Digital sound processors are stereo head units that digitally control all the functions of the vehicle’s audio system. If you want to adjust the frequency response of a particular audio track, you’ll use a digital sound processor. If you want specific frequencies sent to your amplifiers, you’ll use a sound processor. You’ll use the processor to switch between input systems or units. Some digital sound processors have a frequency graphing display so you can see the changes you’re making as you go along. A digital sound processor can also be used to alter the type of listening environment as perceived by the audio system. You can make it sound like a concert hall, a stadium or a cozy club.
For more info, see 3 Facts about Car Speakers