Unless you have been a bit of an home stereo buff, some of the technical terms for specifying car audio and video systems can be off-putting. This glossary will help define the most common terms.

Antishock Memory

A feature of DVD, CD and MiniDisc players that stores audio data in a memory buffer that is output if the main signal mistracks.

Antitheft Protection

A system designed to prevent theft; including detachable faceplates, masking face plates, or systems disabling that means the unit will not operate if disconnected from the battery without a custom code.

Biamplification

A system that uses separate circuits of amplification to feed individual speaker components.

Changers

A CD cassette, MiniDisc, or DVD unit that stores more than one disc and serves them for playing based on input to the CD/DVD receiver unit.

Channel Separation

The amount of sound that crosses from one audio channel to the other ,measured in decibels. The higher the decibel, the better the separation between channels. Also known as stereo separation.

Digital/Analog Tuner

An analog tuner uses conventional circuitry with a tuning knob or thumbwheel and pointer to show the frequency to which you are tuned. A digital tuner, commonly referred to as a synthesized tuner, uses a digital display read-out and presetting radio stations for touch tuning.

Direct Access

Using a numeric keypad assigned to specific tracks to jump directly to that track.

Distortion

A measurement of the deviation from the original signal to the reproduced signal. Distortion produces a harsh sounding effect.

Dolby Digital

Formerly called AC-3, Dolby Digital is a method used to digitally encode up to 5.1 discrete channels of high-fidelity sound in a single digital bitstream. The .1 channel refers to the "low frequency effects" (LFE) channel and carries only low frequency information, normally directed to the subwoofer upon playback. The encoded signal can be any number of channels, from a mono signal to the maximum of 5.1 (front right, center, front left, surround right, surround left, subwoofer) although it has now become common to use Dolby Digital to refer to any 5.1 signal.

Driver

The part of a speaker that actually produces the sound.

DTS [Digital Theater Sound]

Achieves the same end effect as Dolby Digital, but does so in a slightly different way. The system was originally engineered for movie theaters. The two systems are incompatible when it comes to decoding, but after the signal is decoded, it appears the same to the receiver, allowing many new receivers to incorporate both. DTS is not as widely accepted in the market as Dolby Digital and minimal software is available that uses the system.

Dynamic Range

The difference between the loudest peak and softest part. Also can refer to the spectrum range between the edge of distortion at the high end of a units performance and the noise floor.

Equalizer

A circuit or program that changes the frequency response of a signal, boosting or cutting selected ranges from the overall response.

FM Modulation

Takes the audio output of an FM-modulated add-on and generates a radio frequency (RF) signal that is received by your car's FM radio tuner.

Frequency Response [FR]

Also known as Frequency range. This is the spectrum of frequencies produced by a product, within a certain tolerance. A frequency response of 30Hz to 18kHz (+/- 3dB) means a product produces all frequencies from 30Hz to 18kHz without dropping or peaking three decibels. The signal below and above that range does drop off more than 3dB, so the range is really the usable spectrum of sound

Head Unit

The user interface of a piece of equiement mounted in the dashboard. It contains the controls for the components, either internally or remotely installed, such as a changer mounted in the glove compartment or trunk.

Impedance

Impedance is the amount of inductive resistance a speaker offers to the flow of electrical signals from a receiver. It is measured in ohms. Most speakers have an impedence of 4 to 8 ohms, with some ranging from 2 to 16 ohms.

OEM

Stands for Original Equipment Manufacturer; the factory-installed system that comes with a car. Many car manufacturers are using name brand products, either standard or part of an upgrade package.

Player

A player can hold and play only one CD or DVD at a time. A changer plays many discs.

Power Ratings

Speakers are rated according to wattage. This is not so much the output of the speaker, but the amount of power you can safely deliver to th speaker without risking damaging it.

Similarly, too little power can also damage a speaker. Attempting to play a low-powered amplifier too loudly can distort the sound waves, which in turn can destroy a speaker.

Power ratings are listed as either "Peak" or "RMS" [Root Mean Square]. Peak refers to the amount of power the equipment can handle momentarily. RMS is a measurement of the average sustainable power; a better indication of the overall performance.

Preamp Output

An RCA jack that sends the signal to an external amplifier, before the signal is amplified internally.

Sensitivity

The efficiency in which a speaker converts electrical power from an amplifier into acoustic sound energy. The higher the sensitivity number [in decibels], the more sound you will get at a given volume setting. Sensitivity is usually measured from one meter away from the speaker with an input signal of 2.83 volts [1 watt into 8 ohms] which results in a sound pressure level.

Signal-to-Noise Ratio [SNR]

The ratio between the audio signal and noise floor. A low number indicates that the noise and music are closer together. The higher the number, the less noise is apparent.

Subwoofer

A large speaker component that produces extremely low-frequency signals, usually below 180Hz. It's the sub-woofer that rattles your house windows from boom-boxes two blocks away.

Surround Sound

When sound is split into a number of frequencies [high and low sounds], and played through multiple speakers. Surround sound aims to simulate the natural sounds from many locations.