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Differential Audio

Most professional audio facilities use differential audio or balanced audio within their plants.  The main reason for this is noise rejection, which was discovered by the early pioneers of wired telephony back in the late 1800’s.  Balanced audio is created by generating two audio signals that are 180 degrees out of phase using either a transformer or an active device.  These are usually labelled High and Low, + and – or something similar.  Those two audio signals are then transmitted across some distance and recombined at the far end, again by a transformer or some active device.

Noise rejection, differential signaling. "DiffSignaling" by Linear77 - Own work. Licensed under CC BY 3.0 via Wikimedia

Noise rejection, differential signaling. “DiffSignaling” by Linear77 – Own work. Licensed under CC BY 3.0 via Wikimedia

When an interfering signal is picked up, it is transmitted along both sides of the balanced audio circuit until the signals are recombined.  During the re-combining process, and common mode interference is cancelled out, as it becomes 180 degrees out of phase with itself during the re-combining process.

Differential signaling is used in analog audio, digital audio (AES/EBU), HDMI, Display Port, USB, Ethernet, POTS lines, ISDN, T-1/DS-1, E-1, etc.   It is a fairly simple concept, but one of the basic building blocks in broadcast studios.

When a studio project was completed at a disused studio/transmitter site location, a certain amount of RFI was being induced on the studio microphones by the unassociated FM transmitter in the next room.  The problem with microphone level audio is the relatively low level of  microphone output, which requires a good deal of amplification.  The amplifiers in this console have active balanced inputs, which might not be exactly 180 degrees out of phase.  In this installation, microphone level audio was run about 20-25 feet on standard microphone cable then it was converted to Cat 6 cable before going into the console.  It may have been better to use the shielded Cat 6 cable for the longer runs as it likely has better common mode rejection than standard mic cable. Another option might have been Star Quad cable.  However, none of those things were done.

Western Electric was the manufacturing arm of Bell Telephone.  In their day, they made some really good equipment.  One such piece is the WE-111C repeat coil.  These can be configured for either 600/600 ohms, 600/150 ohms, 150/150 ohms,  or 300/300/300/300 ohms impedance ratios.  Since this is microphone level audio 150/150 ohms is the appropriate setting.

WE 111 repeat coil, one of the best such transformers ever made

Over the years, I have found many of these transformers discarded at various transmitter sites and studios. There are five microphones feeding this console. I mounted five of these coils in a sturdy metal enclosure and wired them with RJ-45 jacks to be compatible with the Studio Hub wiring equipment used in this studio installation.  I also grounded each unit to a piece of copper strap, which is connected to a grounding lug on the side of the unit.

Western Electric 111C repeat coils mounted in box

Western Electric 111C repeat coils mounted in box

I swept the coils from 20Hz to 20kHz:

WE 111C coils, 20Hz sweep

WE 111C coils, 20Hz sweep

WE 111C coil 20kHz sweep

WE 111C coil 20kHz sweep

This shows a 0.4 dB difference from 20 to 20,000 Hertz, thus they are all nearly flat which is a pretty cool feat of engineering.  I would estimate the age of these transformers is between 50 to 60 years old.

These coils isolate each microphone from the microphone preamp in the console.  This completely eliminated the FM RFI and solved the problem.

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5 comments to Differential Audio

  • Bill Frahm

    Was one of many wiring up a studio complex for a public FM/FM/AM station. My task was the main console. Got to the mics and all I heard from them was a cacophony of many audio sources plus buzzes and click. Seems that EVERYTHING including the mics at -50 dB, went through the massive patch bay! We tried transformers and various grounding schemes to no avail. I’m standing there studying the punch blocks for the over sized patch bays, and notice there’s no grounds on any of the shields! Spent an hour grounding things. Don’t think even a 111C would have helped that mess. They eventually put pre-amps on their mics at the source.

  • Gregg Richwine

    I cringe when I hear of people tossing 111C’s away. I haven’t had too many hard cases of RFI or hum issues, but every one of them was fixed using 111C’s. For smaller line-level issues that don’t need the electrostatic shielding of the 111C, I’ve used the transformers from old SA DATS receiver cards.

  • Chuck Gennaro

    Those 111C’s would pass FM composite quite well. Not uncommon to find one coupling an Optimod into an older exciter in place of the Orban “wideband interface” kit. They’ll do some rudimentary EQ of long audio lines by using a 150:600 match. The uses are nearly endless.

  • Lou

    Bell (WE) was really on top back then. Did you know they used to feed broadcast quality differential video over the poles for the network feed to TV stations? The A2A system used 124 ohm twinax. It was limited to about 5 miles or so where the TV station was in close proximity to the Bell central office. That way an expensive microwave link wouldn’t be tied up for a such a short hop.

  • Don Mansfield

    There’s something about the 111C that’s just elegant. They are all old as the hills by now but one just can’t find anything better. Wish someone still made them.

    When I worked in Washington we had over a hundred broadcast circuits coming in and going out. There was an entire wall of equalizers and 111C. (We even had bi-directional 15k circuits.) The faculty is gone now. I always wondered who go the coils.

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