"Most professional microphone use twisted pairs and are balanced cables, but there are many lavaliere microphone that use unbalanced cables from the microphone to a belt pack, or similar application," says Steve Lampen

You probably spend hours reading about different microphones, dynamic, condenser, ribbon. You might even borrow and compare them, try them out. But do you ever think about the cable to connect them? Aren't all microphone cables pretty much the same? Well, no they're not. And this article will outline just a few of the differences. Hopefully, at the end, you will make better choices and even be able to ask some serious questions about microphone cable. You will be a dangerous customer.

There are two basic kinds of cable: single-conductor shield and twisted-pair shielded. The single-conductor cable is for microphones that are unbalanced. Most professional microphone use twisted pairs and are balanced cables, but there are many lavaliere microphone that use unbalanced cables from the microphone to a belt pack, or similar application. They get by with a less-than-ideal cable because it is short, usually a meter or two.

The reason why balanced cables are preferred is that they reject noise. I could write this whole article about how balanced lines work, and how they reject noise. But for now, just realize that the two wires inside the cable, can tell the difference between the audio (the wanted signal) and any noise (unwanted signal) and that the box you plug that microphone into is built to reject the noise. If you're professional stick with twisted-pairs and balanced lines.

You can easily recognize cables that are balanced lines because they usually use XLR connectors. These connectors have three pins in them. Two of the pins (pins 2 and 3) are for the two wires that make up the twisted pair and the balanced line. The third pin (pin 1) is the connection for the shield, the metal layer the covers the pair. Table 1 is a list of all the features you might ask about a microphone cable

Gage size and resistance. The bigger the wire, the lower the resistance. Resistance turns a signal into heat, which then radiates off the cables. You can't feel this happening in a microphone cable, but you often can in a power cord running a high-power device.

So many people think big wires are better. In fact, they make very little difference in performance. One reason is that resistance affects all frequencies equally, so it doesn't affect the sound quality, just a slight effect on the level (intensity) of the audio signal. So you can't hear the difference of different size wires in different microphone cables. In fact, the only reason you might want bigger wires is ruggedness. Big wires are stronger, would take more abuse, last longer on the road.

Flexibility. The key parameter to making a flexible cable is the stranding of the wires. The more strands of wire that make up each conductor, the more flexible it will be. Choice of insulation and jacket materials also has an effect, but stranding is the key. However, there is no ‘standard' for flexibility. Too bad, because if there were it would be easy to compare flexibility of different cables just by looking up the "flexibility number". Unfortunately, the only way to compare them is to get samples and actually handle them.

Be aware, however, that flexibility is often the opposite of ruggedness. The soft jacket material to make a cable like a cooked noodle means that the jacket would easily break or crack when used in even a moderately hard environment. Also, don't confuse flexibility with flex-life, which is a parameter than can indeed be measured. Flex-life is the number of times a cable can be flexed before it fails. Many industrial cables are made to have good flex-life. This does not automatically mean they will be flexible, so you can't use flex-life numbers to indicate flexibility.

Insulation: When an electrical signal passes down a wire, it produces a magnetic field. That magnetic field travels in the insulation around the wire. So the insulation is the key to electrical performance. Changing the quality of the insulation will also change its effect on the signal. Table 2 is a list of the most common materials for insulating wires together with a number that tells us how good the performance will be, something called the "dielectric constant". The lower the number, the better the performance.

On this list, the most rugged material is rubber. Two artificial rubber compounds are listed with abbreviations. You can also get real-hole-in-the-tree rubber although it is quite expensive. A good argument can be made for ruggedness. It doesn't matter how wonderful the cable performance is if the cable is broken. So if you are using microphone cables in severe environments, like collecting sound effects in nature or recording bird songs in the jungle, rubber might be an interesting possibility for you. Belden 8412 is a good example, with real ‘tree' rubber on the twisted pair and an EPDM artificial rubber jacket. At the end of the list of solid plastics is the best plastic ever made, Teflon^TM. There are a number of reasons you never see this used in a microphone cable. First Teflon is very hard to work with in a factory and very stiff when it is put on a wire (extruded on a wire). Second, it is very expensive. As you can see, we can easily get to that level of performance, and beyond, by adding air into the plastic. Foamed plastic can be very flexible, and extremely high performance, though less rugged.

Capacitance. When you have two pieces of metal with a non-conductor in-between, you have an electrical device called a capacitor. Capacitors store electrical charge when electricity passes through them. The two wires in a pair, with the insulation in-between, make a small capacitor. The capacitance of the pair is measured in picofarads.

The capacitance of the cable stores a small part of the audio signal running down that cable. The effect is worse at high frequencies than at low frequencies. This is why, if you see a graph of cable performance, it is not a flat line, it has a slope. This is called the frequency response of the cable. (Every audio device has a frequency response curve.)

Changing the plastic on the wires will change the capacitance. You can easily judge the performance of a microphone cable by simply looking up the capacitance between conductors on the web page or in the catalog of a manufacturer. Be sure you are reading conductor-to-conductor in the pair. And also be sure it is in the same unit of length (feet, meters), so it's a fair comparison. Capacitance is the key parameter to cable performance.

Table 3 now shows the capacitance of a twisted pair with each of the insulations list and examples of cables with those properties.

It is amazing how many recording studios and audio facilities purchase microphone for many thousands of dollars and then use all-PVC microphone cable to hook them up.

