Contrary to Popular Belief, Phase and Polarity Are Not the Same Thing
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Proper handling of signal polarity and phase is an important part of audio production, because unintended comb filtering can ruin a project. However, we can apply these concepts in creative ways as well.
For example, critical analysis of music mixes is an excellent way to learn new ideas and techniques for your own work. Manipulation of signal polarity can be a useful tool in such analysis. Think of a stereo mix as having three basic components: sounds in the left channel (A), sounds in the right channel (B), and sounds common to both channels. We can often learn how music is mixed by listening to these components separately. One way to do this is to cancel out the sounds common to both channels and listen critically to the remaining sounds. This technique sometimes called A-B listening.
To analyze music through A/B listening, connect the outputs of your source, such as a CD player, to two channels of a mixer. Pan both channels to the center, and then reverse the polarity on one of the channels. Any sounds that are equal in both channels of the mix are canceled out, because their polarities in one channel are inverted with respect to the same sounds in the other channel. The mono signal that remains contains the sounds that were panned to the left or right channel, but not both. (Sounds that are more pronounced in one channel than in the other in the stereo mix are reduced in volume in this inverted mono mix, but they are not eliminated completely.) Comparing this signal with the stereo mix, as well as with the complete (noninverted) mono signal, helps you learn more about the reverbs, backing parts, stereo spreads, and other details that are often masked by the more prominent parts of the mix.
Manipulating signal polarity can help fix problems as well. For example, suppose you do a multitrack recording of a band, and you send each instrument to a separate track. Later, while listening to the playback, you discover that you mistakenly sent some of the guitar signal to the synth track.
You may be able to fix the synth track. First mute everything but the synth and guitar tracks. Reverse the polarity of the guitar track and mix it in with the synth track. Change the level of the inverted guitar track until you find one that removes most of the guitar sound, leaving just the synth part. Bounce the result to an open track, and the problem is solved! This technique doesn’t work in all cases, but it might get you out of a tough situation now and then.
Creative use of phase shift is another powerful tool in music production. For example, a common technique for recording a guitar amp is to place one mic right up to the speaker cabinet and a second mic just a foot or so behind the first. The mics pick up the sound from the amp at different times, which means the signals are out of phase. When you mix these signals together, some frequencies cancel each other out, and some reinforce each other. The resulting comb filtering often creates a guitar timbre that a single mic can’t capture. If you don’t like the sound, move the second mic back and forth until you find just the right spot where the comb filtering creates an interesting and useful sound.
The same technique can also be employed in mixing. Suppose you are mixing a tune, and something isn’t quite right about the bass track. The performance is great, but the tone just isn’t cutting it. You’ve fiddled with EQ and compression, but the right sound eludes you. Split the track into another mixer input and insert a delay on it. Set a delay time of less than a millisecond, as low as 0.1 ms, with no feedback. By adding the two inputs together, you create a comb-filter effect for the bass track, again generating tones that might not be achievable with simple EQ.
By using different delay times and adjusting the balance between the original signal and the copy, you can create many different timbres. Don’t forget that you can also reverse the polarity of the delayed signal, which gives you even more tonal variations. This approach works great for lots of sounds, particularly snares, kick drums, basses, and guitars.
Final Cycle
Manipulating signal phase and polarity can be a powerful tool in audio production, in subtle and dramatic ways. Once you understand the difference between these concepts—and know what to expect—you will find other interesting techniques for your work. Just remember to check your mixes in mono!
Jeff Baust is an engineer and freelance composer in Boston and New York City. He is also on the faculty of the Berklee College of Music.
Reprinted with
permission from 
Magazine, November, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved.