Installations 1 – Analogue
(first published in “Sound Pro” magazine)Introduction
Installing sound equipment and interconnecting can be straightforward if good engineering principles are followed. If it is done well, all items of equipment will function to their full specifications. All manufacturers obviously measure the specification figures they publish with the equipment working to its optimum performance. The best installations will allow the equipment to achieve this but a few errors can make even the best equipment work badly! In this article only analogue audio signals will be discussed. In the second part digital audio signals and control & data connections and how both of these can affect the audio quality and the reliability of the system will be considered. The later article "One End Only" (written for Line Up magazine) provides an update which re-considers some of the arguments in the light of optimising screening from an EM compatiblity standpoint.
Degradations
The object of interconnections is to ensure that what leaves the output of one piece of equipment arrives at the input of the next without degradation. This can occur from three main causes. One is the addition of extra signals such as hum, other noise or cross coupling ("crosstalk") from other signals in the installation. The second is frequency selective and can produce resonances involving reactive elements in the input and output impedances of equipment and the capacitance of the cable. Cable capacitance can also create a low pass filter effect. Finally input and output levels must match each other.
Noise avoidance
Crosstalk and noise pickup is normally minimised by screening the circuits, though other techniques can also be applied. The diagram below shows an unscreened, but balanced audio connection.
Any noise or crosstalk induced into both "hi" and "lo" legs is cancelled by the balanced nature of the connections. When the induction is more into one leg than the other, the effect of the noise can be minimised if the impedance of Zo and Zi in parallel is very much less than the impedance between each leg and ground. Good equipment design will mean that a noise voltage induced in even just one leg will pass via the low leg to leg impedance to the other leg and so be cancelled by the balanced input.
Transformers can be an effective way to achieve good noise cancellation, but modern electronic input/output stages can also work well. Unbalanced connections have no such benefits so even greater care is required in the system installation to prevent noise problems.
Choice of cable type
For balanced connections, a two core and screen construction is most commonly used. Cable specifications often quote mechanical dimensions and core to core capacitance and these can be worth knowing. However, the likely noise immunity is dependent mainly on the type of screen and the tightness of the intercore twists. For example, if the two inner cores twist tightly, there is a better chance that any noise that is picked will affect both the "hi" and "lo" conductors to a similar extent and will therefore be cancelled by the balanced input stage.
This concept is taken further in a cable that has been used by broadcasters for many years but is not so often found in recording studios. "Star quad" cables use a tightly twisted four conductor structure with the support threads and fillers arranged to ensure that when the end cross section is viewed the conductors are appear arranged in a "star" pattern.
The cable is wired with opposite pairs linked together to give two parallel conductors running the length of the cable. This ensures that the maximum probability of matching noise coupling into both "hi" and "lo" legs. Using twice the number of conductors obviously this makes star quad more expensive and whilst there are advantages for analogue, the greater cable capacitance makes it unsuitable to digital signals.
There are further alternatives such as ribbon cable or multi-pair cable with only an overall screen. Both of these can work well but they make assumptions about what is going to be connected to the cable. For example, multi-circuit ribbon cable will only work adequately if it is driven from a very low source impedance and into a high load impedance. Crosstalk and noise pickup is then kept to acceptable levels by relying on the low output impedance to attenuate any voltage signal that attempts to couple into the circuit and a high input impedance to attenuate any interfering current signal. Unless some overall ribbon screening is provided, ribbon cables are best used only within shielded pieces of equipment. Doing away with individual cable screens saves one third of the cable terminations but only operates satisfactorily if the source and destination equipment have well balanced output and input impedances. It should never be used with unbalanced sources and loads!
All of these cables exist as single circuit types and as multi-circuit cables. The choice of which depends on how many circuits need to have a common start and finish point and on issues such as the size of the cable entrance on multipin connectors.
Cable screens
Choosing the right cable can reduce the risk of noise pickup for both balanced and unbalanced systems. Screening the cable obviously reduces the risk of noise getting into the interconnections but there are several types of screen and the choice depends on the purpose for the cable and on the budget! It should also be remembered that screening is important to prevent signals being picked up on the cable, but it can be equally necessary to prevent radiation of high frequency signals from the cable. This can be particularly important with control circuits and fast edged digital signals.
