There’s been a lot of controversy about effects that use summed stereo inputs vs. independent inputs. Obviously, nobody wants their stereo signal unintentionally collapsed to mono. So, effects with independent stereo inputs are clearly better, right? How has this gone unnoticed for decades and is only now exposed as a cost-cutting measure to cheat unsuspecting consumers?
Except, that’s not what’s happening here — not even close.
Every once in a while, I'll run across an effect that produces a stereo output by inverting the effect on the second channel. While this may produce a "wide" stereo image in headphones or nearfield monitors, the practice of inverting a signal to produce stereo is fraught with peril, because it is not mono-compatible.
One should be able to mono-sum a stereo signal without cancellation, filtering or other effects. A mono-compatible stereo signal can be reproduced on any system, including mono systems and co-located speakers, and retain the quality of the original source. Ensuring mono compatibility is straightforward: simply test each contributing stereo effect by summing it to mono, and eliminate any stereo effect that is not mono-compatible.