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34 Handbook of Intercom Systems Engineering
Some of these are:
1 Mode 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.
2 Level 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.
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:
1 Television camera intercoms to intercom systems.
2 Two-wire systems to four-wire systems.
3 Full duplex systems to simplex systems.
4 When transmission medias change.
Interfacing Issues
There are three tasks to interfacing:
1 Mode Conversion.
2 Level Problems.
3 Signal / Data Conversion.
Table 3.1
Typical system impedances and ranges.
Intercom or
Audio System
Nominal
Impedance
(Ohms)
Nominal
Level
(dBu)
Level
Range
(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