Phase Cancellation - What is it? Detection and avoidance

Phase cancellation occurs when two signals, with differing phase relationships are combined, resulting in a drop in sound level. In practice, this manifests itself as a 'thinning' or comb-filtering effect or in extreme cases, silence - total cancellation of one signal by another.   Signal A Signal A (phase shifted) Sum - Phase Cancellation   Common causes of this effect are multiple mic. setups at recording (eg. near and far mic., top and bottom snare drum mic. etc.) and FX algorithms which change the phase relationships within the audio. Checking for phase cancellation issues is most easily done using audio analysis, using a correlation meter, or a vectorscope. Problems can be easily and immediately identified at source and adjustments made, even if the listening environment is noisy or confused. Summing the signal to mono and A/B listening tests with the stereo signal can also be used to expose problems, although it can be difficult to adjust mic. arrays to best effect without the benefit of an analyser to see the results of small changes. Avoiding phase cancellation is best achieved at the recording stage, aligning sources to minimise the effect. Once a recording is committed, phase alignment tools can be used to adjust individual tracks, although the effectiveness of this depends upon the amount of cross-talk inherent within the recording. Choosing FX with highly mono compatible algorithms also eliminates the need for future corrections, and leaves the stereo effect achieved intact if adjustments are not required later.