The two channels of a stereo signal need to be in phase with each other. But are there times when it doesn't matter?
I received a piece of work from an Audio Masterclass student today, where he said he had flipped the phase of one of the channels of the drum overheads, because he preferred the sound that way.
Conventional audio wisdom would say this is wrong, unless one of the microphone cables was incorrectly wired in the first place, which happens more often than you might think. (There's a reason for this that I'll save for a future article.)
Let's take the case of a stereo recording made with coincident microphone technique. In this case, the two channels of the signal are mostly in phase, with the differences in the signal coming from the differences in the left and right sides of the sound in the room.
Flip the phase (or more correctly 'invert') one channel and you'll get a stereo signal that is mostly out of phase. The left channel will be pushing your left ear drum inwards while the right channel is sucking it out. (My old physics teacher told me never to use the word 'suck' in connection with air pressure. But since everyone knows what it means, and trying not to use it takes twenty times as many words, I'll use it for convenience.)
A stereo signal that is out of phase sounds wrong because this is something that never happens in nature and our ears and brains have no way of making sense of it.
But what if the microphones are spaced rather than coincident, as they normally are for drum overheads?
Now the phase relationship between the two channels is much less precise. The sound from the ride cymbal arrives at one mic before the other, as does the sound from the crash cymbal in the reverse direction. And look how many cymbals drummers have these days.
When the phase relationships of the original sounds are confused like this, it doesn't matter so much whether you invert one channel or not. There's no reason not to try it. If you like it, then you won't be doing any harm. But you really have to be sure you like it. It could easily be something you come to regret later on. And when questioned by the mastering engineer, who at this point in the process can't do anything about it, what will your response be? Don't forget that the individual drums will be out of phase in the overheads, although once again spaced miking will cloud the issue, and the individual drums will have their own mics, which will be dominant in the mix.
In summary, phase relationships are not always a cut-and-dried issue. Sometimes there is room for experimentation and sometimes what isn't totally technically correct will actually sound better. But you had better listen very carefully.
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