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How to Set Up a Home Studio (Part 2)

Cables

Part 2: Cabling. David Mellor reveals all the prospective home studio owner needs to know about cables.


When the cable-free studio is invented, we shall be free of one of the greatest sources of annoyance to the studio owner or musician. Until that day arrives (don't hold your breath) we must find ways to minimise the problems that a multiplicity of cables can bring. David Mellor explains all the home studio owner needs to know about cables.


In case you haven't had the pleasure of getting tangled up in cables yet, here's a reminder of some of the difficulties that can crop up: unreliable connections; incorrect routing between equipment; interference; loss of high frequencies; knotted or tangled cables; and unwanted cables underfoot. Some of these seem trivial in the clear light of day, but when you can't figure out what is plugged into what, halfway through an all-night session, then the gnashing of teeth and tearing out of hair have to be taken as symptoms of a blood pressure raised to unhealthily high levels. It's not good for the music either.

There are several levels of sophistication in studio cabling, ranging from no cabling system at all - just a bag full of leads - to a full patchbay system with cabling housed in trunking or conduit. I'll be describing all of these, with the emphasis on sensible systems, in articles yet to come in the series. For now, I'll stick to the cables themselves - how they are constructed, and what the advantages and disadvantages of each type are.

MYTHOLOGY



Figure 1. LAPPED SCREEN TWIN is common-or-garden everyday cable. Reasonably priced and suitable for most purposes.

Usually when someone writes an article about cables, it's in a hifi magazine, and they are rabbiting on about 'linear crystal' and 'oxygen free' and other such mysteries. I have to go along with the man behind the company that makes the world's most expensive mixing consoles, when he said that if people thought they could hear a difference, that was OK with him. I think there is enough to worry about getting the basic engineering principles right. To make sure that measurable - and clearly audible - deficiencies of cables are minimised. This is a straightforward matter, once you know what those deficiencies may be. Let's take a look at Figure 1, which shows a two-conductor audio cable.

As you can see, there are several components. Each signal conductor is made of a number of fine strands of copper. Around each signal conductor is a layer of plastic insulation - one is normally coloured red, the other black. Around the two signal conductors are more fine strands of copper - known as the screen. Keeping the innards in and the outside world out is another layer of plastic insulation. This type of cable is known as a 'lapped screen twin' cable. Get your microscopes out to look at the fine detail...

The signal conductors each consist of between 25 and 50 copper strands, each approximately 0.1 mm in diameter. The material is copper because it is a good electrical conductor, and also easy to form into a wire. A large number of thin strands are used, because this is a more physically flexible arrangement than having just one thick strand. If there were, say, 30 strands each 0.1 mm in diameter, the conductor would be simply described as 30/0.1mm.

Figure 2. BRAIDED SCREEN TWIN cable is more physically robust than lapped screen, making it more suitable for use in hostile environments - such as on stage. Some versions have two layers of braid. A braided screen also provides more protection against interference than lapped. It is harder to attach a connector to it.

The insulation around the signal conductors is typically PVC, although other plastic materials may be used. The screen consists of 50 to 60 strands of copper wire, each once again about 0.1 mm in diameter. When it is wound round and round the signal conductors, it is called a 'lapped' screen. When the strands are woven together it is known as a 'braided' screen, as shown in Figure 2. The screen is normally connected to earth and keeps electrical interference away from the signal conductors.

The outer insulator is PVC once more, and the overall diameter is about 6mm. This is a standard audio cable and is colloquially known simply as 'Mic' cable, although it can equally well be used for line level sources.

Now that we know the construction, what else is there to know about this cable? A key question that has to be uppermost in the mind of an engineer is the capacitance of the cable. Capacitance in a cable can be compared to holes in a hosepipe. When you water the lawn, most of the liquid comes out of the end of the hose as it should, but some leaks out of the hose, runs along its length, and on to the ground.

In the cable, some of the electricity in the signal conductor can leak through to the screen, which is of course connected to electrical ground (earth). It is always the high frequencies that are first to leak through, so if you had a long cable of high conductor to screen capacitance, you could expect to obtain a dull sound. There is also capacitance from conductor to conductor, which has precisely the same effect. These two characteristics are measurable and must be included in the specification of a cable. (See sidebar for an explanation of the effects of capacitance.) Cables also possess electrical resistance and inductance, but these don't really make too much difference in the lengths of cable typically used in a studio. If you're laying a Transatlantic telephone cable, however, the situation is different.

