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You'll find two types of bass restoration processors. The first type works with factory systems, flattening out any preset low-frequency boosts or cuts so you can get the best possible performance from the subs and amps you've added. The second type can be used with any system to enhance bass response by analyzing the music and filling in low bass fundamental tones lost to compression or poor recording.
An analog equalizer uses knobs and sliders to adjust frequencies, while a digital equalizer uses a menu screen that gives you much more adjustment flexibility and precision. Digital equalizers also let you store different EQ settings, very useful if you listen to a wide variety of music or take part in car audio competitions.
Digital Signal Processors
DSP modules are a hybrid product, often combining features of EQs and active crossovers with enhancements such as adjustable listening positions and soundfields. DSP modules can be connected directly to external amplifiers. In some cases, their output can be routed back into a receiver, making an external amp unnecessary. These units often take the form of a "hide-away" module, installed behind the dash or under a seat. Some high-end EQs have built-in digital signal processors.
An EQ booster is different from the normal passive EQ. The EQ booster has a built-in amplifier in addition to the equalizer. If you want to have precise control over your sound but don't want to go all the way and install an amp, an EQ booster is a good compromise. The built-in amp usually has about the same amount of power as a good receiver, but you get the added benefits of an equalizer.
A graphic, or passive, EQ is the standard type of equalizer that usually has 5 to 12 slider controls on the face. These sliders let you boost or cut certain frequencies, giving you a high level of control over the tonal quality of your music. The width of each band is fixed and cannot be adjusted (unlike a Parametric EQ).
Line drivers allow you to boost the voltage of the preamp-level signal from your car stereo to the highest level accepted by your amplifiers. A higher voltage signal travelling the length of your vehicle will pick up less noise from your car's electrical system.
Listening Position Control
Normally, the music is focused in the middle of the car (the "sweet spot"), but you are on the left side of the car. Listening position control allows you to adjust the stereo image from left to right to compensate for this "side-biased" listening.
Parametric equalization or tone controls allow you to set not only the amount in dB by which a certain frequency band is boost or cut, but also the width and/or center frequency of this band. This gives you extremely precise control of the tonal balance in your vehicle. Parametric equalizers are more versatile than graphic equalizers, which have fixed center frequencies and bandwidths.
A spectrum analyzer is a series of vertical light bars that "dance" according to the output of different frequency bands. Digital equalizers include a spectrum analyzer on the display to give you an idea of how your response curve looks. A spectrum analyzer also adds a bit of flair to your system as it moves to the beat of the music.
Subwoofer Level Control
Many equalizers have a built-in volume control for your subwoofer. It boosts or attenuates the signal going to your subwoofer, letting you raise or lower the bass as needed.