The top peaks of the left channel corresponds to bottom on the right, they are now “out of phase”. Of course, this sounds normal in stereo. When the signal reverses polarity, the signs will change so a signal of -5dB will become 5dB and vice versa.
If out of phrase stereo is converted to mono: +2dB (left channel) + (-2dB right channel) ~ 0dB or almost no volume when played in mono because the wave are “cancelled”. This describes the “phase cancellation” problem. In actual scenario, they are not perfectly out of phase, so you can still hear mono signals but they are VERY weak unlike what they should sound in stereo.
Phase cancellation is also a technique used to remove vocals from a song. In that technique, the stereo waveform with vocals consist of two channels (left and right). Then one channel is inverted; after summing up the left and right channel; the center elements are gone due to phase cancellation.
UPDATE: A comment from below emphasizes that phase cancellation requires a delay. The illustration above is a perfect example of polarity inversion.
How to prevent this?
As a quick guide for the impatient especially for those using microphones in their recordings, you can read this very nice tutorial on how to correct or fix out of phase cancellation in recordings.
1.) Do not use long delays between left and right channel. Chances are, these long delays can cause phase related issues. Be careful with the effects you use, make sure they are not introducing phase related problems. This is very common when adding delay on guitars particularly in mixing electric guitars using double tracking.
2.) Check with your audio recording software the phase between the left and right signals. You can zoom in to make sure the left and right channels and matched /in phase.
3.) Check your speaker wiring. Make sure that left and right wires are in-phase with each other. Refer to your studio monitor manual.
4.) Learn when to record in mono or stereo like in vocals.
If you are using a different recording software, there are features that allows you to check the mix in mono. Some audio interface like Focusrite Saffire allows you to check the mix directly in mono by simply pressing the mono button.
5.) Place the microphones properly (read the tutorial on “How to Correct Out of Phase Cancellation in Recordings“ ) and monitor the phase of the resulting wave. You might be tempted to use 2 microphones to record a guitar track simultaneously (one using DI for the left channel and one using microphone to get sound from the guitar amplifier – recorded to right channel).
This might be good for great stereo sound (delays are nice). But be careful, these delays can cause phase mismatch and introduce phase cancellation problems resulting to poor mono mix version of your audio track.
Content last updated on June 17, 2012