Now is probably as good a time as any to examine why there are two conductors in this cable instead of just one conductor and a screen. In professional audio, it is normal to use balanced connections between equipment. Balanced wiring reduces hum and interference and makes it simple to connect any piece of equipment to any other without the likelihood of problems arising. It costs more, but professionals are prepared to make that initial investment, because it saves time and money in the long run.

The balanced system works by having the same signal in both conductors, but one inverted in polarity - like connecting a battery the other way round. Balanced equipment has transformers, or the electronic equivalent, which sort everything out at both input and output. The advantage is that any interference (noise) that gets into the cable is cancelled out in the balancing/unbalancing process.

I don't want to delve too deeply into the whys and wherefores of balancing right now. What I do want to point out is that there is no such thing as balanced cable. It's the equipment that is balanced, and two-conductor cable is suitable for use with balanced equipment. It is also suitable for use with unbalanced equipment, as found in the home studio. But this is getting off the basic subject of the cables themselves, so I'll leave that until another time.

CABLE MECHANICS



The mechanical properties of a cable are as important as the electrical properties. Ever had one of those cables that can tie itself into a knot without human intervention? We all have, and we know that it is something to avoid.

The mic cable I described above is suitable for use as a free cable, that you would connect for a particular purpose, then disconnect and coil up for storage. The two features that make it good for this application are flexibility and coilability. Flexibility comes from the fine stranded wire used, and also from the soft plastic insulation. Coilability - I don't think it's a scientifically measurable quantity - comes from the relationship between the flexibility of the cable and its diameter. There is a point where a cable can be too flexible and too small in diameter for its own good, and creates the knotty problem described above. Six or seven millimetres is a good diameter for a mic cable.

Cables of lesser diameter can be used for installation work. That is where the cable will be wired up and left undisturbed. It doesn't matter what its handling properties are like as long as it's reasonably easy to hook up in the first place. You won't touch it after that.

There are two types of cable that I find very useful for installation. The first I use basically because it's cheap. There's nothing wrong with that as long as it is still OK electrically. It is a single conductor cable (therefore only suitable for unbalanced connections) with a 7/0.2mm conductor and a lapped screen. The overall diameter is 3mm and the cost is a mere 12p or so per metre. Its big advantage is that it is ideal for making into a loom. A 'loom', as you are probably aware, is a collection of 20 or 30 individual cables fastened together and all going to the same place. I'll be explaining how to make one next month.

Figure 3. FOIL SCREEN TWIN. A brilliant invention, very suitable for fixed installations, but not good for repeated flexing. Very easy to prepare and solder.

The other favourite installation cable is known as 'FST', standing for Foil Screen Twin (Figure 3). The brand I use is available with two 7/0.2mm conductors. But instead of having a copper wire screen, it has an aluminium foil screen. To connect the screen to earth there is a 'drain' wire, which is an uninsulated 7/0.2mm wire in electrical contact with the screen throughout the length of the cable. It is actually a much better cable than the one I described above, having a lower capacitance, making it good for use in long runs. It is more expensive, of course, around 30p per metre in 100m lengths.

FST cable is fairly stiff, which actually makes it very easy to install inside racks. (How to wire a rack? Details next month, or perhaps the month after, there is so much to talk about on this subject). The one thing you can't do with FST cable is to use it as you would an ordinary mic cable. It tends to kink and would soon become difficult to work with.

PRACTICALITIES



You are wiring up a studio and need some cable. What types of cable do you need? I'm going to run the risk of being unpopular with some manufacturers by talking about the brands of cable I use. There are alternatives, and I would urge you to investigate all the possibilities before deciding for yourself what to use. But once you have found cables that suit your purposes, stick to them. Being consistent can save a lot of time and energy. But whatever you do, avoid unbranded cables. There is little saving to be made, and the operation of your entire studio depends on these bits of copper and plastic.

Figure 4. CONDUCTIVE PLASTIC SCREEN is arguably less robust than lapped screen cable, but it provides a better coverage over the inner conductors for interference protection. It is slightly easier than lapped screen cable to solder to a connector.

For mic cable (remembering that this is the accepted term for cables that follow the use-then-coil-and-store routine) I invested in ten lengths of Musiflex, at around 60p per metre. Why ten? Because it is available in ten different colours and, believe me, this is a great help in finding your way around a bird's nest of cables. Musiflex is slightly unusual in that it has a conductive plastic, rather than a copper, wire screen. It's just as good in a physically non-hostile environment (ie. don't use it on a theatre stage if you can help it) and it's a brilliant coiler.

Also as a general purpose cable, I use Musiflex's relation, Phonoflex. It's similar in construction, but single conductor and thinner. The simple reason why I have to use it is that you can't get Musiflex through the hole in the back of a phono connector! It also coils very well for a thin cable, without kinking.