Shielding. Shields are metal layers put around the twisted pair to help prevent interference from getting to the pair. It can also help prevent the signal inside the pair from getting out and interfering with other pairs or other cables around it. There are two basic shields. One is made with many small conductors wound or braided around the twisted pair. The other type of shield is made with a foil wrapped around the pair.

Foil is very good at high frequencies, above 10 MHz, but it is only useable where the cable does not move while in use. The foil can shift position, open and close, when the cable is flexed. This can be heard as ‘capacitive noise' since it affects the capacitance of the cable. Since microphone cable is indeed intended to be flexed while in operation, a foil shield is a poor choice.

The other kind of shield uses multiple conductors. It can come in a number of forms. The simplest is to wind the wire(s) around the pair. This is called a spiral or ‘serve' shield. While it is very flexible, it also has a problem when the cable is moved or flexed. The serve shield will open up, allowing noise to get into the pair, or allowing signals inside the cable to get out. Serve shield have a second problem. Since they are essentially continuous windings of small wires, that also describes an electrical component called an inductor. Because a serve/spiral has significant inductance, it is appropriate only at low frequencies, such as analog audio.

Some of the problems with serve/spiral shields can be reduced by having two spirals wound on top of each other, each at right angles to each other. This is called a Reussen shield. It still maintains flexibility, and can increase the coverage and the layers help ‘short out' the inductive effect, but this design only moderately help the problem of shield openings when flexed.

The historic best shield is a braid shield. In this construction, groups of wires are wound in and out, very similar to the way hair or fabric is woven. This dramatically reduces the ability of the shield to ‘open up' and is very low inductance, since the wires continually cross each other. Such a shield is very effective up to hundreds of Megahertz, way beyond analog or even digital audio.

There is also a new shield type called a French Braid. It is somewhat like a Reussen shield as it takes two spiral shields, but then these two spirals are braided along one axis. This provides the flexibility approaching a Reussen shield, but with consistent coverage of a braid shield, and excellent high frequency performance.

The truth about shields. Many engineers are surprised to learn that any shield around a pair is less effective than often believed. In fact, below 1,000 Hz, in the low frequency section of audio, there is no type or configuration of shield that has almost any shield effectiveness at all. Even if you put a twisted pair inside a solid steel conduit, you can only get approximately 30 dB of noise rejection at 50 or 60 Hz.

Above 1,000 Hz, braid or French Braid can have reasonable to good shield effectiveness and noise rejection. But the truth is that most of the noise rejection in any shielded twisted-pair cable comes from the twisted pair itself, run as a balanced line, and not from the shield.

Starquad. Since it is the balanced-line that provides most of the noise rejection, then the best balanced-line will reject the most noise. The idea balanced-line twisted-pair would be one where the two wires occupy the same space at the same time. If this were possible then any noise that hits the two wires would be as close to identical as possible and the rejection of that noise by the balanced devices at each end of the cable would be close to perfect.

But, of course, you can't have two wires occupy the same space at the same time, can you? Amazingly, yes you can. And that configuration is called starquad. Virtually every manufacturer of microphone cables makes a starquad. In Belden's case, we make three different sizes: 1192A (full size), 1172A (low-profile), and 1804A (ultra-miniature).

In any starquad cable, there are four conductors instead of the two you might expect in a twisted pair. These four wires are spiraled around each other. Often they are color coded to help combine the correct conductors. The wires opposite each other need to be combined into one wire. Thus the four wires end up as two wires, what you would need for a balanced line. But if you look closely to what you have done, you will realize that each of the resultant conductors did indeed occupy the same space at the same time.

The ability of starquad to reject noise, especially low frequency noise such as 50/60Hz power cables, or lighting cables, is quite amazing. These cables routinely reject these low frequencies by 50 dB or more, better even than solid steel conduit. So, if your microphone lines run over (or parallel to) power cables, or if you are hanging audience microphones from a lighting grid, or similar applications, starquad might be seriously considered.

There is really only one downside to starquad. Since you are combining conductors, you are creating much larger capacitance than normal microphone cable. Even though they use higher-quality plastics, starquad cables are often up to 50 pF/ft. or 164 pF/m, thus limiting their effective distance. We'll talk about capacitance and distance at the end of this article.

One other caution about starquad cable is that this is not two twisted pairs. It is four wires spiraled together. If you use the four wires in the cable to run two signals, you do not have two twisted pairs, and you have thrown away any noise rejection that you might have been able to get. Only by combining the conductors does this cable reject noise.

The ideal microphone cable. So the ideal cable, as you can see, really depends on your application. If you need ruggedness, stick with rubber (or similar) jackets such as Belden 8412. If you need the ultimate performance, look for foamed insulation and low capacitance such as Belden 1800F. If you want low noise pick-up, go for starquad cable, such as Belden 1172A.

How far can you go? This really should be a question you should never need to ask. The reason is that microphone cable is made to flex while in operation. That means that only the first few feet will actually be moving. You can then transition to a much cheaper and sometimes better performance cable, a line-level cable, or multipair snake cable.

Still, there might be a chance that you have to go a hundred or a thousand feet without a transition. In that case Table 4 will be of interest. It shows how far you can go on a cable given two numbers. One number is the capacitance of the cable, listed at the top of each column. The second number is one you might not know. It is the source impedance of the device that is feeding that cable. If that device is a microphone, the source impedance is the same as the output impedance of the microphone.

Hopefully this article has given you some insight into choosing microphone cables and questions you can ask to get the most for your money.