THE music industry is now battling over which technology -- MP3, Real Jukebox or some other scheme -- should become the standard for listeners who want to download small, secure, good- quality audio files from the Internet.

Composers, on the other hand, have had a standard workable digital form for years. Musical Instrument Digital Interface, known as MIDI, gives composers, using a computer and special software, the ability to simulate the sound of an entire symphony orchestra or quickly change a composition's key, instrumentation and tempo and hear the results instantaneously. And it doesn't require a single performing musician.

MIDI is an industry-standard protocol that lets composers easily connect a computer with a keyboard and musical synthesizers.

Before MIDI was unveiled in 1983 by a group of musical instrument manufacturers, electronic musical instruments used different methods to connect to one another, often making it impossible to use two from different manufacturers. MIDI changed all that, analogous to the way that SCSI and USB allow all printers and hard disks to connect with PC's using standardized cables.

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To compose and edit music on a PC, a musician purchases a low-cost MIDI interface box for less than $100, which connects to a computer via the serial port, or installs a MIDI card. Music sequencing software like Cakewalk Pro, Steinberg Cubase or Mark of the Unicorn Digital Performer is used to record the notes, as well as the composition's instrumentation, key and length.

To play music in real time, a controller, which can be a MIDI-compatible keyboard or wind or string instrument, is added. The setup will also include at least one synthesizer, which actually creates the sounds. Some synthesizers manipulate samples of different instruments that have been stored in its memory. Those sounds are then fed to an amplifier and speakers to become audible.

With this MIDI-compliant package, a composer today can create compositions that to most listeners sound as if they are actually being played by scores of musicians seated in a concert hall.

''MIDI gives composers the musical equivalent of a word processor,'' said Thomas Dolby Robertson, founder of Beatnik, which produces technology to deliver high-quality music and sound over the Internet.

A MIDI-compatible computer can control up to 16 different channels on each of an infinite number of synthesizers that can be chained together, giving composers the ability to create music with innumerable layers of sounds.

Unlike MP3 or the other musical compression formats, MIDI files are not composed of musical sounds; rather, they are digital instructions telling the synthesizers what notes to create, how long to play them and with what intensity.

MIDI's small file size makes it ideal for CD-ROM games, in which space is at a premium. A game player hears music because the disk's embedded MIDI data instructs the PC's sound card to create certain notes. But those notes are not embedded in the game itself.

Composers, equipped with very expensive synthesizers, use MIDI to create compositions that would be impossible or extraordinarily expensive to perform with live musicians.

Jeff Rona, who has composed musical scores for the movie ''White Squall'' and the television shows ''Chicago Hope'' and ''Profiler,'' uses MIDI and computer sequencing software to create compositions performed by virtual orchestras. Changing the sound from a violin to a cello or tuba is a simple matter of the PC instructing the synthesizer to use a different instrument.

Mr. Robertson samples unusual sounds, like crickets and trees falling in the forest, and then using MIDI commands instructs the synthesizer to substitute those for cymbals and bass drums.

Because of the complications of human speech, the MIDI standard will remain confined to controlling tones, like instrument sounds and sound effects, that can be accurately sampled and then reproduced electronically by synthesizers.

''Voice sampling is rarely done,'' said Tom White, president of the MIDI Manufacturers' Association. ''Speech is extremely complicated. There are so many nuances, mouth shapes and tones that it's very difficult to create virtual speech from a recorded sample.''

Some MIDI software has a ''humanizer'' element that subtly alters tempo and dynamics so that the music does not sound machine perfect. But can music created using synthesizers and sampled tones, and controlled by MIDI devices, ever sound exactly like the real thing?

''MIDI-created music can't, because physical sounds are intrinsically linked to underlying physical gestures,'' said David Mash, vice president of information technology at the Berklee College of Music in Boston. ''If you play a violin sound on a keyboard, you can't play the keyboard as if it's a violin.

When you're one person, trying to perform 15 different parts of a composition by yourself, you've got to try and inject those missing personalities that would be there in a live performance. It's very difficult to do so. Still, it's a lot cheaper than hiring a whole symphony orchestra.''

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