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Chapter 3 - Design of Party-Line Intercom Systems 37
TELCO = Telephone-lines in two-wire mode
Two-wire = Clear-Com, ROH, HME, R-Columbia, Protech, Theatre Techniques, Telex**,
some television cameras
Four Wire = RTS
™
ADAM
™
intercom, Philip Drake, Link, McCurdy, Ward Beck, ADM,
Farrtronics, PESA, Audix, Datatronics, all triax television cameras, some multi-core
television cameras, Radio-telephones, Telephone-Line circuits, Wireless Intercom systems
Carbon Mic Interphone = RCA, Daven, Video Aids, General Electric, Colorado Video,
many low-cost television camera intercoms
* Per Station
** Telex(r) Phase 2 = 300 ohm, 5 mW balanced line.
Some Practical Considerations
Headset Cable Lengths
The dynamic (low level) headset cable carries signal levels that differ by as much as 34 dB
+ 52 dB = 86 dB. Ordinarily, there are three types of unwanted coupling possibilities:
resistive (through a common ground), capacitive and inductive. Since separate grounds are
carried back to the microphone preamplifier and headphone amplifier, the common ground
resistive coupling is, in this design, negligible. The capacitive coupling can be made non-
significant by a 100% shield in the cable. The inductive coupling mode dominates in this
design, and can be offset in several ways:
• The distance between the microphone and headphone pairs can be increased, while the
mutual inductive coupling is decreased by the use of “ribbed” cable (two cables molded
together side-by-side).
• Both the microphone cables and the headphone cables can each be tightly twisted.
• Two or four separate cables can be run. A balancing transformer on the microphone
circuit may be used. Estimated, Safe Operating Distances are as follows:
• Single cable, two shielded twisted pair: 10 feet.
• Dual ribbed cable, two shielded twisted pair: 30 feet.
• Separate cables, shielded twisted pair in each: 50 feet and more.
• Balanced microphone input: up to 100 feet depending on cable used.
Table 3.2
Intercom comparisons.
Intercom
Type
Nominal
Impedance
(Ohms)
Impedance
Range
(Ohms)
Output
Type Mode
Estimated
Peak TX
Power (mW)
TX/RX
Levels
(dBu)
TW 200 50 to 400 Un-Bal two-wire 5 0 to -10
TELCO 600 600 to 900 Bal two-wire 1 0 to -10
Two-Wire 150 to 200 100 to 1k Un-Bal two-wire 0.7 -10 to -20
Four-Wire 600 600 to 10k Bal four-wire 7 +8
Carbon Mic 150* 4 to 150 Un-Bal two-wire 2 0 to -30