Mono compatibility and the correlation meter
A stereo recording is mono compatible if it still sounds right when the two channels are added together. That matters more than it seems: car radios in poor reception, phone speakers, club systems and public address systems all collapse stereo to mono at some point.
Why a stereo signal can fail in mono
Add two channels and you get the sum of the two waveforms. Wherever the two signals carry the same content with opposite polarity, they cancel, fully if the levels are equal and partly otherwise.
- Time differences (spaced microphone pairs) shift one copy against the other, which gives comb filtering when summed. Notches appear at the frequencies where the delay is half a period, see Comb filtering.
- Out-of-phase content (a reversed polarity cable, a wide-stereo effect that inverts one side, a Blumlein pair with sound from the sides) cancels in mono.
- Very wide stereo synthesis can leave large parts of the signal in the difference channel only, which disappears when summing.
The correlation meter
A correlation meter shows the relationship between left and right on a scale from −1 to +1:
- +1: identical signals (mono).
- 0: unrelated, no net relationship.
- −1: same waveform, inverted. The mono sum cancels completely only if the levels are also equal: with R = −0.5 · L the reading is still −1, but the mono sum is 0.5 · L.
Occasional dips towards zero are normal in stereo music. Sustained negative readings warn of mono problems or a polarity error.
A goniometer (vector scope) draws the left and right signals against each other. Mono appears as a vertical line, a wide stereo signal as a wide cloud, and an out-of-phase signal as a horizontal line.
Checking by ear
Switch to mono regularly. Listen for:
- loss of level in whole instruments or reverberation,
- hollow or phasey tone,
- instruments that change level relative to others.
Techniques that stay safe
| Technique | Mono behaviour |
|---|---|
| Pan pots | perfect (the level of a centre source rises, see Pan laws) |
| Coincident pair (XY, M/S) | very good |
| Near-coincident (ORTF) | good, small differences |
| Wide spaced pair (AB) | weaker, comb filtering |
| Stereo-widening effects | varies, check them |
Why the sides matter
In mid/side terms, mono is the mid channel. The side channel is the part that disappears. A side as loud as the mid is not a problem in itself: a source panned hard left has equal mid and side levels and still comes through in mono, 6 dB lower than the same signal sent to both channels. The warning sign is a side that is louder than the mid, which means anti-phase content that does cancel. Keep an eye on the ratio, see Mid/side stereo.
Draft: written by an AI assistant, awaiting technical review.
Found a mistake or have an idea for this article?
Suggest a change