(MONEY Magazine) – Roll over, Beethoven, and tell Mick Jagger the news. Just about anyone can sound symphonic with the remarkable device shown below. And if your favorite opening bars are not to the master's Fifth but rather to the Rolling Stones' Brown Sugar, this same electronic keyboard can help you get off some hot licks on that tune too. From a preschool pianissimo to concert crescendos, electronic keyboards are quietly revolutionizing the sound of music. Using an internal computerlike sound chip, a single keyboard can produce a vast repertoire of musical sounds from a delicate violin to an eight-piece punk band. And now that scores of these versatile melody makers sell for well under $500 in discount department stores nationwide, a stampede of parlor pianists is heading home with an unprecedented capacity for music under their arms. Last year Americans bought some 3 million keyboards -- roughly twice the number sold in 1985 and nearly six times the 1984 total. Prices have been falling about 20% a year as makers achieved economies of scale and retail sales shifted from music stores to discount outlets. For Chicago pharmaceuticals executive David Dezelan, buying a portable keyboard was a way to ensure that daughters Kaelin, 10, and Suzanne, 7, would practice their piano lessons during the family's Florida vacations. Says David: ''At first it was enough of a novelty to keep their attention; later on they became more creative with it and now they'll even bang out a few tunes when their friends come over.'' In the hands of a professional, such banging can pay off, says Jeff ''Skunk'' Baxter, a classical pianist and former Doobie Brother, who cut the instrumental track on the hit tune Pour It Out with a hand-held keyboard. Like any other musical instrument, keyboards simply allow their users to manipulate a series of notes. Picking out a tune on one is just as hard as on a piano. But depending on how sophisticated a machine you buy, the ways that you can enhance, modify, accompany, orchestrate, syncopate and record your basic tune represent a giant step away from traditional instruments. For one thing, you can play several different instrument sounds at once to make your solo act sound like the work of an entire combo. By far, the major manufacturers of inexpensive keyboards are the Japanese firms Casio and Yamaha, though a handful of other makers, including Technics and Akai, also sell models for nonprofessionals. In all, there are some 50 models to choose from, ranging from pocket-size musical toys selling for less than $30 to 70-pounders the size of an ironing board. To find the model best suited to your talents, or lack thereof, you must first decide what you want the keyboard to do. Do you want to get your 10- year-old started on piano lessons, or are you looking for a 16-piece band- in- a-box for a budding rock musician? Then you need a basic understanding of those knobs and buttons on the console. Some merely set the type of sound you play -- piano, trumpet, violin -- but others plunge you into the fundamental physics of sound waves.

