MIDI stands for Musical Instrument Digital Interface. MIDI is both a means for communicating between digital instruments and playing them via a computer based program.
Electronic instruments are really synthesizers. A PC sound card is also a synthesizer. The difference between a MIDI instrument and a full synthesizer is that the instrument may only have the voices that are appropriate to the kind of instrument it is. On the other hand, a synthesizer has a wide range of voices. For example, the Casio MIDI horn, which looks like a little saxophone, can only make sounds like a number of different kinds of horns, and some reeds.
The General MIDI specification defines a number of voices, and gives them names, like flute, tambourine, piano, etc. The standard does not specify how each instrument should be synthesized, and has no parameters for how it should sound. Each voice sounds like what the card designer thinks the instrument should sound like. Thus, if you wanted you could create a MIDI synthesizer whose piano voice sounded like a violin. Your MIDI file would play to the piano voice, and you would hear a violin. However, the program would be playing correctly.
A limitation of synthesizer MIDI cards is how accurately the hardware can synthesize a given sound -- and how accurate the mathematical model of the sound is that the card uses. This is why you can play the same MIDI file on different cards and have it sound different, even though both are playing the file correctly. The MIDI standard does provide for ways to compensate. This is what some of the settings in your Windows MIDI mapper are about. Some DOS MIDI players allow you to apply compensation on the fly.
Recognizing that sound synthesis will only be as good as the mathematical model of the sound to be synthesized, designers have moved on to what are called wave tables. A wave table is a sampling of sounds of real instruments for each of the MIDI voices. Nobody would have enough memory or disk space to store every possible sound of every instrument represented in MIDI. Wave table synthesizers start with real sound samples and manipulate them to achive the specific note called for, etc. Starting from a real sound the generated sound comes out "better."
MIDI files are edited and often created with a program called a MIDI Sequencer. There are a number of major commercial and shareware sequencers available. They allow you to create midi files, and manipulate them, note by note. Most of them also display the music notes on the computer screen, and some allow you to print the score from the midi file. In addition, at least one sequencing program, SmartScan, allows you to scan a printed score into the computer to produce a midi file.
MIDI files are computer programs which instruct a synthesizer to produce sounds. WAV and AU files are actual recordings. On the plus side, they can include voice. MIDI does not synthesize the human voice. On the minus side, everything over a few seconds of complex music can produce HUGE files. As a rule of thumb, you should assume that with a 22 KHz sampling rate (half that used by a CD player) you will produce 10 meg of data per minute of playing time in a raw WAV file. Voice WAV files are often sampled at 11 KHz to save space. The WAV format allows some compression, and AU files provide better file compresson.
To create WAV files you need a program called a mixer, and sound sources. Most sound cards come with a simple mixer. The card has a line input connection for an amplifier, and a microphone input connection. The mixer program will also mix analog sound with sound from an onboard MIDI sequencer.
Once you have WAV files, you can use a wave editor to actually cut samples from one file to another, and to modify the sound of the file. The display of the editor looks like the trace of an ocsilloscope, except that you can freeze it and mark the part of the trace you want to work with.
Updated 14 December 1999