Before we jump into DSP (Digital Signal Processors) let's start by understanding their forebearers: ASP (Analog Signal Processors).
The simplest ASP you're likely familiar with is a tone control, like a bass and treble set. Long ago, all stereo amplifiers and preamplifiers had tone controls. Tone controls were typically used to boost either the bass or the treble–or both–for recordings and loudspeakers. As the hi-fi industry grew, simple tone controls turned into complex graphic equalizers. Here's a picture of a popular Soundcraftsmen from years ago.
These complex controls offered greater flexibility
tuning the system, as well as the room, and had become part of the natural landscape of consumer audio–though not too many Audiophiles embraced them because of damage to the sound. Running through groups of op amps and filters isn't exactly the purist's path. Add in massive phase shift when the tone controls are engaged, and what you gained in smooth amplitude was at the expense of good sound.
However, Audiophiles did embrace a graphic equalizer at one time, the much coveted Cello Audio Palette. Here's a picture of it.
Envisioned by Mark Levinson (the man) and designed by Dick Burwen (the engineer) the Audio Palette was blessed by the Audiophile community. It was functionally the same product as the Soundcraftsmen, but sounded much better because of the skill of the designer.
