from imitations that sound like imitations to some that are essentially indistinguishable from the real thing. It is reasonably safe to say, however, that the accuracy of even low-end MIDI imitations is better than that of the best electronic organs of the analog era. New electronic organs however use the new technology also and thus their imitations are as good as those of any other MIDI equipment.
MIDI makes it possible for a person who is knowledgeable about its implementation to create elaborate, flawless orchestral arrangements of even the simplest of songs. It allows a person sitting at one MIDI compatible keyboard to control and play simultaneously a number of other MIDI compatible instruments, and it also allows a person to add numerous different instrumental parts to songs, fix up any errors, and select from a wide variety of instrumental effects and sounds whatever he wishes to incorporate in the final production. MIDI can be used in real time playing, and it can also be used to produce complex arrangements which then get played back later on in their entirety after the MIDI musician has first "laid down" tracks or the various parts of his final arrangement.
With the right computer software installed, you can play a song on a MIDI keyboard, and then have the computer's printer print it out as sheet music. If you hit a wrong note or have slightly sloppy fingering, this is not a problem either as MIDI allows complete editing and error correction. If you meant to hit Middle C but instead you nicked D by mistake, you can go in and move D down to C. If after you listen to the MIDI playback and you decided you would like to have the song play in a different key from the key you played it in, you can type in a simple command and instantly the song is shifted up or down as necessary to the key of your choice. This is a boon for those keyboard players who play everything in the key of C but can't transpose to other keys by normal playing.
If you wish to play a polka at a fast tempo but your fingering is not up to par, no problem. Just play it as slowly as you want, and then set the tempo to as fast as you would like and instantly the song plays at the new tempo. With all of these wonderful capabilities, it is no wonder that MIDI has become a very important part of music production today. It is safe to say that probably at least ninety percent of the musical scores that accompany movies and TV programs today are, in spite of their big, lush orchestral sound, probably the work of just one or two musicians using MIDI equipment in a studio. And when the time comes to play this music and have it sound like it was being played in an acoustically correct concert hall instead of a bedroom sized studio, modern digital signal processors come to the rescue, creating room and hall ambience at the touch of a few buttons.
MIDI keyboards and other related equipment which are capable of generating musical sounds have achieved an incredible level of realism in their instrument sounds. Some of the best equipment available can mimic various instrumental sounds so well that on a recording it is virtually impossible to distinguish between the MIDI instrument sounds and the actual instruments that they are imitating.
When you press a key on a MIDI keyboard, this action sends out some information. It tells a compatible synthesizer or tone generator to start playing a certain note. It also tells it what pitch to make the note, and what it should sound like and perhaps even how loud it should be depending on how hard you hit a key, just like a piano. When we let go of the key, this tells the note when to end. All of these "instructions" are bits of information. This is fundamentally very different from the operation of earlier electronic organs, such as the Hammond, where pressing a key applies an existing audio frequency or frequencies ultimately through an amplification system and to loudspeakers. The signals coming out of a MIDI keyboard are not musical signals at all. If you converted MIDI signals directly to sound, the result would not be even remotely recognizable as music, because the signals from the MIDI keyboard are not audio signals but coded bits of digital information.
When the MIDI keyboard signals arrive at the synthesizer or tone generating section of the system, which may be entirely remote from the keyboard and possibly even the sound card in a computer, then the synthesizer responds to the MIDI information signals by generating the required instrumental sounds which it usually recalls from an electronic memory that holds samples of the various sounds. In most cases, these MIDI sound samples have been taken from actual musical instruments which is why the realism of MIDI equipment is so much better than that of the earlier analog technology of electronic organs.
This separation of time is something that takes a little getting used to. For example, we could set up a MIDI keyboard, play a song and hear absolutely nothing as the MIDI keyboard sent its information to a computer. Then, we could store the MIDI file in the computer, and perhaps even a week or a month later, we could have the MIDI file operate on the computer's sound card, and then we would finally hear what we played previously. This is totally different of course from conventional instruments where you hear what you play when you play it, but then a MIDI keyboard may not even have the capability of producing any musical sounds at all, although most of them have on board synthesizers so that you can indeed hear what you are playing. So there is your most rudimentary introduction to MIDI!