Flip the phase! ...even though that's impossible
(originally posted September 1, 2012)
I remember being very confused with phase and polarity for quite awhile. And why was I confused? Being told to flip the "phase" so many times you can't help but be confused. Seeing a button labeled "phase" or "phase reverse" on equipment and in equipment manuals only kept me confused. I have learned to ignore other engineers when they use this phrase because I know they really mean to flip the polarity. This confusion still exist in the audio world and I don't think it will ever cease to exist.
So, whats the difference between phase and polarity? I'm glad you asked.
Phase is a time relationship between two sources. If phase is time there is no way you can flip it because you can't change the time of something - you can delay a signal but that is a different subject. If you could flip time I would like to flip time and go back to the large bowl of ice cream I just ate.
Polarity is taking the positive and negative electrical signal from the source and flipping them around. You do this to help compensate for time differences between signals. If a signal is 180° out of phase with another signal then the two signals will cancel each other out. If you flip the polarity of the second signal when they are combined the cancellation no longer occurs.
Here is a snapshot taken from the workbook "Recording 101"
courtesy Shon Moore, LLC
The polarity was flipped on one of the signals which you can see since the two signals are a mirror image of each other. There is a 180° difference between the signals.
Here is another signal
In the magnified area you can clearly see that the two signals have the same issue as the two signals in the previous picture. The signals are a mirror image of each other. But unlike the previous picture this one does not have the polarity flipped on ether channel, it is 180° out of phase. You can tell it is 180° out of phase when you look at the beginning of the two signals.
The signal on the bottom starts later than the one above it, making it out of time (phase) with the top signal. To relate this to recording an instrument, if two microphones are setup to capture a professional Kazoo player and one of the microphones is setup at a further distance away from the musician the signal will arrive at the second microphone later than the first one making it out of phase when you combine the two signals to playback.
So what do I say when someone yells out "Flip the phase!"? In my company they are escorted out immediately but when working for someone else the correct answer is "OK". You know they mean flip the polarity so just hit the button and continue what ever you're doing. Hopefully you can explain it to them one day.