Balancing Act
Jay Rose

How you connect mics and other audio equipment can make a big difference in the quality of your track.

Why wires are noisy

If your tracks are plagued by noise and low-frequency hum, it's probably because of a wiring problem. But the solution is simple, relatively inexpensive, and has been around for a hundred years. Balanced audio wiring was first used by the phone company, to send calls over hundreds of miles of low-quality wire without picking up too much noise. Today it's used by every audio professional for much the same reason.

The problem is that wires are also antennas. When you plug a mic into a camera, or a DAT player into your NLE, you don't just get the desired signal. Any nearby electric fields are also picked up on the wire, adding a slight voltage which the equipment can't distinguish from the desired audio.

You can't avoid these fields. They're created by any other wires that carry a current. This includes video, timecode, and data cables, which can add all sorts of high-frequency whines and whistles to your track.

Manufacturers reduce this antenna effect in a signal wire by wrapping a shield around it, usually a copper braid or metal foil. The shield is connected to ground and shorts out the interference before it can reach the signal wire in the center of the cable. That's why phono, BNC, and cable TV plugs have a center pin and outer metal shell: the pin is signal, and the shell carries the shielding that in the wire.

A typical Unbalanced shielded cable.

Bah, Hum.

Of course the biggest currents in most places are in the wires that supply electricity to lights and and wall outlets. They radiate a lot. Cable shields aren't very effective with this 60 Hertz interference from the power-line frequency. Fortunately, it takes a much longer antenna to pick it up than the higher-frequency interference from video or timecode cables. In a small editing setup with short wires, the amount of 60 Hz pickup is very little compared to the audio voltages, so interference is minimal. But on shoot with long microphone cables, or in a complex post-production setup (my studio has about 3500' of analog wiring), the hum can be a major problem.

Furthermore, the same shielding that protects against high-frequency noise can contribute to hum.

About ground loops

People hum when they don't know the words. Audio circuits hum when they don't know what silence should sound like. The sensitive circuits that boost your camera's audio before it's recorded, or shuttle sound around your editing suite, need a reference they can be sure is zero volts. They compare the input signal to this reference, amplify or proces the difference, and generate an output voltage that's also compared to the reference. Designers designate one point within a piece of equipment (often connected to the chassis or grounding pin of the power plug) and call it "ground": all voltages inside the equipment are measured with respect to it.

That's fine for a single piece of equipment, but when you hook two devices together, both have to agree on the reference. Since the cable shield has to be grounded at least at one end, the usual scheme is to use it to connect the two devices' reference points together. It works in very simple systems.

But remember, that shield is picking up hum from the building wiring. And if the shield is carrying current -- something unavoidable if it's part of the audio path -- it has a slight voltage drop. Both these factors mean that the two devices are going to have slightly different references, and the difference is constantly varying. The input circuit can't tell that this variation isn't part of the signal, so it amplifies it. Again, in a simple NLE with short wires this interference may be tolerable. But in a complex room or studio shoot, it becomes hum and noise.

It's called "60 Hz hum", but it's not just 60 Hz

When power-line frequencies leak into an audio circuit, they generate harmonics. The 60 Hz base signal also hums at 120 Hz, 240 Hz, and up the band. That's why filters don't do a good job removing hum... you have to fix it at the source

Complex setups have other problems as well. If there are multiple ground paths, they combine to make a very efficient loop antenna for the 60 Hz noise. These "ground loops" are almost impossible to predict, since you don't know the internal details of your equipment, and can crop up even in very simple setups... particularly if both pieces of audio equipment also share a ground connection through their power plugs' grounding pins. In a practical video studio, the situation is apt to be far worse: the non-audio cables -- RS-232 and RS-422 control, video wires, and even cable TV -- all have their own grounds.

Balanced Wiring

Professional equipment solves this problem by using two closely-spaced conductors twisted together. Audio is balanced equally on these wires, flowing in a positive direction on one wire while in a negative direction on the other. Equipment looks at the voltage difference between those wires, and ignores everything else. A grounded shield is still used to prevent high-frequency noise, and it might form a hum-gathering antenna -- particularly if there are other ground connections. But since the ground isn't part of the audio path, nobody cares.

