Telex 38109-977 manual Typical Interfacing Problem, Interfacing Issues

Some of these are:

1Mode differences. There are several not directly compatible modes of operation: two

wire mode, four wire mode, full duplex mode, half duplex mode, simplex mode. Examples: the TW System is two-wire full duplex, the ADAM™ matrix is four-wire full duplex, the telephone is two wire full duplex except some long distance calls are half duplex (both people cannot talk at once), a walkie talkie is simplex, AudioCom is two-wire full duplex, Clear-Com is two-wire full duplex, office intercoms are often simplex operation.

2Level and Impedance differences. System voltage levels range from - 40 dBu to + 21 dBu with peaks to +28 dBu (where 0 dBu = 0.7746 volts). See Table 3.1, for typical ranges.

Table 3.1 Typical system impedances and ranges.

	Nominal	Nominal	Level
Intercom or	Impedance	Level	Range
Audio System	(Ohms)	(dBu)	(dBu)
Telephone	600 to 900	-15	-40 to 0
Old Clear-Com	200, 10k	-30	-45 to -15
New Clear-Com	200, 10k	-14	-14 to +5
Audiocom	300, 10k	0	-8 to +1
RTS™ TW	200, 10k	-10	-10 to -1
Recording Studio	600, 10k	+4	-6 to +24

There are different modes of intercom operating modes because each mode offers a different advantage for different needs and situations. For example, two-wire is quick and easy to hook up, while four-wire is easier to interface to other systems.

A Typical Interfacing Problem

A television camera uses a triax cable to connect the camera to the rest of the electronic system because a triax cable allows operation over longer distances with more consistent quality. This is because the triax cable uses radio frequencies to transmit information both ways on the cable. This is, in effect, four-wire (two path) communication. The following implementations often need interfacing:

1Television camera intercoms to intercom systems.

2Two-wire systems to four-wire systems.

3Full duplex systems to simplex systems.

4When transmission medias change.

Interfacing Issues

There are three tasks to interfacing:

1Mode Conversion.

2Level Problems.

3Signal / Data Conversion.

34 H a n d b o o k o f I n t e r c o m S y s t e m s E n g i n e e r i n g