Mono compatibility in music: Does it matter?

It’s a topic that has come up a few times lately, so here’s my take on it…

First, let’s try defining mono compatibility in this context: a two channel audio signal, recorded or mixed to stereo, in which its elements remain audible when summed to mono (same signal in both left and right channels), causing no significant imbalance to the mix or cancelling to its parts or its tonality.

Despite mono compatibility arguably being less important than it used to be, it does still matter. Ultimately it’s simply good engineering practice. A few reasons, in no particular order:

1. 1. FM radio is broadcast via sum/difference signals as opposed to left/right. In situations of weak radio reception the sum (mono) is all you’ll get, as it’s the stronger signal of the two.
2. 2. Better playback compatibility and likelihood of a cleaner cut for vinyl.
3. 3. In general, lossy encoders will handle the signal better (as more signal is common to both channels) with less sonic artefacts than a largely uncorrelated “out of phase” signal. Try bouncing out a mono mix, convert it to, say, 256kb/s mp3 (not that we’d ever condone doing such a thing). Then go back to that mix, polarity invert one channel and convert that to the same bit rate (256kb/s). Compare the two results and the amount of masked lossy artefacts in each! (Leave a comment below).
4. 4. Often, music simply sounds most “engaging” when its sense of depth is retained. This usually comes from a strong front and centre sonic image, with a sense of foreground/middle ground/background, which also helps create contrast and separation with the right amount of stereo width elements.

Of course, in a true stereo recording, mono compatible means phase-coherent, which in everyday terms simply means a more realistic sound – truer to the source.

1. 5. There are no guarantees on how a permanently installed sound system in a venue, etc, may be set up, (e.g.: how far apart ceiling speakers may be). The more common the left and right channels are, the less loss there will be in such situations.
2. 6. Ever seen kids share ear buds, or listened to a streaming service or radio station via a smart phone speaker? (I know… but it’s happening).

If your music is mixed to sound great in mono it will likely sound great in stereo, whereas the reverse is less often true. If it sounds good to you in mono, with no parts or tonality significantly cancelling/disappearing then it should be fine, but any such errors really must be addressed at the recording and mixing stages. The main thing is that you’ve checked for it and corrected it where needed – it’s inherently part of what we check for in mastering.

As for mastering with stereo width processing? Honestly, I’d have to say I’ve used that maybe twice in as many years – and for the sake of bringing overly wide sounding mixes inwards a little to better match other tracks!

What are your experiences with mixing music for mono?

- Adam Dempsey