The Mystery of the Missing Audio in my Video
First the Mystery
Watch this brief clip. For best results listen on speakers. The flaws are not as obvious on headphones.
What did you hear, Or not hear?
Well… not much. We saw Kim speaking but at best the audio sounded as if it was in a tunnel and at worst … you might not have heard her at all. Very sad … she was saying some deep stuff.
Can you guess what the problem was?
Did you think there was a glitch in the audio export of this clip from editor app, in this case, Final Cut Pro X?
Wrong: That cannot be the case.
Why: Because the audio does not completely drop out in the example. You can still hear the background music! Finished video files do not support multiple audio tracks. There is no way that one layer simply doesn’t get exported from the editing application and the other does (unless of course it’s muted, but that was not the case). It’s all or none for audio in a video file.
If you guessed “phase cancellation:”
Pat yourself on the back, sit back in your chair with your hands behind your head and gloat over your impeccable knowledge of audio production. Post it on social networks, you might even add the triumphant cackle emoji. (Someone needs to make one)
If you did not … read on. This may happen to you.
So what is phase cancellation and how did it mess up Kim’s voice?
Scary, semi-technical definition: Phase cancellation occurs when two identical audio signals combine. When this happens the opposing waveforms “cancel” each other out making the resulting signal sound funny or in-audible.
Here is a brief, simplified, yet still confusing video targeted at budding audio professionals. (Something that video editors need to be). Watch it and then I will give you an explanation of what actually happened.
And…. How to fix it. without re-shooting your video. YES!
Good stuff… but you are probably still scratching your head.
How it Happened
The video was shot using a wireless lavalier mic for close micing. I hope you are doing close micing. See my other post “Audio: The Ugly Stepchild of Screencasting” to find out why. After trouble-shooting the mics I discovered that one of them, the one that was used for the video at the top of the page, had an option on the receiver labeled “Audio Output Selector.” that selector allows for two settings: Balanced or unbalance (dual mono). IT WAS SET TO DUAL MONO! That was it!!!! Dual mono means that the camera was receiving TWO IDENTICAL AUDIO SIGNALS VIA ITS STEREO INPUT.
TWO IDENTICAL AUDIO SIGNALS!!! Guess what….. when TWO IDENTICAL AUDIO SIGNALS are combined. They cancel each other out!! You just read that a few minutes ago.
Problem detected……now what?
The good news is it’s wicked (sorry…I spent numerous years living in Boston) easy to fix phase cancelation. The bad news is, only pro level editing apps offer the feature that you need to fix it.
To simplify: I activated a plug in in FCPX called “Gain.” This plugin has a phase reversal switch. To fix phase cancelation problems you only need to reverse either the left or right channel of the audio signal.
Watch this quick tutorial:
How to fix phase cancellation ing FCPX
How can I avoid this problem?
This problem is not likely to happen if you are only using the mic that is built-in to your camera. These mics are made to be foolproof. They are also, often times, made to be “quality audio proof” (the ugly step child rears his head again) which is why you should always use a “close micing” technique. Particularly if you are doing interviews with people.
Know your Mic
Most auxiliary or wireless mics come with many options. You need to defy all of your instincts and READ THE MANUAL. At least look at the diagrams. Know what all of the buttons do. It’s fun… you get to push buttons. Most of the time, the factory default settings work best. An Audio-Technicha ATW 1701 was used for this shooting. A great sounding mic for the money.
Make a “pre-shoot” check list.
Many of you teach video production at the high school or college level. This means that you may have many inexperienced, yet well-meaning, students shooting video. Once you find the optimal settings for your equipment, make a check list to be gone over prior to shooting. An make sure audio/mic settings are included. Video shoots are often chaotic so this step can really help you avoid problems for both video and audio (its ugly step child).