The two installation cables are Medium Single Round (what a name!) - that is the cheap cable I described above, available from a company called Electromail - and FST once again.

The one kind of cable I have no use for is multicore cable. This type of cable consists of several - perhaps as many as 32 - individual two-conductor-plus-screen cables, surrounded by one outer jacket. Much of the professional sound industry thrives on multicore cable, so why isn't it suitable for a home studio?

Most types of multicore are intended for use with multipin connectors. It isn't possible to fix the individual inner cables to phono, jack or XLR connectors as they are not, by themselves, mechanically robust enough to be exposed to the outside world. They need to remain snug in their outer insulation, or inside the hard shell of a multipin connector.

Some multicore cables have inner cables which can be split up and attached to individual connectors. But they are stiff and heavy, and really only suitable for use with firm-fixing XLR connectors. They would impose an excessive strain on a feeble phono. Also, when you work out the cost, you don't save much by using a multicore. It's much better to gather together individual cables into a loom (see next month).

CABLE CAPACITANCE

Just like the leaky hosepipe analogy, any output in your studio system has to provide enough electricity to drive both the input it is plugged into and the leaks - the capacitance - in the cable.

Any output has what is called an output impedance. This is, roughly speaking, a measure of how much current it can give, and is measured in Ohms (the more ohms, the less current output). A typical value of output impedance might be 800 ohms.

The capacitance of a cable also creates an impedance, measurable in ohms according to this formula:

Z=1/2πfC

where Z = impedance, f = frequency, and C = capacitance.

A typical cable might have a capacitance of 200pF (pF = picoFarad - a millionth of a millionth of a Farad) per metre. So one metre of this cable would have a leakage impedance due to this capacitance of approximately 80,000 ohms at 10kHz. The more cable, the more capacitance, the less impedance and the more leakage. So 100 metres of cable would have an impedance of 800 ohms, the same as the output impedance of the equipment. Guess what? You have just lost 3dB of signal at 10kHz, more at higher frequencies. I don't need to say that this is a bad thing, and in the real world there are other issues, such as input impedance, which complicate matters. But in practice, small studios need not worry too much about capacitance in cables, as long as it is not unreasonably high, and cable runs are not unnecessarily long (the interference will get you first!).


STORING CABLES

Figure 7.

Cables get very chummy when they are stored together, and like to inter twine themselves, making it difficult for you to sort them out. Here are three methods of keeping them separate:

1. Hang them on coat hooks, sticking strictly to the rule 'one hook, one cable'.

2. Use a short loop of rope or string (as shown in Figure 7) and hang several on a hook.

3. When you coil a cable, wrap a short length of masking tape around it. You can hang it on a hook, bung it in a box, or store it any way you like and your cables will stay tidy.


OTHER CABLE TYPES

QUAD. The ultimate in interference rejecting cables, for use in very electrically noisy environments, particularly where lighting dimmers are active. Diagonally opposite pairs of conductors are connected together, helping balanced equipment cancel out interference signals almost completely.

SPEAKER CABLE. Looks the same as 13 amp mains flex to me.


CABLE SUPPLIERS

Canford Audio, Crowther Road, Washington, Tyne & Wear NE38 0BW. 091-415 0205.
Electromail, P0 Box 33, Corby, Northants NN17 9EL. (0536) 204555.
(Electromail is the consumer arm of the RS Components empire. Their catalogue costs money, but it is essential reading for any sound engineer).
Studio Spares, 61-63 Rochester Place, Camden Town, London NW1 9JU. 01-482 1692.
Cable Technology, Unit 9, Enterprise Court, Park Farm, Wellingborough, Northants NN8 3UW. (0933) 674800.
Future Film Developments, P0 Box 3dG, 114 Wardour Street, London W1A 3DG. 01-434 3344.
(Klutz cable) Beyer Dynamic UK Ltd, Unit 14, Cliffe Ind. Estate, Lewes, East Sussex BN8 6JL. (0273) 479411.


Series

Read the next part in this series:
How to Set Up a Home Studio (Part 3)



Previous Article in this issue

MIDIsoft Studio Sequencer

Next article in this issue

Home Recording Techniques


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Jan 1989

Donated & scanned by: Mike Gorman

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Topic:

Home Studio

Maintenance / Repair / Modification


Series:

Setting Up A Home Studio

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9


Feature by David Mellor

Previous article in this issue:

> MIDIsoft Studio Sequencer

Next article in this issue:

> Home Recording Techniques


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