An audio system (equalizer, compressor, expander, et al) used to modify some characteristic of the signal passing through it.
"I have a gigantic smile on my face having just listened to the CD you made for me. With the source material being an 8-track, there was no telling how the finished product would turn out. But the sound is clear and sharp, both high end and low end come across beautifully and the stereo separation is perfect. Without the original masters tapes ..."