Originally Posted by krabapple
Well of course they are. Human hearing is perfect, didn't you know?
Way back when we built this device:
The most famous of the alternatives was the "Grunge Box" which added selectable amounts of even order distortion. One interesting thing is that this particular circuit adds the same amount of distortion without regard for signal level which should make it easier to pick out the distortion. If memory serves, the two lowest settings were impossible to hear with music, but could in some cases be heard with pure tones.
Of course on a THD analyzer, even the far less sophisticated devices of the day, the distortion looked huge.