Lapped screen cables can be rapidly terminated and may easily be designed to have high flexibility. However, high flexibility can encourage their use in applications where they may often be flexed such as for microphone leads. They are not good for this because repeated twisting can cause the lapped screen to develop gaps. Even small gaps can begin to allow pickup - or radiation - from the cable.
The screening from double lapped screen cables is less likely to degrade after long periods of being flexed but foil and braided screens are most likely to maintain their performance. Unfortunately, simple braid designs tend to have gaps and unless a simple braid is combined with some other techniques, high frequency leakage can be a risk.
Foil screens provide a continuous screen but tend to be less flexible than lapped types. However, they can withstand considerable flexing, without any change in performance. Obviously terminating a conductive foil cannot be done directly, so cables contain a "drain wire". This is a non-insulated wire, in electrical contact with the foil which can be used for the screen connection.
Screen connections
Cable screens function by providing a convenient path to ground for the interfering signals and thereby reducing the risk they will couple into the audio circuit. All conductors have an inherent inductance and this can be sufficient to allow a significant voltage to be developed across even quite short lengths of cable screen. To get maximum benefit from the cable screen, it is necessary to connect it at both ends to the equipment ground connection. The traditional principle is that there should be only one ground path between any two parts of an audio installation so these two demands must be reconciled.
Consider the traditional wisdom of avoiding dual earth paths. This diagram shows all the four "earthy" elements of a typical piece of modern mains powered audio equipment. To ensure that the overall shielding is effective, the manufacturer should have linked the "XLR pin 1" point to the outer shielding of the equipment, which is normally also the protective case. For the safety earth to be effective, this must be linked to the safety earth in the mains cable.
The manufacturers of rack mount effects devices, tape recorders and amplifiers usually link the analogue and digital grounds internally, though alternative connection methods may be sometimes be provided for use in systems with a separate technical earth. Unless a comprehensive earthing plan is in place that requires them to be separated, any optional earth links should normally all be installed.
Earth loops
Earth loops are created when there are two alternative earth paths between two parts of a system. Simply connecting grounds through both the left and right signal paths between a mixer and an amplifier will create a loop. If both devices are also connected to a mains supply a further loop is created.
Whether a loop causes any adverse effects depends on whether a current flows in it. This can happen for two reasons. The first is if other currents are creating potential differences between two parts of the loop. A noise current can flow between the two "earth" points and through the cable screen. Most often, any noise from this will be related to mains frequencies and their harmonics, but this can also act as a loop aerial to pick up other interference.
This leads us to the second reason current may flow and is related to basic electrical theory. If a loop of wire has an alternating magnetic field passing through it, a current will be induced in the loop. The size of the current is directly proportional to the cross sectional area of the loop. If the loop is created only by two cable screens in a pair of cables running side by side, it is unlikely that the area will be sufficient for serious problems to occur, unless the cable was extremely long. However, when one side of the loop is through mains wiring and the other through signal wiring, the loop area can be very large. Problems are therefore most often due to loops which include mains wiring.
Breaking earth loops
It is essential for reasons of safety that all mains powered equipment has a mains (safety) earth connected. Installation problems must never be resolved by removing this connection. The traditional way to overcome a loop such as that shown above is to link cable screens at one end only. Most of the examples have assumed that the connections are balanced and floating but they remain valid in unbalanced systems. Balanced connections can make systems more resistant to signal degradation from which it follows that even more care is needed when circuits are unbalanced.
Loops through cable screens can be broken at either the input or output connections. Mistakes are less likely to occur if absolute consistency is used and the diagrams above assume that every output screen is broken. An equally reliable result can be obtained if all input screens are broken and output screens left connected.
An obvious exception to this principle is for microphone circuits as microphones do not normally have any earth connection except that through the mic. cable. Breaking mic screens will certainly cause problems with phantom power!
Electronically "balanced" outputs of some equipment may require a variation the recommendations above when connecting to unbalanced loads. This will occur when the design does not operate satisfactorily with one leg of the output grounded. Symptoms of this being a problem can include oscillations or output distortion.
The simple answer is then to disconnect the "lo" side of the balanced output, though this will often produce a 6 dB loss of signal level. A better answer is to use some method of balancing the unbalanced input, for example, a 1:1 transformer or proprietary interface unit.
Loops or full screening?