Keyboards, no matter how simple or complex, share certain basic characteristics. Almost all will sound a note when you hit a key (a few require a separate speaker) and most will play at least one prerecorded tune at the touch of one button. Most operate for several hours on batteries; a plug-in adapter for longer playing times is usually a $15 option. Both the quality of the sound and the number of features tend to rise with the price. A crude but useful generalization: below $100 or so, you get a nifty toy; between $100 and $250, buttons proliferate, the number of instrument sounds increases and tones become noticeably clearer. Above that you will likely find formidable instruments worth keeping for years. (For a list of standard features in each price range, see page 176.) Still, paying extra for features you will never use can be a costly mistake. Says Jim Aikin, associate editor of Keyboard Magazine: ''Many people who buy home keyboards get no more use out of them than they do from a health spa membership.'' In your quest for the perfect keyboard, focus first on the obvious -- the keys themselves. On small models, you find from 29 to 49 mini-keys, each no bigger than your pinky finger. Tiny keys are one reason why the Casio PT-1 ($30) and Yamaha's PSS-30 ($35) are essentially toys. Mid-size keys, which are about two-thirds the size of standard piano keys, are wide enough for practicing or casual entertaining, and standard-size piano keys are best for serious students and aspiring performers. Made of plastic, the keys on some models feel and move a lot like real piano keys; others are closer to the feel of a door buzzer. As a rule, you need at least 49 keys -- four octaves -- to play rock or pop tunes; 61 is considered the minimum for classical pieces. You cannot play chords on some keyboards. Called monophonic, these models sound only one note at a time no matter how many keys you strike at once. Monophonics, however, come with a variety of prerecorded chords that can be woven into your melody at the touch of a button or a key. To play chords with your own hands, you need what's called a polyphonic keyboard. Prices for models capable of sounding nine-note chords begin at around $120 with the Yamaha PSS-170. Canned rhythm tracks, from the beguine to the waltz, can be laid in at the touch of a button. Keyboards priced below $100 have 10 or so prerecorded rhythms expressed in sounds such as snare and bass drums, clapping hands and bass chords. More expensive models provide a wider selection of rhythm sounds and let you vary the tempo. Several models, including Casio's MT-500 ($325), have electronic drum pads for tapping out your own beat in a variety of clear percussion sounds, including bells, bongo drums and cymbals. Embellishments such as glissando, portamento and vibrato are one-touch features on models priced above $100. Also common is a sustain feature that holds a note, much as a piano foot pedal does, and a chorusing feature that detunes a note slightly to make it sound like several musicians are playing it. Thus can the song in your heart be set free by an electronic keyboard. How it sounds, however, depends on the type of speaker or speakers -- if any -- built into your model. Inexpensive monaural keyboards tend toward the kazoo end of the sound-quality scale, unless they have a jack, which permits a hookup with your home stereo speakers. Even so, a stereo keyboard sounds noticeably better than a mono model. Some keyboards have no built-in speakers at all. You need a pair of headphones or separate speakers to hear your creation. Parents and siblings of the almost-talented take note. When you go shopping for a keyboard, don't get flimflammed by talk about digital vs. analog sound. An analog keyboard uses electrical voltage to control its sound-generating components. Voltage variations create sound waves that are analogous to -- but not duplicates of -- those made by an instrument. Digital music employs mathematical snapshots of notes, in effect duplicating them electronically in the form of computer-digestible numbers. Because it is a duplicate, a digital copy is apt to sound more like the original instrument than an analog approximation. In general, digital boards allow you greater control in modifying their sounds and thus offer more features. They usually cost more as well. Digital keyboards, however, are not inherently better than analog; other design factors also affect the sound you hear, so don't succumb to digital dementia. The best judges of a keyboard's sound are still your own ears. If you want to record and play back tunes, plan to spend at least $200 for a keyboard with a four- or an eight-track sequencer. A sequencer records, say, two melody lines, chords and a rhythm track and holds them for instant recall. You can replay one track, work on it awhile and then put it back into the tune. A built-in synthesizer confers astounding musical powers: it gets physical with the notes. You can construct or redesign any sound by altering the shape and size of its characteristic sound wave. For example, suppose that your keyboard's factory-installed violin voice is too sluggish. Just use your synthesizer controls to shorten the notes' attack (the way the individual sound begins) and also cut the decay (the notes' fadeout). One inexpensive model with an on-board synthesizer is Casio's SK-1, a monaural unit that sells for about $100. For about $90 more, you could get a stereo synthesizer, Yamaha's PSS-470. The Casio CZ-101 and CZ-230 models, which sell for about $250, provide dozens of preset sounds that can be altered with their built-in synthesizer. What if you want to create your own sounds to fiddle with? Then look for a keyboard with a feature called a sampler. With this on board you can record any sound (a beautiful high C or a moose call) and replay its distinctive qualities on any key -- that is, in several octaves, replete with sharps and flats. If you are comfortable this far into the realm of samplers and synthesizers, you should consider a keyboard that also has a feature called MIDI, the acronym for musical instrument digital interface. MIDI connects other keyboards to yours, capturing their sounds for use in your master keyboard. MIDI can also link your keyboard to a computer. With appropriate software, a personal computer displays the notes you play and prints them out as sheet music. Or the computer can feed back instructions, turning your keyboard into a player piano. Many brands of musical software are available for Apple, Atari, Commodore and IBM-style computers from $50 to $500. Instead of expanding all musical variables, electronic pianos, a highly specialized variety of keyboard, focus their new technology almost solely on fidelity. The tones and touch of many electronic pianos are now remarkably similar to those of their stately wood-and-wire cousins. In addition to Casio and Yamaha, companies that make electronic pianos include Ensoniq, Kurzweil, Roland and Siel. Some have speakers, others require headphones or a separate amplifier and speaker. A model with 61 full-size keys is usually sufficient for most compositions, but you may need 76 or 88 keys for demanding classical pieces. Usually, electronic pianos have four or five piano-like voices to choose from: vibes, honky-tonk, harpsichord and two acoustic. Excellent fidelity -- near that of a good FM radio -- is part of the trade-off for the high price and dearth of features. Touch sensitivity -- keys engineered to feel and respond like acoustic piano keys -- is a must for serious pianists, although this feature can add anywhere from $50 to $300 to the price. Touch sensitivity means that the speed with which you strike the keys determines the loudness and duration of the note. If you are beyond the hunt-and-plunk level or have good reason to think your child will soon be, it may be worthwhile to spend $900 to $1,200 for a professional model. Nevertheless, several electronic pianos do admirable yeoman's service for much less. Yamaha's YPR-7, for example, which has 61 touch-sensitive keys and a built-in monaural speaker, sells for about $300. For about $100 more, the YPR-9 provides stereo sound and a built-in recorder. In roughly the same price range, Casio's 61-key CPS-101 gives you 10 preset voices, including strings and pipe organ sounds, stereo sounds through built- in speakers and MIDI.

BOX: KEY CATEGORIES Most electronic keyboards cost between $50 and $500. Look at the listing below to see what features you can expect to get in different price ranges. Your actual cost may be 20% to 30% below list price.

Under $100 -- Keys: 29 or 49 mini- or mid-size -- Voices: four to eight preset instruments -- Chords: two-note and preset -- Rhythms: eight to 12 auto-rhythms -- Speakers: one built-in -- Memory: 100 to 400 notes -- Examples: Casio PT-1, PT-87, Yamaha PSS-30 and PSS-130

$110 to $250 -- Keys: 44 or 49 mid-size or full size -- Voices: 100 preset instruments -- Chords: eight- and nine-note polyphonic -- Rhythms: 10 to 20 auto-rhythms -- Speakers: built-in stereo -- Memory: 200 to 600 notes -- Other: sampler (a few models) -- Examples: Casio SK-1, CT-360, Yamaha PSS-470, PSR-12

$250 to $500 -- Keys: 49, 61 or 76 mid-size or full size -- Voices: 40 to 60, FM quality -- Chords: eight- and nine-note polyphonic, auto chords -- Rhythms: 12 to 20 auto-rhythms -- Speakers: built-in stereo -- Memory: 400 to 1,500 notes -- Other: drum pads, synthesizer, sampler, MIDI, auto harmonize -- Examples: Casio MT-520, CT-510, Yamaha PSS-570, PSR-32

Electronic pianos $400 to $700 -- Keys: 61 or 76 full size -- Voices: five to 14 instruments -- Chords: eight-note polyphonic -- Speakers: built-in stereo with headphone jack -- Other: damper pedal, MIDI, touch-sensitive keys -- Examples: Casio CPS-101, CPS-201, Yamaha YPR-7, YPR-9