Recording - October 2010 - 6

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Sound and polarity Hi. I love your magazine, and I find it extremely informative. However I’m relatively new to the magazine— and recording altogether—and I don’t fully understand polarity/phase and phase cancellation. I understand polarity when it comes to electricity, but in terms of sound, it just seems like a sound wave is a sound wave. How does it work? How do I listen for phase cancellation? Is there maybe an article coming up, or one you have online you could direct me to?
Ryan Quinney via email

Ryan, Thanks for writing. We’ve done a bunch of articles on polarity and phase cancellation, at least one or two of which should be available in our website’s online resource library. But while I have your attention... You understand that an electrical signal can be represented as a fluctuation of voltage above and below a “zero” line (the ground reference of the circuit), and that inverting polarity simply makes the positive voltages negative and the negative voltages positive. Remember that while our ears hear sound waves directly, our audio machinery can’t; sound waves have to be turned into electrical representations (and back) by transducers—microphones to turn sound into voltage, speakers to turn voltage into sound. A sound wave is a complex collection of changes in air pressure, where air gets compressed or rarefied; these pressure changes get turned into a fluctuating electrical signal. (Paul Stamler’s article on microphone design in our June 2010 issue explains this in detail.) OK so far? Now, one confusing thing is that many people routinely interchange the terms “polarity inversion” (which is correct) and “phase inversion” (which is not). Let’s talk about what phase really is, first. It’s how far along you are in the cycle of a waveform. The start of the wave is represented as 0 degrees, then you go around a circle (360 degrees) to get to the start of the next wave. 180 degrees is the halfway point, and for many waveforms you can roughly simulate a 180 degree phase shift with a simple polarity inversion. Why would you want to do this? Read on. If you draw a regular waveform on a piece of paper, then make an exact copy of it on another piece of paper, and lay one over the other and hold them up to a bright light, you can “see” the effect of changing phase by sliding one piece of paper back and forth over the other one. Where a peak on one wave overlays a peak on the other, adding them together will make a higher peak; where a peak overlays a valley, they will partly or completely cancel out. If you mix two versions of the same sound source, one time-delayed from the other, that delay causes a phase shift, and the two waves mix together to make a new waveform that can be radically different from the original. It’s like running the wave through a filter with lots of closely spaced peaks and dips, like the

teeth on a comb. This “comb filtering” can be easily heard by running a signal through a flanger. This can really mess with your sound; two mics picking up the same signal at different distances can partly cancel each other, making the sound thin and weak. Now, changing the phase on a waveform just by turning a dial (without distorting the sound) is difficult but not impossible in the analog world; you can do it with the Little Labs IBP or the Radial Engineering Phazer or the SM Pro Audio P-Control. With such a box, you can dial in the phase of one mic signal vs. another so they don’t cancel out in weird ways. It’s a useful thing, but too expensive to build into every channel of a mixer. So what many mixers do instead is to have a polarity inversion switch—an inexpensive approximation of a 180 degree phase shift. Inverting polarity can get you in the ballpark for solving a wide variety of phase problems (most commonly, matching two mics above and below a snare drum or inside and outside a kick drum, since a positive movement of the drumhead for one mic will be a negative one for the other), but it’s not perfect. Many people now do this alignment by applying small delays to individual tracks in their recording software. I hope this helps clarify matters a bit. Thanks for your question, and thanks for the kind words. Good luck!—MM Ubuntu saves the day Just after we published our article on the Ubuntu Linux operating system and its application to music computers (August 2010), we got this letter from frequent contributor Paul Vnuk Jr.: Hi Mike! As you know, I am one of those cross-platform Mac/PC guys, but a few days ago it was Ubuntu to the rescue... I was downloading an album cover for a rare 1950s jazz album in my PC’s MP3 collection and it had a malicious Trojan Horse virus attached to it. Of course I had been careless and had not updated my virus database in about two weeks; this Trojan bluescreened my PC and successive reboots and hack troubleshooting resulted in an unmountable Windows boot disk. Of course it had also been about 3 months since I backed up any data in this computer... all of my art photos and recent studio invoices and such might be gone forever. I was going to simply pull the hard drive and use an adaptor to turn the drive into an external drive to rescue my data, but the drive was missing the necessary jumper to turn it into a slave drive. Just when I thought all was lost, an internet article mentioned a way of rescuing the data by creating an Ubuntu boot disk. So that’s what I did, and the process was really quick, painless, and best of all, worked flawlessly! First I used my Mac to download the Ubuntu system files and burn them to a CD as an ISO image. Next I put this CD in my wounded PC and booted into Ubuntu from it. In the Ubuntu environment I was able to access my old drive and rescue every one of my files and transfer them to a removable thumb drive. Once my data was safe I was able to use my backup software and restore my PC to a virusfree, fresh install state, and then reload all of my lost files. The moral of this tale? Update your virus software often, be leery of hidden Trojans even when downloading seemingly innocuous files, back up even more often than you do now, and when all else fails, try Ubuntu to rescue your rear end! Thanks, Ubuntu.—PV

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Recording - October 2010

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