Checking Phase (Polarity) Integrity
By Anderton |
by Craig Anderton
Digital audio workstation software, or two-track audio editing programs (like Magix Sound Forge or Steinberg Wavelab), can serve as test gear. Let's look at how you can make sure your studio hardware connections, and even samples, are properly in-phase.
Before going any further, note that in most cases we’re really talking about a change in signal polarity, which means that the entire signal is phase-flipped, regardless of frequency. Phase shifting can be a frequency-dependent phenomenon. Although most musicians and engineers understand what the term “phase reversal” means, “polarity reversal” is technically a more accurate term.
Flipping a signal’s phase may or may not mean too much by itself; that’s a matter of debate. Some people believe you can definitely hear a difference with instruments like drums. For example, with a real kick drum, the first rush of air pushes out at you. If this signal goes through a system that doesn’t change phase, the speaker will push air out to re-create the sound of the kick. But if the signal flips phase, then the speaker will suck in to move the required amount of air. The result will still sound like a kick drum, but some people say they hear a subtle difference.
Phase problems are not uncommon in the studio. Balanced cables can be miswired, some vintage gear used pin 3 hot instead of pin 2, some guitar effects play fast and loose with phase anyway, and even new gear can have a design problem crop up from time to time that flips phase.
In any case, there’s no debate that mixing an out-of-phase signal with an in-phase version of the same signal can cause an obvious weakening and “thinness.” This occurs a lot when using two microphones, because depending on their spacing they can pick up a signal’s waveform at different points, thereby creating a phase difference. Problems can also occur with parallel effects. For example, if an echo signal is out-of-phase and mixed with an in-phase dry signal, the echoed signal will tend to cancel the dry signal to some degree, resulting in a thinner sound.
As to detecting an out-of-phase condition, phase meters are expensive—but DAWs and digital audio editors make a pretty good substitute. You can determine not only whether a device’s output is in-phase with its input, but in some cases, even tell whether a signal’s phase was reversed somewhere along the line.
Fig. 1 shows how to hook up your “test setup” for mono signals. Split the input signal and send one split to the input of the device being tested, and the other to the software's left channel. This is your reference. Then feed the output signal of the device (or chain of devices) being tested to the software’s right channel. You can also split this off to an amplifier if you want to hear what’s going on. For stereo, assuming you're working with a DAW, the split would feed two different tracks.
Fig. 1: Phase tester setup for mono signal sources.
One application is testing a mixer to make sure all outputs are in-phase. Patch a sound source with an asymmetrical waveform into the input, then test the output at a variety of points: master out, submaster out, monitor out, sends out, direct out, etc. Another application is verifying phase integrity of older effects and guitar stomp boxes. Fig. 2 uses PreSonus Studio One to show a comparison of the input and output for a guitar stomp box delay; the lower waveform shows less high-frequency response, but the two waves are in phase. If they were out-of-phase waveforms, the peaks and valleys would have the same shape, but go in reverse directions—in other words, if a waveform rises at the input, it falls by an equal and opposite amount at the output.
Fig. 2: Comparing the phase relationship between an effect input and output.
Vintage guitar effects are notorious for phase problems, and are well worth testing. It’s also a good idea to test the entire input-to-speaker chain to make sure nothing’s amiss. In particular, make sure there isn’t a phase difference between the left and right channels, as that could have disastrous results on a mix.
I reversed some instrument samples to hear if they sounded any different in-phase or out-of-phase. Interestingly, this did seem to make a difference with some sounds, but I didn’t do anything rigorous like conduct a scientific double-blind test. Check it out and judge for yourself.
Fig. 3 shows an example of absolute and flipped phase with kick drum. The top track shows a kick one-shot from a sample library; note how it starts by going negative. The bottom track shows a different kick drum sample from a different library. Note how it starts by going positive, which presumably reproduces the initial kick hit pushing air out and going positive, as opposed to sucking air in and going negative.
Fig. 3: Two different kick samples showing absolute phase: out of phase (upper) and in phase (lower).
It seems that you can identify the absolute phase of most percussive sounds similarly—look for an upward slope at the beginning of the signal. However, I must stress that these are just a few examples, and some signals do start off naturally with negative transients.
FIXING PHASE PROBLEMS
If you find that phase reversal does matter, the same program that identified the problem can also provide the solution. Just about all modern audio software lets you select an audio region and reverse the polarity, so you can indeed "fix it in the mix" - as long as you know that the problem exists.
So boot up your computer one of these days, and take the time required to check out the phase integrity of your system. You never know what evil lurks in the wiring of cables. -HC-