The Basics: Car Signal Processors
Signal processors, or digital signal processors (DSPs), are the final stage in fine-tuning the sound in your automobile. The term covers a variety of components--by definition, digitally affecting the sound to alter it from the input signal. Signal processors can refer to an equalizer/crossover unit, a surround sound decoder, speaker steering (allowing you to turn on and/or off specific speakers) and delays for the speakers, sometimes referred to as digital time alignment.
An add-on equalizer might be just what you need to finally make your car audio system live up to its full potential. An equalizer lets you finely adjust the sound that gets fed to your speakers, allowing you to compensate for deficiencies in the frequency response of a speaker or an audible shortcoming in a component. Equalization also lets you attempt to control a car interior's typically troublesome acoustics. In very basic terms, equalization boosts or cuts certain ranges of frequencies from an audio signal--a very sophisticated tone control, with more than just bass and treble adjustments.
Stand-alone crossover units remain in the realm of high-end audio gear. While a few amplifiers do some sort of crossover-type frequency division, there are times when an audiophile wants and needs an external crossover unit. It's easy to see why--an external crossover unit usually offers the ability to fine-tune adjustments in a way no amplifier or head unit can.
How to Shop
Your reasons for purchasing a signal processor really determine what features you should look for. As most cars already have the amplification needed for four channels of speakers, some signal processors include amplification for the additional center channel and subwoofer that are required to reproduce 5.1 Dolby Digital and DTS surround sound. In addition to these set formats, some signal processors will add an artificial surround sound field to a regular stereo signal. These make the music sound like it's being played in a stadium, jazz club, or whatever the engineers decide to call their programs. These come as prenamed presets, however, you might want to take control and program your own personal sound fields and store them in user presets. Some processors have a wonderfully useful feature that compresses the audio signal. By reducing the overall dynamic range, the softer passages of music aren't lost under engine noise, and the loud peaks won't blow out your speakers.
A feature that is found on some higher-end processors is an independent equalizer for the left and right speakers. This feature helps to compensate for the sometimes non-ideal placement of speakers that are found in many vehicles. Digital time alignment also compensates for the placement of speakers, since you will rarely find yourself sitting in the perfect sweet spot evenly placed between all four speakers. This works more effectively than just using volume to control the balance of the speakers. Time alignment uses digital delays; psychoacoustics to fool the ear into thinking the speakers are centered on the drivers seat.
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