A balanced shielded cable.

Most modern circults do not use balanced wiring internally. The internal wires or printed-circuit traces are so short that noise pickup isn't a problem. But they balance the signal before it leaves, and unbalance any incoming ones. This is very easy to do with op amps or transformers.

Balanced wires also reject noise that isn't coming from a ground loop. The two conductors are twisted closely together, so any interference radiated into the cable is picked up equally by both. But remember: the equipment is looking for a voltage difference between those wires. Noise is the same on both wires, so the equipment can't hear it.

Another advantage of balanced wiring

If a single-conductor shielded cable acts as an antenna, why doesn't two-conductor balanced wiring act as a double antenna?

Answer: it does. Noises from nearby video or computer cables are picked up on each conductor. But remember, a balanced audio input cares only about the voltage difference between the two wires. Interference is radiated equally into each wire. Since the interference is equal on each, there's no voltage difference from it! The balanced input can't even see that the noise is there.

Or to put it into a chart: 

Conductor           Audio Signal    Noise     Total on wire
    Black               +1 v          +1 v     +2 v
    White               -1 v          +1 v      0 v

Transmitted difference   2 v
                           Received difference  2 v
This noise-immunity of balanced wiring is why it's also used for high-speed computer networks. Category-5 cable contains four tightly-balanced pairs of wires. In fact, if your balanced input and output circuits are good enough, you can use Cat-5 cable for professional audio wiring!

"Star Quad" is four-conductor shielded balanced cable. The four wires form a tighter, more consistent pack than two wires can and can resist even more noise. If you're using Star Quad, you must tie the two pairs of similarly-colored wires together at each end... reducing it effectively to two conductors. Don't try to use it as two balanced pairs for two different signals: this won't give you any noise-reduction benefits at all.

You can't tell a wire by its connector.

Balanced wiring is sometimes called XLR wiring because it often uses that kind of three-pin connector. But it's dangerous to assume that an XLR plug or jack is balanced. Although most manufacturers reserve these connectors for balanced circuits, there are exceptions. Check the equipment specs.

You can also get into trouble assuming that other kinds of connectors aren't balanced. Some manufacturers save money and space by putting balanced signals on three-conductor phone jacks. From the front panel, these look identical to two-conductor unbalanced jacks. But if you connect a three-conductor phone plug -- also known as Tip-Ring-Sleeve, or TRS -- it can carry a balanced signal. Again, check the specs.

And a balanced connection works only if both ends are balanced. If either of the two internal wires are shorted to ground at either end, the whole thing becomes unbalanced. This can happen easily if:

So don't do these things.

Got balanced wiring on a TRS jack (found in many compact mixers)?

The worst thing you can do is split it to two channels with a stereo Y cable. You'll hear a signal on each end of the Y, but the two signals will be of opposite polarity. If you then listen in mono, they'll cancel each other and the sound will disappear!

Connecting balanced and unbalanced equipment

As noted above, as soon as you make an unbalanced connection at one end of a wire, the whole thing becomes unbalanced. So for maximum noise immunity, you should use balancing adapters.

If you want to connect a balanced mic to a prosumer camera, use one of the small transformer adapters from BeachTek or Studio 1, or put a mixer (with balanced inputs, of course) near the camera. This is the only way to run long mic cables to a boom or wired lav without risking noise pickup.

Connecting an unbalanced consumer output to a balanced professional input -- say, from a sound card to a professional VTR -- is slightly more complicated, because the voltage levels are different as well. For this situation, the best bet is to get a balanced interface for the computer. This is a small powered box with an unbalanced input at -10 dBV (usually on an RCA jack) and balanced output at +4 dBm (usually on an XLR male). Run a very short cable from the source to the RCA jack, and as long a balanced XLR cable as you need to the pro equipment. Similar interfaces are available to connect balanced sources to unbalanced equipment. They cost between $50 - $75 per channel

© Jay Rose 2001

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