Still Vibrations wrote:May question is, what is the correct way to convert stereo to mono?
Stereo has two audio channels of related-but-different information, whereas mono is a single channel of audio.
The standard way of reducing stereo to mono is to sum the left and right channels, so all of the original information in both source channels is carried into the mono signal.
If the two source channels are identical (coherent) -- eg, a solo vocal panned centrally -- the mono sum will be 6dB louder than either source channel on its own, and some stereo-to-mono converters always attenuate the mono output by 6dB to prevent this level rise causing overloads.
If the two source channels are similar but different (incoherent) -- eg. a normal stereo mix with panned instruments and reverb, etc -- the mono sum will typically be around 3dB louder than either source channel on its own... and again, some stereo-to-mono converters attenuate the output by 3dB to prevent overloads...
The potential problem, though, as you mentioned above, is that if there is information in one source channel which happens to have the opposite polarity of that in the other channel, they will completely cancel when summed to mono, and so that information -- which would be audible when listening in stereo -- will disappear from the mono summation.
While total polarity inversions are unlikely (although it can and does happen), this problem does rear its head often when there are large phase shifts (close to 180 degrees) between information carried in the two channels at some specific frequencies, and that is very common with some stereo reverbs and some stereo chorus type effects, as well as some stereo widening effects.
What tends to happen in these situations is that the reverb will appear to be much drier in the mono sum compared to the stereo version, or it may sound coloured or 'nasal'. The latter may also apply to stereo chorus type effects too. The same can also happen with some stereo keyboard instruments -- especially sampled pianos -- when mono-ing the output of a stereo keyboard. (That's why most stereo keyboards have separate mono piano sound options).
If you don't like what happens when you sum a stereo source into mono there are two alternative ways of trying to improve the situation.
The first and easiest is simply to only use one channel of the stereo source. This usually works fine stereo reverbs and chorus effects processors where both channels carry the same information but with complex and varying phase relationships. It won't work with things like stereo piano outputs where the two channels replicate the effect of separate spaced mics so that one channel has more low end and the other more high end, etc. Taking just one channel here will give a radically different tonal balance...
The other option is to use an all-pass phase shifter in one channel before the mono sum to introduce a fixed phase shift at all frequencies -- typically 45 or 90 degrees. This usually has the effect of reducing (but not completely removing) any phase cancellations that render a normal stereo-to-mono sum unacceptable.
In your case where you just want a mono output from your effects box, I'd just hook up one output channel!
Hope that helps.