Telex 38109-977 manual Some Practical Considerations, Headset Cable Lengths

Table 3.2	Intercom comparisons.
		Nominal	Impedance			Estimated	TX/RX
Intercom		Impedance	Range	Output		Peak TX	Levels
Type		(Ohms)	(Ohms)	Type	Mode	Power (mW)	(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

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.

C h a p t e r 3 - D e s i g n o f P a r t y - L i n e I n t e r c o m S y s t e m s 37