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Chapter 3 - Design of Party-Line Intercom Systems 41
user stations will normally continue to operate if one can only figure out a way to flip the
switches and touch only the knobs.
Moisture / Contamination Protection
If, in the field, a soft drink or something like it is spilled into the equipment, the equipment
can be dismantled and cleaned gently with clean water. After the equipment is dry it can be
returned to service. If this happens fairly often, residues in the water can be deposited on
the equipment. It should be noted that a build-up of contaminates and humidity can cause
audible noise on the intercom line. If it is likely that the equipment is continually to be
exposed to contaminating liquid, suitable plastic covers should be employed. It may also
be necessary to add a plastic coating as described above. When using equipment in the rain
always protect the equipment with plastic covers - also, make sure all cable connectors are
lifted out of the mud or snow and protected with plastic bags. Rain, mud and snow in
connectors can cause considerable audible noise in any communications system.
Magnetic Fields: Hum Problems
When the balanced type of intercom equipment is used, it is still possible to induce hum
into the system by placing or locating user stations or system interconnects near a hum
source, such as, power transformers or electrical switch panels or lamp dimmers. When the
microphone switch is turned on and a dynamic microphone headset is used, the dynamic
microphone is a sensitive antenna for magnetic fields. Often, operating personnel will go
on a break, leave the microphone on and lay the headset on equipment with power
transformers or near TV cameras or monitors with vertical deflection yokes. This is the
reason for the system microphone turn-off scheme (Mic Kill).
SUMMARY
(Defining and Meeting Your Needs)
1 Application Block Diagrams are a good starting place to define a system.
2 The generic block diagrams show a basic small system and how things plug together.
3 A generic system could used in a small television studio production, an outside
television field production (such as ENG and EFP) or an industrial test of a large system
(such as an aircraft).
4 A generic system can be created using almost any Party-Line system. Audiocom, Clear-
Com, and RTS TW systems block diagrams are shown.
5 A switch on the Audiocom PS2000 two channel power supply can combine channels
into one large Party-Line.
6 Equipment available from any one of the three illustrated manufacturers intercom
systems can be assembled into a two-channel system.
7 A two-channel system can be used for a small TV operation (Studio or Truck) or cable
access. One channel can be used for the director and crew, and the other channel can be
used as a public address or stage announce system. The stage announce system can cue
talent for the show, or allow the director to talk to the performing crew and talent during
rehearsals.
8 All three manufacturers make equipment suitable for theater applications use. Again,
one channel of a two-channel system can be used to cue the actors.