Car Stereo Signal Processors
Signal processors are the workhorse for your signal curve
Signal processing covers a wide area especially these days with the digital domain able to perform specific tasks to specific frequencies. An equalizer is a signal processor but so are DSPs, and crossover signal processors can save your car stereo system from eminent self destruction.
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DSP simply stands for Digital Signal Processor; digital signal processing is as old as microprocessors in stereo systems. The conventions in car audio signal processing have long been in favor of cheesy DSP settings.
When used responsibly and subtly, a DSP can bring out favorable sound in any system. If overused, it can morph a good sound system into a muddy mess with overextension of mid-bass and left/right channel delays for "wide-sound' illusions.
What they do to a cheap car audio system is attempt to give the illusion the system provides more power than it does. By over-driving the lowest portions of the frequency spectrum their audio system is mostly capable of reproducing the mid-bass around 200Hz, masking the total lack of anything lower.
These are the harmful implementations of DSP that will bulk up a particular portion of the frequency spectrum to mask a sound system's deficiencies. Settings will have names like Ambient or Wide, or they might have descriptive names like Hall or Concert. Other less harmful DSP settings will toy with the volumes from each speaker like a fader, and basically act like a fader pre-set. For instance, if you had a setting that emphasized sound to the driver rather than passengers, this could be accomplished without cheesy frequency effects. Simply alter volumes from the left/right and front/rear channels to put the driver in the center of the sound field.
More advanced DSP settings like those created by Dolby Labs and DTS include Dolby Pro-Logic II and Neo:6. These settings are finding their way into car audio and have great potential for tweakers. But even these aren't going to make a bad sounding system good or a good sounding system any better, they're just effects.
Crossovers are a form of signal processing that is necessary to save your speakers. A band pass/band stop filter system will prevent highs that cannot be reproduced from reaching subs. They will also prevent low frequencies that can rip apart a tweeter from ever reaching anything but a sub or midrange. This level of signal processing is necessary for more efficient use of your amplifiers. What good is an amp's work in sending signals to a speaker to which it can only do harm and will produce no sound? It's simply wasted energy from the amp. A good set of crossovers are needed to distribute the frequency spectrum correctly across your car stereo system. Failure to distribute frequencies correctly can result in blown speakers and overloaded amps.
EQs look impressive in your car stereo and can help contour the sound of your system. This can help mask weaknesses or bring out strengths in your system or just customize sounds for particular tastes. A car stereo EQ is most often abused and will ruin sound quality. But any EQ with a frequency analyzer in your dash is one of the coolest advances in music since the invention of bongos.
Any car stereo head unit with a bass and treble control has an equalizer. It's just a two band EQ, meaning only two bands or frequency ranges are being controlled with a /- db variable using the control on your stereo. For most musical reproduction you should try to leave your bass/treble controls alone; bypassing them is always a good idea if the option is available. Adding a bit of bass and treble on a car stereo system can bring out deficiencies in the acoustics of the cab portion of the car.
Outside subtle use of the EQ or to bring out specific deficiencies, EQs are mostly gimmicks. Whether they're built into a head unit which digitally manipulates the frequency spectrum or part of an extra component, you should never feel you absolutely have to have one of these. However, bouncing lights on a spectrum analyzer are just so damned cool.