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36 Handbook of Intercom Systems Engineering
Interfacing Practice
Interfacing Television Camera Intercom Systems to TW Systems
General Camera Configuration Information for Television Cameras (except ENG
units)
Television cameras used in broadcast and industry usually have two parts: a camera head
and a camera control unit (CCU). The camera head assembly usually contains the lens
equipment, camera electronics, and triax adapter (if used). The CCU contains additional
electronics for processing video, the other end of the triax adapter, an interface for
microphone audio, and the intercom interface. The intercom interface usually incorporates
switches and electronics so that the intercom can be two-wire or four-wire.
The Problems in Interfacing to Cameras
There are two problem areas in television camera intercoms:
1 The electronics in the camera head.
2 The intercom interfacing electronics at the CCU.
Some possible problems with the camera head intercom electronics are as follows:
• Inadequate headphone drive (Not loud enough for athletic contests and studio
shows)
• No limiter in the microphone preamplifier (level variations are too much)
• The headphone and the microphone share a common circuit return conductor
(headphones oscillate when volume is turned up)
• The Triax Adapter / electronics does not give the camera intercom enough
headroom, so there is a trade-off between signal clipping and signal to noise
ratio.
• The microphone on/off switch does not disconnect the microphone preamplifier
thus adding noise to the system.
• Some possible problems with the CCU intercom interface electronics are:
• An earth ground is applied to the wiring usually in two-wire mode (causes hum
loops in the system)
• The four-wire input to the camera is not bridging impedance
• The two wire “RTS0153 Systems compatible” interface loads the line
• No safety capacitors are installed in the CCU, thus causing burnt transformers if
connected to the intercom line
Alternatives for Interfacing to Television Cameras
1 Bypass the camera, tape a microphone cable to the camera cable, and plug a TW belt
pack in at the end.
2 Use the existing camera intercom, interface it to the TW system with a Model SSA324
or SSA424 interface (if camera intercom is four-wire).
3 In multi-core connected cameras, use the camera wiring to allow a TW belt pack to be
plugged into the camera head. This allows the camera operator to use a portable User
Station mounted on his belt or attached to the camera body. (
Note:
This requires significant modification to the camera head and CCU)