Q. After years of successfully recording with stereo mikes and a Canon XL1, we recently purchased an Audio-Technica lavalier microphone and an Azden shotgun mike. When we record with these mikes, we use the standard input for the camera mike on our XL1. We go from an XLR, connected to the mike, to a 1/8-inch stereo adapter plugged into the camera. So far, so good. We can record on two channels (left and right), and dub directly onto a stereo VHS that plays fine on any stereo VHS VCR or DV deck.
The problem: when we take the same VHS tape that just played fine in the stereo VHS VCR and demo it in a mono VHS VCR, like the ones that come as part of a TV-VCR combination, the sound is gone. What’s happening here?
Charlie Chapin, Ph.D.
Stockton, CA
A. You’re in luck. We know exactly what you are talking about after a first-hand post-production fiasco we recently encountered right here at Videomaker as we finished up the latest in our series of instructional videos.
Like you, for some shots, we acquired our sound using a mono XLR microphone adapted to a stereo 1/8-inch plug that went into the camcorder. The audio sounded perfect in the edit bay, fine when played back on a DV deck and sounded great when we got the VHS tape back from the duplication house. The problem, of course, was when we played it back in a monophonic VCR, in which case the audio from the interviews dropped out. The background music bed was fine, however, which gave us our first clue. How could a stereo music bed be fine on a mono VCR but a mono voiceover on both channels was not?
After a bit of head scratching, we had one of those "Aha!" experiences. "Phasing!" one of our editors said. "I bet the two channels are out of phase, and when they’re combined they cancel each other out.
In short, balanced XLR audio cables reduce or eliminate line noise by passing an audio signal down two lines (instead of just one in an unbalanced cable). The two lines in the cable are 180 degrees out of phase, which means that any interference that enters the cable (e.g. a 60Hz electrical hum) will be exactly cancelled by the out-of-phase signal on the other line. In our case, the adapter we used took the signal from one line and fed it to the right channel and then took the 180-degree phase-shifted signal from the other line and sent it to the left channel. When played back in stereo, you can still hear both signals from each speaker (although you could probably find dead spots in the room where the two signals cancel each other). When the two signals combine for playback in mono out of a single speaker, however, disaster ensues and the audio is lost.
The solution is to get the right adapter for the right job, something we preach in every issue. One place were we have found the right equipment is www.equipmentemporium.com/audiofor.htm.
But what do you do with all of that out-of-phase footage when it’s too late to go back and reshoot (which it almost certainly will be if you don’t notice the problem until you get your tapes back from the dupe house)? Go into your editing application, isolate the two channels (left and right), delete one of them and duplicate the other. In Adobe Premiere, right-click the affected audio clip and select Audio Options: Duplicate Left (or Duplicate Right).
For more information on balanced/unbalanced audio, see Balancing Act on page 43 of our July, 2002 issue.
Q. What does BNC stand for?
C.P. Bauer
Atlanta, GA
A. There are many tales and urban legends about the origin of the BNC connector (used for some component video connections) and what the initials stand for: British Naval Connector, Bayonet Neill-Concelman, Bayonet Nut Connector and Baby N Connector to name but four. Despite what you have been told and despite what your engineering textbook may say, the origins of the name are not very clear.
The connector was (is?) probably used by the British navy, but it was invented by Americans Paul Neill and Carl Concelman. And if you named it after the creators, wouldn’t it be called a Neill-Concelman Bayonet (NCB) connector? We’ll just call it BNC and leave it at that.
