Los Senderos Studio
Phase
 Issue 63 December 2014 
Three-Phase

Words from the Glossary

Our glossary of recording studio terms continues to grow. It now has over 5,200 entries. As we continue to explore terms from the glossary, this month we take a look at phase.

Phase

This month's terms: phase, polarity, phase cancellation, three-to-one rule, antiphase, phase shift, phase reversal, phase coherence, phase distortion, in phase, out of phase, comb filter, phasey, phasing, flanging.
(Note: Click on the term to view its definition in the glossary.)

Polarity and Phase Shift


Phase is one of those terms you hear bantered about in recording studios and audio circles, but that many people really do not quite understand. Phase is how far a wave has passed through its cycle, measured in degrees. One complete cycle of a wave is 360 degrees. The wave can be a sound wave, electricity with alternating current, audio wave, or any other wave. When two waves of the same frequency are exactly aligned in time, they have a phase difference of zero degrees and they are "in phase." When two such waves are mixed together, their intensity increases. This is called phase reinforcement. Two waves not lined up exactly are "out of phase." When the two waves are exactly 180 degrees out of phase (called antiphase), phase cancellation occurs, causing the signal to dissappear. And that is why we are concerned with phase in the studio.

An example would be recording a guitar with two microphones. If the distance between the two microphones is one-half wavelength of a particular tone, then the signal of that tone received by the first microphone will be 180 degrees out of phase with the signal from the second mic. When combined, the two signals will cancel one another. Because a guitar plays more than one note, some notes will be increased while others will be decreased. A plot of such a recording will show peaks and troughs in intensity, which is called a comb filter, because the peaks and troughs resemble the teeth in a comb. Such as recording is said to be phasey as the sounds seem to come from no particular direction.

One way to correct the problem is by using the 3 to 1 rule. It says when two microphones are used to record a source, the second microphone should be placed at three times the distance from the first mic as the first mic is from the sound source. But it is only a rule of thumb and sometimes that does not work. Another solution is to reverse the polarity of one of the signals. This can be effective, particularly on something like miking the top and bottom of a snare drum. For this reason, many consoles and digital audio workstations have a button marked "phase inversion" or "phase reversal." However, that is a misnomer as what it really does is change the polarity, not the phase. It should be marked "polarity inversion" or "polarity reversal." When the phase difference remains constant over a given time period, it is said to be phase coherent.

Another problem sometimes encountered in the studio is phase distortion, which sometimes occurs when a plug-in or effects processor causes the phase of the signal to shift. To avoid this problem, one should seek out software and hardware that is phase linear, the ability to process an audio signal without causing any phase shift.

Not everything to do with phase is bad. Sometimes it can be used to create a special effect. Two of these effects are phasing and flanging. Originally flanging was created in the days of analog tape by mixing signals from two tape recorders and slowing down one of them by pressing a thumb on the flange of one tape reel. That is how it got its name. The effect produces a swishing sound that feels like the sound is swirling around you. The two effect are similar, except with phasing the signal is first passed through an all-pass filter, which results in phase differences that depends on the input signal frequency. The resulting in peaks and troughs are not harmonically related. With flanging the signal is not filtered and the phase shift is uniform across the entire audio spectrum, so that the peaks and troughs that are harmonically related.

So there you have everything you wanted to know about phase and more.

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Music Trivia

Irving Berlin wrote "White Christmas" in 1942 for the movie Holiday Inn, and it won an Academy Award for the best song of that year. The Oscar was presented to Irving Berlin by Irving Berlin. The Bing Crosby recording of "White Christmas" is still the best-selling record of all time.
"Jingle Bell Rock" by Bobby Helms entered Billboard's Top 100 Chart on December 23, 1957. It remained on the chart for six weeks and climbed all the way to Number 6.

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