This is the work of Light, and was posted by him:
PHASE
First of all, you are confused on the difference between phase and polarity. They are two VERY different things, though they are frequently taught in a way as to make the difference less than obvious. 180 degrees out of phase is really, mostly, a theoretical concept. It is only possible at one frequency at a time, and as all the sounds we record (sine waves are boring) contain more than one frequency, the theory is pretty much useless.
At any rate, phase is an issue of time, polarity is an issue of electrical direction. The following post contains a graphic of this.
https://homerecording.com/bbs/showth...?threadid=62002
First, a couple of terms you should know.
The cycle of a wave is the time it takes for a signal to start to repeat. During this time, a typical signal will start at zero, reach its peak positive amplitude, pass through zero, reach its peak negative amplitude, and reach zero again. The number of cycles in a second equals the frequency (pitch) of the signal.
The period of a signal is one half of the cycle. It is defined more precisely as the time it takes a signal to go from zero to peak amplitude (positive or negative) and back to zero. You will note it only goes to one peak, not both.
Phase is the time difference in two waves which are the same. For our purposes, it is the time difference between two electrical representations of the same sound.
The degrees of phase shift is how much of a cycle difference there is between two signals. There are 360 degrees in a cycle, or 180 degrees in a period.
A phase switch is a myth. It is also the name marketing people with no knowledge of audio give to the polarity switch on many pieces of gear.
This is the image within the above post.
https://homerecording.com/bbs/attach...p?postid=495077
In the top pair of (poorly drawn) sine waves, you have exactly one complete cycle of two signals, which have reversed polarity. They are NOT out of phase, as they begin and end at the same point in time. The first and second half of the period is the same in both signals.
In the second pair of sine waves, you have one complete cycle of two signals, 180 degrees out of phase. You will note, that, during the second period of the first wave (which is the first period of the second wave) it looks exactly like the second period of the two signals which have their polarity reversed. However, in the first period of the first signal, the second signal has not yet started.
When using sine waves, the importance of this difference is not obvious, because with two sine waves, reversed polarity and a 180 degree phase shift sound the same. This is not true, however, with more complex wave shapes.
The third pair of signals, which are triangle waves, have their polarity inverted. As you can see, these two signals have a zero sum. You will also note that they are more complex signals. The amplitude is different in each cycle. This is a more natural acting signal (though it is still a manufactured signal).
If you look at the last set of signals, of which there are three, you will see the real problem with using sine waves as a teaching aid for phase/polarity. The first of the three is the same as the pair of signals above. The second is the same signal with its polarity shifted 180 degrees. Because of the complex nature of the amplitude, these signals do not create a zero sum signal.
None the less, if you had a perfect 180- degree phase shift, the polarity switch will "fix" the issue. Any two signals, when summed, will sound (marginally) different if the polarity is reversed on one of them, and it is good to try both ways on every signal whenever you are mixing. The difference will, of course, be most pronounced with two signals which are similar.
The polarity switch will NEVER actually fix a phase issue, because while it will make the peaks of one frequency line up, it will have the opposite effect on another frequency in the same pitch. It may sound better with the switch in a reversed position, but you have only created another issue. (This may not make sense if you do not understand the Harmonic/Overtone Series. Even as long winded as I am, I don't have the energy to explain this right now. You can probably find everything you need to know by doing a web search for "Overtone Series.")
I hope this helps.
(You will note I am not calling them phase problems. Some phase issues will sound good, some bad. You will never know for sure until you have tried it both ways.)
Here are a couple of other posts on related topics.
https://homerecording.com/bbs/showth...?threadid=59380
https://homerecording.com/bbs/showth...?threadid=57636