Cadac F-Type - Manual (Page 97)

APP-I
Revision F2005-2 F-Type
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In live sound applications, it is normal for the console to be situated a long way from
its sound sources and amplifier/loudspeaker system. This inevitably means cable
runs of many hundreds of metres. In order to minimize noise and interference pick up
on the cables, it is generally agreed that the “balanced line” connection system is
essential. On the R-Type console, all inputs and outputs are balanced and this
includes the
insert points.
Great attention is paid to the construction and layout of all printed circuit boards used
in the console. It is relatively simple to design high performance circuits that work
perfectly on the bench, but it is somewhat more difficult (and costly) to engineer the
printed circuit boards in a way that allows this performance to be repeated in produc-
tion. CADAC use layout techniques that have been developed over the years to mini-
mize variations in production quality and in the case of logic circuits, to absorb any
interference that may be generated inside the console.
The part of a console that is most prone to interference pick up is the mixing bus.
Mixing bus wires have dimensions large enough to allow them to be one quarter of a
wave-length of many medium wave radio transmissions. In other words, a mixing bus
can be an antenna for radio reception. Since the airwaves are becoming ever more
polluted with radio transmissions, CADAC use a balanced bus system for all mixing
requirements.
In general, analog audio circuits do not transmit unwanted signals into the surround-
ing air space (unless a poor design or component fault causes a circuit to produce
oscillation). On the other hand, using a computer (embedded processor or controller)
to handle any form of automation always produces a wide range of unwanted high
frequency radiation. Some artifacts from this can cause interference to the audio
system and radiate interference to external equipment. CADAC designs are engi-
neered to minimize any potential interference generated inside the system and to
prevent radiation into the environment. The techniques used also have the advan-
tage of making the system immune to external interference.
On the 1st January 1996, new European legislation known as the EMC Directive
came into force. The EMC Directive requires that all electronic equipment manufac-
tured or imported for sale in the EEC must not emit electromagnetic interference that
can impair the performance of other systems or sub-systems. Similarly, the product
must also be immune to a wide range of natural and man-made electromagnetic
interference in its operating environment. Many countries outside the EEC are also
developing or have already put into place similar legislation. In order to conform to
these rules, a number of changes were required to the basic design of our consoles
and modules. One of the main changes is:
All cable ‘screen’ connections are bonded to the frame.
The figure below shows the required connection scheme for XLR connectors.
Please note that in the case of ‘line’ or ‘free’ connectors (XLRs on cables), the 'pig-
tail' formed from the cable screening braid should be as short as practical and sol-
dered to the
shell ground tag. Pin 1 is then connected to the 'pigtail' approximately
halfway between the shell ground tag and the start point of the cable screening braid.
The twisted pair should be left 'twisted' right up to the signal solder tags.