The relevance of mono compatibility music in mixing, mastering, and playback is a topic that regularly crops up. It’s also inherently part of what we check for at the mastering stage.
As opposed to a single channel mono (monophonic) recording, we can define mono compatibility in this context: two channel audio, recorded in or mixed to stereo, in which its elements, when summed to mono (the same signal in both left and right channels) cause no significant imbalance to the tonality or musicality of the mix.
Why mono compatibility matters
Ultimately it’s simply good engineering practice to ensure mono compatibility. A few reasons, in no particular order:
- 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.
- Better playback compatibility and likelihood of a cleaner cut for vinyl records.
- Lossy encoding, such as MP3, AAC, Ogg Vorbis as used by streaming services, handles audio better when more of the signal is common to both channels. As a result, this means less sonic artifacts than largely uncorrelated “out of phase” signal.
- Music often sounds more “engaging” when its sense of depth is retained. This usually comes from a strong front and center sonic image, with a sense of foreground/middleground/background. This also helps create contrast and separation against the stereo width elements.
- There are no guarantees on how a venue’s sound system may be installed, such as how far apart ceiling speakers may be, or is it one channel stereo? The more common the left and right channels are, the less loss is heard in such situations.
- Ever seen people sharing ear buds, or listened to a streaming service or radio station via a smartphone’s speaker?
Of course in a true stereo recording, mono compatible means phase-coherent. In everyday terms this simply means a more realistic sound, truer to the source, with a strong phantom center image. Our hearing is very sensitive to phase – tiny differences in time between sound sources arriving at each ear. As such, it’s fundamentally self-preservation – how we sense the direction of sounds.
What to do
The main thing is that you check (listen) for mono compatibility and correct it where needed. With experience, you’ll hear it long before you see it on a phase meter. Any such errors really must be addressed at the recording and mixing stages, via mic placement or careful time-alignment of tracks in the multi-track mix environment.
Music mixed to work well in mono will likely sound great in stereo, while the reverse is not always true. If it sounds good to you in mono, with no parts or tonality significantly losing energy, great! It can be subjective.
A reality check
As for professional mastering typically involving “stereo widening”, I’ve used that perhaps thrice in as many years – sometimes only to bring overly wide sounding mixes inwards a little to better match other tracks. And yes, I said thrice.