A wedding videographer came to me last week with an audio problem in a wedding video he was editing. Just before their first dance as husband and wife, the happy couple had taken to the dance floor with handheld mics to sing a torch ballad over a karaoke track. A huge crowd-pleaser, this would have been a "highlight" of the wedding video. Unfortunately, on tape the backing track was so loud that the lyrics were completely unintelligible—in fact, you couldn't hear the man's voice at all. Could I save the track? And his reputation?
In a nutshell, no, not this time. The voices were so faint in the two-track mono mix and the electric piano so loud that no amount of EQ could bring them forward. The client even sent me the original wav file of the karoake track, hoping that if I mixed it out of phase (reversed polarity) with the botched recording, it would cancel out some of the music track, leaving the voices more intelligible.
For the cancellation trick to work, the music in the end recording would have to be almost identical to the original karoake file. It wasn't: the karaoke was in stereo not mono, and it ran slightly faster than in the final recording.
Moreover, there was tons of ambience in the duet recording, because the karaoke playback was picked up not only directly from the audio mixer at line level, but also from the loudspeakers in the hall by the singers' microphones. So the final recording is muddied and colored by the reproduced sound from the loudspeakers and all sorts of reflection from the walls and floor, which, of course, is not present in the original track.
This also partially explains why the music was so much louder than the vocals: the music was being picked up twice, once as a direct feed, and secondly from the singers' microphones.
What should the operator have done? Since the goal was apparently not to make a stereo recording—no attempt was made to preserve the original left-right stereo of the instrumental track in making a two-track mono recording—the operator should have fed both sides of the original instrumental track to track 1 of the camera, and the microphones to track 2.
This simple split-track technique would have allowed for the voices to be properly balanced with the music in post production. It's done all the time in recording dialog for films and TV on 2 tracks, especially when there is a mic on a boom or fishpole, and the actors are wearing lavs. Boom goes to track 1, lavs to track 2.
When shouting and screaming are anticipated in a scene (perhaps not at a wedding), the same audio is printed on both tracks but at a reduced level—say, 10 dB lower than normal—on track 2. So if the scream is distorted on track 1, the sound editor can lay up the lower, undistorted scream from track 2, and match the levels during mixdown.
It's simple and it works. It just requires a little planning ahead, but when you only have one shot at it, split-track recording can save the day. And your reputation.