After countless hours of careful acquisition and editing, you go to play your finished work on a television set and part of your audio is gone. What happened?
Audio phase is one of the most misunderstood topics in the audio world. This may be because it's a complicated issue and the math behind it all is something only a physics geek could love. Regardless, audio phase and, more importantly, phase cancellation, is important to every video producer who cares about the sound on their projects. Phase cancellation can easily ruin the dialog or voice-over track when your client plays the video, so let's do something about it.
Signals and Waves
Have you ever been in a restaurant or store, listening to your favorite song, only to wonder where the vocals went? This is a classic example of phase cancellation. How does it happen? The simple answer is, someone, somewhere hooked things up wrong. Let me explain. Acoustic sounds — like your voice or an instrument — compress the air around them. This creates invisible waves or ripples in the air. These compressed waves reach your ear, and the eardrum translates them into electrical signals for your brain to decode. Now, let's substitute your ear with a microphone. The mic's diaphragm moves along with the air vibrations and converts that movement into a small electrical signal for you to record. It's actually very easy to see representations of the waves if you zoom in on an audio recording in your favorite editing program.
A single signal going directly into a camera or mixer is no big deal, but recording in stereo is potentially problematic. Here's what happens: balanced mic and line cables carry two versions of your audio signal — one in correct phase and one 180 degrees out of phase. At the end of the signal chain (mixer, recorder, etc.) these versions are combined into one accurate, noise-free channel of audio. Add a second channel recording the same source and there's a possibility that one of your cables or adapters is wired opposite of the others. If you combine these two out-of-phase signals in a mixer or to a mono TV, one cancels the other. The math is pretty easy: if you have +1 and add -1, the result is zero.
This was more of an issue when low budget or guerilla cinematographers commonly made their own XLR cables but this is less common now. The out of phase problem could, however, arise when an XLR cable is connected via an adapter to a camera with an 1/8-inch mini stereo connector. The out of phase signals are separated into distinct channels and not put back in phase in the camera.
Here's the big problem; if you're listening to phase-cancelled audio on stereo speakers, you may not notice. You can still hear everything — although it may not be as sharp and clear as normal. A quick test is to play the audio while moving your head from right to left. If the sound is in phase, it will be clean and clear wherever you move. If the sound is out of phase, it will be indistinct and hard to locate between the speakers. Or you can mix your sound down to a mono signal, either in the software or with cables. Listening in mono will tell all.
Fixing the Mix
Of course, the best way to eliminate phase cancellation is making sure you correctly wire your audio cables and adapters. When making this connection be sure to use an XLR mono-stereo mini adapter with audio bridged to both channels. If you do make your own balanced cables, make sure your lines are not crossed.
But what if you discovered a problem after the shoot? Don't panic, there are ways to repair the damage in post.
If you're working on an audio-only recording, or prefer to export your audio from the edit timeline, it's easy to repair phase cancellation in your audio editor. Open the stereo waveform and highlight the left or right channel. In Adobe Audition, choose Effects, then the Invert option. Your editor may be different, but all major applications have a similar option. Save the corrected WAV file and you're ready to drop it into your video production.
The same trick is available in your video editor too, but it's a little more complicated. First, make a copy of the audio track with the phase cancellation. Use the Fill Left effect for the channel you will keep and Fill Right for the channel you want to change. If your NLE can split an audio track into two separate channels, that's even better. Next, apply an Invert filter to the bad channel and you should have correct, uncancelled audio. This option also increases the complexity of your mix since you have to manage edits and volume envelopes for two independent channels. And don't forget sync. If timing for the repaired channel is even a little off, you'll have weird echoes and strange stereo problems. If you have a great deal of audio to repair, consider exporting it all to an audio editor to make the changes.
Here is the short of the long: Phase cancellation is bad and can cause your voice tracks to vanish on playback. It only happens when you're recording two or more audio channels and you can fix it before or after the shoot. Make sure you wire your equipment properly and you may never have to deal with phase cancellation. If not, all is not lost.
Contributing Editor Hal Robertson is a digital media producer and technology consultant.
[Sidebar: Reality Check]
In my edit suite, I use a good set of surround speakers, complete with subwoofer. This system produces very nice sound and is a great reference point for mixing and balance between audio sources. However, I can't forget that many of my viewers will hear these projects over a simple mono speaker or through a mono sound system. To combat potential problems, I also route the stereo mix through a VCR and hook the output to a $60 mono television. This way, I can quickly check for phase cancellation. If it looks and sounds good there, I'm done.