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 a stereo effect is fraught with peril.
This is one of the more common examples, but it is not the only way in which a stereo effect can be non-mono-compatible. Other examples are stereo signals that differ by only a short relative delay or phase shift.
While this problem is not common, it is insidious — only appearing when it affects another part of the signal chain.
What Is Mono-Compatibility?
Mono-compatibility means that you should be able to sum the two channels of a stereo signal to mono without unintended effects. The resulting mono signal should sound like the original stereo signal, except without stereo imaging. None of the original stereo signal should be cancelled out, and no weird "phasing" or "flanging" (comb filtering) should be heard.
Why It's Necessary
You may be thinking "This doesn't apply to me, because I never sum my stereo signal to mono." But let me ask you the following:
- Will you ever be recording your stereo signal, possibly for distribution?
- Will you ever be listening to your stereo signal through something other than headphones?
If you answered yes to one or both of these questions, then your stereo signal needs to be mono-compatible. To understand why, let's review a couple playback scenarios.
Scenario 1 — Mono Playback Systems
After you record and distribute your music, you have no control over the playback system. In fact, many playback systems are mono:
- night clubs, bars and restaurants
- portable systems for weddings, parties, etc.
- small portable Bluetooth speakers
Many of these systems use a sum rather than only the right or left channel. Picking one channel is risky, because you can never be sure what may be missing. On the other hand, professional recording engineers know to produce stereo signals that are mono-compatible. So, it is generally safer to sum stereo to mono for these systems, rather than picking one channel.
Scenario 2 — Far-Field / Co-Located Speakers
Stereo systems can sound mono when the listener is sufficiently far away from the speakers relative to the distance between the speakers. The farther the listener is from the speakers, the more they will sum acoustically; until at some point, they sound like a mono source.
Many small, portable playback systems co-locate the stereo speakers, so that they are right next to each other. Often times, musicians with a live stereo rig will place speakers next to each other. Co-located speakers have a difficult time producing a good stereo image and, unless you're right on top of them, will exhibit acoustic summing.
Any time you have the potential for summing, whether electronic or acoustic, your system needs to be mono-compatible to ensure that no unintended effects are produced.
Now that we've established the importance of mono-compatible stereo, how can you ensure that your stereo rig produces a mono-compatible signal? Simply sum the stereo output. It should sound just like the stereo signal, but the stereo image will be collapsed to mono. Summed stereo effects should not sound weird or cancel out.
If an effect is missing or sounds "off," the problem may not lie with that particular effect. Rather, it may be one of the previous effects in the chain that is wreaking havoc. You'll have to go through each stereo effect, enabling one at a time, to find the one that does not properly sum to mono.
Once you find the effect or device that is not mono-compatible, you really only have two options:
- Don't use it in stereo. If possible, move it before the stereo split in your chain. Or, you may be able to run it in a mono sidechain (in parallel) that sums back into your stereo signal. This may require additional hardware.
- Remove it from your signal altogether.
You may be tempted to leave this device in your chain and deal with the consequences. If you must, you have been warned.
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.