CDs are convenient, durable and sound great. Less than half the size of an old vinyl record album, a CD holds nearly twice as much music - 74 minutes - and you don't have to turn it over to hear the entire recording. It's almost impervious to harm compared with fragile vinyl and will sound as good as new after years of listening.

The remarkable sound quality of CDs is the result of two factors. The first is the digital encoding of sound, which can produce a dynamic range nearly as wide as that in a live performance. The second is the freedom from the mechanical limitations of conventional record playback. All you hear is the music, without hiss, snap and pops.

Features

The CD player offers a range of features, most common of which are programmability, time indicators and instant search.

Programmable CD players allow you to play tracks in a sequence other than that in which the CD was recorded, repeat one or more tracks or play tracks in random sequence.

Elapsed time and time-remaining indicators display time remaining in either a single track or in an entire recording.

Instant search takes you to a specific track or time, such as 22:43 into the recording.

CD players offer numerous features that are not available with a conventional phonograph turntable. Many of these features may add to the cost of a basic player, and you may want to consider which features you're likely to use.

All CD players incorporate a digital-to-analog converter (DAC or D/A). This circuit converts the digital 1s and 0s comprising the music into an analog wave that can be translated by your amplifier and, ultimately, your speakers. The better the D/A converter, the better the sound.

Many higher-end CD players have a direct digital output jack, which allows you to connect an outboard D/A converter, or to connect your CD player directly to a digital recorder, a digital audio tape (DAT), MiniDisc or CD-R recorder.

Variations

CD players vary primarily in size and disc capacity. The basic single-disc player holds and plays one disc at a time. There are many types of CD changer models, which hold and play several CDs. The most popular type is the carousel changer, a turntable-like CD player that can hold as many as five CDs on a single circular tray. Another variation is the cartridge changer, in which six or more CDs can be loaded into a plug-in magazine. CD jukeboxes can hold and play anywhere from several dozen to a couple of thousand discs. Most affordable jukeboxes will hold from 50 to 200 discs.

Each of these multi-disc players offers features such as multi-disc programming and multi-disc random play. Some changers allow you to change several discs while one is playing. When considering a CD jukebox, one feature to look for is daisy chaining, the ability to interconnect several high-volume multi-disc players together to create a large jukebox.

The random or shuffle play feature does not work the same on all changer models. The random play on a cartridge player will take more time between songs because it takes longer to change discs than a carousel player.

Some players random-play each track once, then stop when all the tracks on all the CDs in the changer have been played. Random play on other machines will be truly random, meaning you may hear the same track more than once while not hearing another track at all.

A variety of car models are available either as add-on stereo components or combined in a single unit with a tuner. Car CD players can be mounted into or underneath the dashboard, and some are detachable, so you can protect against theft. If you're interested in a CD player for your car, see the Web pages on Autosound.

The most common portable CD player is the personal model, a flat unit barely the size of a CD jewel case, to which you attach headphones. Many of these personal CD players protect against shock in the form of a 10- to 40-second buffer. The buffer memory records several seconds of music, and if the player is jostled, the music is played from the memory, resulting in uninterrupted play.

CD technology

Technical differences among players cause specifications to vary and to affect the way music sounds on different players.

A compact disc is a thin polycarbonate wafer 4-3/4 inches in diameter. Instead of the spiral groove on a vinyl long-playing record, a disc contains millions of microscopic pits in a highly reflective smooth surface covered by a coat of clear protective plastic.

Each pit represents part of the music in the form of a binary digital code. Arranged in patterns on the surface of a CD, the pits are read by a low-power laser beam in the player. The average analog record groove is about one millimeter wide, the thickness of a human hair. Thirty CD tracks can fit side by side in the same space. Each pit measures just 0.00002 inch in width. Stretched out in a straight line, the pits in an average recording would measure nearly four miles.

The laser reads the pits from the center of the disc outward to the edge and converts the digital code into a digital signal, into electrical energy and eventually into audible sound. Since the laser beam does the tracking, no solid object comes into contact with the playing surface. Thus there is no wear on the disc and no distortion from a worn or chipped stylus tip.

Four technologies separate average CD players from great ones: the number of laser beams used, oversampling, the D/A converter and error correction.

• All CD players use a single laser whose light is split into two or more beams either before or after it's reflected from the CD. The purpose of multiple beams is to provide constant mechanical positioning of the laser so that it stays on track. Most CD players offer a three-beam pickup.
• Some higher-end CD players sample the waveform at two, four or eight times the standard 44.1 kHz rate. The advantage of this "oversampling" is that any distortion can be filtered out. (Compare sound from different players side-by-side in a store to determine if any difference you hear is worth the extra money.)
• Once a digital music signal is created, it must be converted to an analog waveform for your speakers to reproduce the music. Every CD player has a built-in digital-to-analog converter for this purpose, and better players have higher-quality DACs.
• Digital technology can compensate for tiny flaws in the disc by means of logic circuitry. If one or a group of microscopic pits on the disc is obscured or missing, an error correction circuit reconstructs them. The better the error correction capability, the better the music will sound.

Accessories

The biggest differences between CD players may be in the way each player interprets digital data and converts them into an analog signal for reproduction by your speakers. Outboard digital/analog converters (DACs) perform better than the ones built into most CD players, and there is no chance of extraneous noise. To use an outboard DAC, your CD player must be equipped with a direct digital output jack.

Personal CD players can be turned into mini home audio systems by adding small accessory speakers. These contain small amplifiers and can run on batteries or AC power. The speakers connect to the CD player via the headphone jack.

As with the other features, there are also a variety of storage options available for your CDs. Jewel case storage units are either open shelf-type or slotted. Open shelf systems include bookcase-like units and/or dowel racks. Slotted storage cases hold each CD in a single slot.

A third type of storage allows you to dispose of the jewel cases and store both the CD and its pamphlet into plastic sleeves. These sleeves are stored in a photo album-like case, or a Rolodex-like open file. In the open file system, the plastic sleeves holding the CDs can be removed and transferred easily into a carrying case for portable use.

Maintenance

If a CD player skips, the problem is usually dirt on the disc. A layer of clear plastic protects the surface, but the pits still can be obscured by dust, scratches, grease or oil. The laser beam must have a clear shot at as many of the tiny pits as possible to reproduce the music perfectly. Skin oil from your fingertips on the disc surface can confuse the laser and distort the sound or set the CD skipping.

Handle discs with care by holding them by the rim or by a finger through the center hole, rather than touching the surface. Contrary to popular belief, CDs are not completely invulnerable. Keep them away from extreme heat or cold and return them to their cases or sleeves when you're not listening to them.

Dust and fingerprints can be removed from the disc surface with a clean, lint-free, soft, dry cloth. Never use a household cleanser or solvent because these are more likely to destroy a disc than clean it.

Use radial cleaning: straight strokes outward from the center of the disc to the edge, and from the outer edge toward the center, which is the way the player reads the disc.

There are many commercial CD cleaning kits or devices available.

Because the CD drawer opens and closes, the laser pickup can accumulate dust. A CD lens cleaner, an actual CD with a small brush mounted on the surface, gently wipes the laser pickup clean.