Multicore systems
Multicore cables are used between the stage and the mixing board required to process the sound for professional live sound technology. These cables are made up of several sound wires, stranded and shielded in pairs, and surrounded by a common insulating jacket. The cable is designed for tough road operations.
Based on individual installation requirements, in dependence of location and stage construction, it has to be flexible and torsion-free. Regardless of whether the cable heats up to 50 °C at the open-air festival during the summer or cools down to -20 °C on the truck in the winter, these properties should be maintained. Of course it should also resist tension and the heavy boots on stage.
The cable is specified to transmit audio signals. Problem-free transmission of sound signals broad level spectrum should be possible. And that also depends on the connected sound sources.
Signal sources on the multicore:
Dynamic microphones achieve a sound level of approx. -50 dBu to approx. -35 dB at a source impedance of 200 Ohm, in dependence of the acoustic pressure of the instrument or the vocal voice to be recorded.
Condenser microphones have an integrated amplifier, thus the transmission range is approx. -40 dBu to -30 dBu, in dependence of the acoustic pressure. Here the multicore also has to transmit the phantom supply from the mixing board, 48 VDC from the amplifier. Proper grounding has to be provided.
During stage operation DI-boxes are the interface between high-level LINE-outputs from e.g. keyboard or instrument amplifiers. To avoid humming loops, systems with integrated transmitters should preferably be used here. Depending on the source, DI-boxes can achieve a sound range of approx. -40 dBu to -20 dBu. Here it always must be considered that the transmitter level should be stepped down to prevent too large a deviation in sound level between the individual conductor pairs. This prevents annoying crosstalk effects especially on multicore plug connectors.
LINE-level outputs are at high level range, i.e. nominally a sound level of approx. +4 dBu, and depending on the accuracy of modulation, up to +12 dBu with short peaks (transients) should be expected.
Crosstalk
All sound technicians know this phenomenon - when they switch on their headset via PFL to a mixing board channel that should only transmit the LINE-signal for the
synthesizer, and end up listening in on quite a bit of other stuff in the background. Of course, acoustic crosstalk cannot be avoided altogether, depending on the number of microphones and their set-up location on stage. As a consequence of this experience a lesser number of microphones may be used and they can be replaced by DI-boxes.
But aside of acoustic crosstalk, static or inductive crosstalk, only noticeable on the mixing board, is also possible. Different factors are responsible, which, if they are considered ahead of time, will not be able to completely abolish crosstalk, but at least reduce it to a minimum.
First: For a multicore cable the specification
- stranded and shielded in pairs -
does not necessarily abolish the crosstalk effect to 100 %, but it gets rare with a consistent use of balanced cables between source and mixing board. Mixing boards have, depending on manufacturer and price range, crosstalk damping of max. 90 dB between the individual channels (at 1 kHz and nominal level).
The multicore system does not have to be any better than that, but is also should not be any worse.
The eye of the needle is the plug connector at the end of the multicore system:
Fig. 01 72-pin connector; crimp contacts
As shown in the picture above, the plug connector housing is made of plastic (see Appendix). Each wire of the individual cable inside the multicore is crimped onto a socket or plug contact. These contacts are then inserted into the guide holes on the plastic housing and locked in place. The axis-symmetrical design results in the fact that the plastic housing on one hand isolates the contacts from each other, but that simultaneously a multiple capacitor is created. Regardless of how we position the individual wires, theoretically the same happens as if we intermesh the individual wires with small capacitors.
This may sound dramatic, but during practical operation it will only become a problem if we, as described above, allow excessive level deviations on the individual conductors. Due to the geometric construction the capacitive values for the individual capacitors are almost the same, and in dependence of the frequency they only amount to a few pF.
We have learned that a capacitor is impervious to D.C. voltage and that its capacity reactance decreases if the frequency increases. However, the crosstalk damping values on our plug connectors at a frequency of 1 kHz are sufficient, which is significantly better than what can be said of most mixing boards.
Also, an insufficient quality of the mechanical craftsmanship on multicore plug connectors can also create significant crosstalk effects.
Our wiring diagrams are available under Support/Wiring diagrams.
Cable shielding - centrally cross-meshed or separate?
The optimization of crosstalk damping is reason enough to prefer separate grounding.
The multicore cable could be understood as an extension cable for the balanced sound cables that connect the sound sources on stage to the inputs on the mixing board in the hall. All conductors inside the multicore are stranded and shielded in pairs, and they have their very own insulating jacket.
For that reason it is logical and rational to stay with the advantage of the extension from plug connector on stage via mulitcore all the way to the plug connector on the mixing board.
A further argument is based on the frame potential, meaning the point in the system where the shield/shields are connected to the ground, and thus inevitably to the ground for the low voltage power supply at the event site. Normally this should be the input on the mixing board. It is the central location where all the shields of a multicore system are inevitably meshed with each other. Here all shielding connections are connected to 0 V = frame potential internal power supply and thus protective ground, regardless of whether we are talking about XLR or jack.
Only panels used in broadcasting facilities and those within the higher priced range have an option for independent grounding (FPE), which however is also connected with the protective ground for the building.
If balanced cables with separate, i.e. individual wire shielding, are consistently used for routing the XLR-input socket on the stagebox will precisely match the mixing board input that it is connected to. If a capacitor microphone is connected and the phantom supply for this channel is activated, the operating voltage for the microphone will only flow through this channel.
Separate shielding is even more advantageous for the connection of mains-operated sound sources. In most cases you can assume that the protective ground on the Schuko plug does not exactly match that of the Schuko plug on stage in the hall where the mixing board is set up.
If a mains-operated unit is now connected to the output on the stagebox, an equalizing current may be created via the shielding on the audio cable, resulting in a typical humming loop. If the shielding is separate, most of the time only one channel is affected. It is relatively easy to find the fault and a connected DI-box with GND-lift (Pin1 XLR separate) will eliminate the problem. But if all shielding on the stagebox or in the plug connectors is interconnected and several mains-operated sources are connected, then the humming noise may multiply. Thus, ground equalizing currents are not only discharged via the panel, but also via the shielding among the connected units.
A very dangerous trick is commonly used by "stage practitioners". Very often they tape off or disconnect the protective ground on units or power cables. This may lead to immediate success, but it also means LETHAL DANGER.
The argument that meshed shielding saves money, because only the
a - conductor and the b - conductor on a cable need to be crimped and more sound wires per plug connection can be utilized, is a fallacy. Neither the plug, nor the cable or crimping contacts cost a whole lot of money. It is human labor to twist, solder, and connect the shielding that makes up the major cost of a cable. Very often the shielding is connected to the protective ground on the plug connector. Where else? But here the safety aspects are neglected once again. The manufacturer identifies the connection on the plug as CHASSIS = protective ground. The plug housing is conductive and must, accord. to VDE 0100, be connected to the protective ground. All other options are prohibited, and this rule must be observed. Shielding has no business being connected here.
The construction of stagebox and multicore systems should not only consider signal transmission. This is especially true for a number of amateur and hobby musicians who often loose the fun in showing off their talents in public because of the - so-called crappy technology. We are talking about tough live operations that cannot afford hours of trouble shooting, which can easily be avoided with careful planning and a bit more, wisely spent money.