In a perfect world, any speaker could take any audio signal and do a flawless job of reproducing every frequency range. But we don't live in that alternate universe: our speakers have to abide by the laws of physics, and so we have subwoofers, woofers and tweeters. Getting the right frequency range to the right speaker is the job of a crossover. Chances are that's not news to you, and the next question you might ask is, "Why use active crossover signal processors when I can get a speaker cabinet with a passive crossover built in?"
That's a great question, and the answer is a simple one: an active crossover adds a lot more flexibility. Here's why... in a typical setup, you've got your amplifier connected to your speaker, and the internal passive crossover handles the incoming frequencies within the cabinet. Simple enough, but there's one catch: all of those frequencies are passing through the same amp. Even if you're connecting different speakers to different outputs on the amplifier, that's still going to be the case.
What if you want to use different amps for different frequencies? This is a crossover signal processor's time to shine. Instead of connecting to an amp and then to the speaker, you can use one of these active crossovers to split the frequencies before the signal gets to the cabinet. Then, each frequency range can be run through its own amp before being sent directly to the woofer, tweeter or sub... or even mixed back together to be handled by the speaker's passive crossover, depending on the speaker setup you use.
The most common processor spec in this section supports 2 stereo channels or 3 mono channels; a few examples of those would be the Peavey PV 23XO and BBE Max-X3. If you need an extra channel, though, take a look at the DBX 234xs and 234s Stereo 2/3 Way Mono 4-Way Crossovers. Maybe you're looking for something really advanced? The Rane AC 23S might be for you. It uses a Linkwitz-Riley filter to align the signal phases for sound output that's distortion-free and amazingly clear.
If you've been wondering what you can do to customize and add extra versatility to your live sound setup, a crossover signal processor could be the answer. When you can amplify different frequencies separately, you open up a huge amount of potential. No matter your personal setup style, they're definitely worth trying out.