27 Feb Mono Compatibility and Stereo Phase Manipulation
Mixing and Mastering engineers can face some unpleasant “surprises” when switching from stereo to mono if their audio in works is harboring any phase or polarity problems between its left and right channels. A common source of these problems is the use of spaced microphone configurations during the recording stage -drums, room ambience, vocal/instrumental ensembles, and piano are often captured in this way, for instance-. If any part of the sound source is not the same distance from both microphones, its sound will arrive at one mic earlier than at the other. In stereo, this time-delay between the two channels actually reinforces the stereo positioning of the instrument for the listener, but in mono the same time delay will cause comb filtering, a noticeable frequency-cancellation effect.
In addition to real stereo recordings, simulated stereo reverb and echo effects can also suffer from comb filtering in mono, as by their basic nature, they combine delayed versions of the same basic sound. If any signal appears in both stereo channels, but one of them has an inverted waveform polarity, there will be a mismatch between the balance of the stereo and mono renditions of the same mix.
Phase analysis is an essential component of the mixing and mastering processing. During the mixing stage, the engineer in charge must check every stereo track from at least two perspectives: simply sound-wise (to make decisions about the degree of phase cancellation to avoid or allow to get the best timbric quality for that track) and summing stereo contents into mono (both for the track as a single element and within the whole mix) to check its audibility level in context. When the mix arrives to the mastering stage, it is a must to rely in a properly calibrated monitor system to be able to hear what’s really happening in the mix in the phase regard. Then, the mastering engineer can take advantage of any of the modern resources to check and correct phase issues; these include phase-manipulation tools as multiband controllers of the stereo image and frequency-based “stereo spreading” hardware or software, along with sophisticated graphic polarity meters that include high-precision vectorscopes and more ‘expressive’ analyzers as those based in Lissajous curves, among others.