What Is The difference Between Polarity And Phase? - Julian Offers A Pedant’s Perspective
Lots of people, myself included, silently correct anyone who refers to “flipping the phase”, “pressing the phase button” or similar. If that is you then you are also a pedant and welcome to my world. As well as being a pedant I also understand that many of the people saying it “wrong” understand the difference between polarity and phase and are using it in the vernacular. It’s fine to use it that way as long as you are aware of the difference. But what if you don’t? What harm does it do to use “phase” wrong?
The issue is that polarity is simple to understand, it’s very tidy and gives predictable results. Phase isn’t discussed as often, is harder to understand and people might conclude that because they understand polarity, they understand phase - no they don’t.
Polarity inversion is the reversal of the positive and negative voltages in an electrical signal. If you swap the positive and negative connections then the voltages will be reversed and everything which was on the positive side of zero volts becomes negative and vice versa. The waveform is turned upside down to become the exact opposite of its un-inverted form. This is incredibly useful for all sorts of reasons.
Phase describes the relationship between two periodic alternating signals, it’s related to delay but it’s not quite the same. It describes the offset between the two waveforms in terms of how far through its cycle from positive to negative and back again it is. Crucially this is frequency dependent so as frequencies vary, so do phase relationships. Phase is measured in degrees with a whole waveform cycle being made up of 360 degrees. This offers a way to talk about these relationships without reference to frequency, delay or anything else.
It is easiest to understand these relationships when talking about multiple versions of the same signal but phase relationships can affect signals which differ, things just get messier the more different the signals are so for clarity examples tend to involve identical sine waves. If you delay one signal against the other you affect their mutual phase relationship. If you delay identical signals by exactly half a waveform then a 180 degree phase relationship exists between them and they will cancel when summed together. This is the same outcome as inverting the polarity but the 180 degree phase relationship has been caused by a different mechanism - Changing the time relationship between the two signals rather than the voltage relationship.
Neither changing the time or voltage relationship manipulates the phase directly. This can be achieved using phase rotators. If you’ve ever wondered what an all pass filter does, it rotates the phase of a signal. Phase rotation goes beyond the scope of this article but I cover it in some detail in this video about Sound Radix Pi, which is a dynamic phase rotator.
Discussions of phase are often confused by people talking about the effect polarity or delay have on the phase relationship of a signal as if changing polarity or mutual delay is the same thing as phase, it’s not but they both have an effect on phase.
In this short free video tutorial, we show you how to check for phase, or should that be polarity, issues in a multi-mic recording and then how to fix it using the Avid Trim plug-in.
There will be three kinds of people who watch James’ video and read his explanation of the issues with this kick drum:
People to whom discussions of phase are new and who find it really useful - You are the people for whom this tutorial was made.
People who already knew about this issue. Members of this group will either be totally unconcerned about, nearly everyone’s, use of the word “phase” to describe polarity or will to some degree mind the two being used interchangeably.
Then there are people like Julian…
Identify The Problem
The first stage to fixing a phase issue is knowing there is one in the first place. James plays back a short clip of 3 kick drum mics used as part of a full drum kit session. The Kick Inside and Kick Outside mics sound fine but when the Sub Kick mic is added the tone and punch of the drum falls apart.
How To Fix Phase
Any plug-in’s phase reverse switch (or should that be really called a polarity switch) can be used to flip or invert the phase, James puts an instance of the Avid Trim plug-in, which is free with Pro Tools, on each of the three kick drum tracks. He puts the plug-in on all three tracks as small delays induced by plug-in latency can really affect the phase relationships between channels. By adding the plug-in to all the channels being tested the latency will be the same for all of them. James then flips the phase on the sub kick track and as if by magic, the life and bottom end comes back to the kick drum recording.
Get It Right On The Way In
It is so important to check the phase or polarity of your mics at the tracking or recording stage but sometimes these problems do slip through the net. This is a quick and easy way to fix problems and restore the true sound your recordings. James demonstrates this fix using Pro Tools but all the major DAW software has free plug-ins with a phase reverse or invert switch and the principles shown here will work in any DAW package or console with a phase invert button.
Something For Everyone?
There you have it. Hopefully something for everyone on the misunderstood issue of phase and polarity with audio signals.