If equipment is to live up to the maker's claims for immunity from RF interference and cause the minimum of radiation, all screens must be connected at both ends. This of course also ensures the maximum number of loops! How this contradiction is resolved depends on the installation. If a complete building is being newly cabled certain steps can be taken to ensure that the loops do not create problems. When real difficulties are likely to be encountered is when replacing part of an existing installation. Starting by applying good principles is obviously desirable, but failings in the old system can affect the performance of the new installation.
Earth loops only cause problems when currents circulate in them. Some manufacturers have prevented loops occurring by making equipment which has the "XLR pin 1" connection (or the equivalent multipin connection) left open circuit! This is not good practice as it can lead to some cables being unscreened and may leave the system free from hum problems but very prone to picking up RF signals! A better, but not ideal arrangement, uses a small value capacitor to link the XLR pin 1" point to chassis. This means that for RF signals, the screen is grounded, but not for mains frequency signals. An essential element of a successful system is a high quality technical earth to all parts of the system.
The later article "One End Only" (written for Line Up magazine) provides an update to these arguments in the light of optimising screening from an EM compatiblity standpoint.
Technical earth design
True technical earth systems will involve a high quality earth source, designed by a consultant to suit the local soil conditions and can be an expensive item. However, the results can justify the cost if it saves troublesome fault tracing later on. Technical earths should normally be based on a "star" pattern. For a multi-studio complex, the technical earth is distributed to each control room. Within each control room the earth is distributed to the mixing console, the outboard equipment racks, tapes machines, amplifier/equaliser racks and possibly to studio wall boxes, depending what facilities they include.
The earth cable should never "daisy chain" from one piece of equipment to another. Each cable should go back to the local "star point". The gauge of cable from the earth source to the distribution buss bar in each studio will depend on the lengths involved but should usually have a cross section of at least six millimetres square. Wherever the earth is to be distributed, it should be done with a substantial metal buss bar, obviously isolated from any locally earthed metalwork.
When there is no possibility to create a true technical earth, a pseudo tech earth system can be devised. This can be kept relatively free of the most troublesome noise signals such as current leaking to ground from "dirty" sources such as the heavy duty electric motors used in lifts and air conditioning plant.
The pseudo tech earth is distributed within the studio area in the same way as a genuine technical earth. The "earth source" for a pseudo tech earth may be the incoming mains earth from the electric supply company. This can be substantially "cleaner" than relying on the earth that comes with the mains to the socket outlets in the control room as these may be based on "ring" techniques, rather than a star system. The technical earth should never take the place of the safety earth. The mains earth must also be distributed with the mains supply and local power safety regulations observed fully.
Matching
The topic of signal levels is a large one which cannot be covered fully here. However, a few interesting asides are worth noting. Many items of equipment have analogue outputs that are of a nominal "+4" level. This generally means that the manufacturer has calibrated the meters to read "0" for a signal level of +4 dBu. This is not an area with universal agreement and there is a rival "standard", most often used on lower cost equipment where the output level is -10 dBV when the meters read "0". -10 dBV is equivalent to about -7.8 dBu. This means that a "-10" machine will therefore give a signal level which is 11.8 dB less than a "+4" machine. This creates the superficially odd arithmetic that:
+4 - (-10)=12 (or very nearly)!
When +4 and -10 systems are to be interconnected a matching network is needed. Signal step down is straightforward and can be done using a network such as this.
The 330 ohm resistor may be reduced if more attenuation is needed. However, many connections are two way e.g. record and replay so amplification is also needed. The most effective method is then to use an active level converter.
Frequency response considerations
Electronically balanced, low impedance outputs are very much the norm on a large range of studio equipment. Unless extremely long cables are to be driven, most good quality cables will cause little effect to the frequency response though there are a couple of exceptions. The output impedance of most electronic drive stages can be considered primarily resistive at most audio frequencies, though output capacitors may have an effect at very low frequencies. When transformer coupled outputs are used, their inductance can have significant effects and some care is required.
The manufacturer will normally have assumed some typical length and type of cable and will have chosen the R-C network across the transformer secondary winding to give a useful degree of damping. However, when driving long or particularly capacitive cables, it can be worth checking the frequency response in the 20 kHz to 300 kHz region to ensure there are no resonant peaks.
Many microphones have transformer coupled outputs and this provides a good reason not to use overlong microphone cables. Remotely controlled mic amps can be a useful resource when long distances are unavoidable.
The second section of the article continues with notes on digital audio connections, control cables and keeping video and other "clocked" signals where they belong and not mixed with the audio!
All material is copyright PHM © 2004.
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