So what does it all mean? How can you use this for fun and profit? I will talk more about the wonderful recorded character of A+B and A-B some other time. Right now, I want to double back to the concern about mono. If you’ve read the above carefully, you’ve probably figured out that like most things I write about, there is no single, right way to do things. No really awesome stereo signal is ever going to be absolutely perfectly compatible with mono. Summing it to mono changes it, usually for the worse (“worse” meaning that it has less entertainment value, impact and/or interest -- the philosophical issue of which is more “accurate” is beside the point here). So, you are faced with once again having to make a decision based on your own opinion (gulp!): Do you think the mono signal is of acceptable audio quality for your production purposes?

Here are some badnesses you can look for (I call them Mono Nonos -- the term has kind of a ring to it, don’t ya think?):

Comb-filtering: arising as a result of time differences between A and B, comb-filtering can dramatically affect timbre in mono. As I’ve mentioned in other articles, the time differences that are localization cues in stereo become timbre cues when summed to mono, thanks to the way our hearing works. How much is too much? When you hear sustained howling/whistling type tones or sustained ringing or a sound quality like you are listening through a vacuum cleaner hose, this is definitely too much comb-filtering. I also think there is too much comb-filtering when the summed mono version of a good sounding stereo signal sounds dirty, grainy, colored, or gritty, if you know what I mean.

Change in level: If the A-B component is very strong in your stereo signal, there will be a significant loss in level when you sum to mono. Sometimes, you will lose just the bass (long wavelengths). I figure if the level changes an easily noticeable amount (say, more than one or two dB), or you notice the bass disappearing, get out the REJECT stamp and head back to the panpots and faders.

Change in ambience: when you sum to mono the stereo image is going to collapse into the phantom image. It shouldn’t just go away. If you hear big changes in reverberance, or spaciousness, then your reverb wash has too much A-B in it and is being largely canceled when you sum to mono, drying out the mix. Once more, back to the mixing board.

One caution here: it’s really tempting to sit there and toggle back and forth between mono and stereo, really getting grooved in to the differences between the two signals. You can end up, however, psyching yourself into a mindset where no mono signal is ever really acceptable unless it doesn’t change at all from the stereo. One major engineer I know of has done exactly that -- he’s gotten so whacked out and obsessed that mono and stereo must sound exactly the same that he uses coincident omni microphones (omni mics right next to each other) as his preferred stereo mic placement. This placement results in what might be thought of as really expensive mono, which of course sounds exactly the same as regular mono except that it is 6 dB louder. I know. I went home and tried it after I listened to said major engineer describe how he did it! The point is, first you need to check back and forth between stereo and mono. If they don’t sound too different, then listen to the mono version of your mix all the way through and see if it retains its musical values and impact. Don’t fixate on how much worse it is than your stereo version. Remember, your listeners won’t be listening while switching back and forth between stereo and mono. You just don’t have to worry about that level of nit-pickery!

So, the big three Mono Nonos are change in timbre, change in level, and change in ambience. To reduce these badnesses, you may have to back off on some of your favorite killer stereo FX. Your ear and your judgment have to guide you here. Like I said at the top (well, if I didn’t, I should have), life in engineering is a series of tradeoffs, and only you can decide how important one particular value is versus another. In any case, mono compatibility means a certain limit in the amount of difference between the stereo version and its mono summation. It’s bad enough that you should have to give up left and right and all that really cool spaciousness, you shouldn’t make the middle sound crummy into the bargain!

Dave Moulton is the A-B signal in the MP&E department at Berklee College of Music in Boston. Alex Case is definitely an A student and Peter Alhadeff’s doctor has assured us that he doesn’t have mono.