Wednesday, September 22, 2010

Microphone cable wiring 101: connecting the ground lug—or not

Trying to repair a broken microphone cable the other day, I noticed that pin 1 of the XLR connector was connected to the ground lug with a small jumper wire, thereby bonding the cable shield to the connector shell. Here's what it looked like:

The cable was a cheapie from Active Surplus, the store on Queen St. West in Toronto with the stuffed gorilla by the door. From the quality of the components and the crummy soldering job, it's a good illustration of you-get-what-you-pay-for, so please consider this blog entry my penance for buying it. I was caught in a weak moment.

By way of contrast, here are quality connectors from Switchcraft (top) and Neutrik, with the ground lugs identified by arrows:

Should the ground lug be connected to pin 1, as in the top illustration, or not? I've read opinions pro and con over the years, so I decided to ask an acknowledged expert in the field, Neil Muncy.

Before I get to his answer, you should know that Neil, a Fellow and Life Member of the Audio Engineering Society, is the author of the ground-breaking 1994 AES paper, "Noise Susceptibility in Analog and Digital Signal Processing Systems," in which he explored the relationship between the physical construction of shielded twisted-pair cable and induced noise in a signal circuit due to cable shield current. This paper was published, along with others by authors including Philip Giddings of Toronto's Engineering Harmonics, in the June 1995 issue of the Journal of the Audio Engineering Society, which has become the most widely accessed issue of the Journal in history.

When he wrote his paper, most commercially available audio gear had pin-1 problems. It was, indeed, difficult to find equipment without it—even the most highly revered consoles had serious pin-1 problems. Since then, a number of leading manufacturers have redesigned their products to correct their mistake, but unfortunately, many have not yet done so.

Neil Muncy is also a member of the task group that developed the standard AES48-2005, "AES Standard on Interconnections—Grounding and EMC practices—Shields of Connectors in Audio Equipment Containing Active Circuitry," the published standard that deals with the pin-1 problem.

Based on all this, I figured Neil should know how to wire up a microphone cable. In close to 30 years, he hasn't failed me yet. Here's his answer to the question, Under what circumstances do you solder the ground lug (aka pin 4) to pin 1?

"The short, long, and infinitely long answers are NEVER, NEVER, & NEVER. To do so would introduce ground loops which could totally compromise an otherwise working Isolated Ground (I.G.) installation, and raise Hell with any front-end equipment that is plagued with Pin-1 Problems. Terminal #4 was introduced by Switchcraft back in the late '50's/'60's to address an application in very high impedance medical interfaces. It has no use whatsoever as far as portable A/V cables are concerned. They are simply extension cords."

There you have it. But if the lug shouldn't be used in general audio applications, why is it still there? Wouldn't it be advantageous for manufacturers such as Switchcraft and Neutrik to produce a line of connectors without the ground lug for normal stage and studio applications that have nothing to do with medical interfaces? I'd like to hear your comments.

Tuesday, September 14, 2010

Plan ahead for split-track recording when making documentary videos

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.