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Q&A Guide To Telephone Interconnects
Telephone Interconnects,
or phone patches, can add a high degree of functionality to just about any two-way radio
system. CES Wireless has been manufacturing top quality telephone interconnects and radio signaling
products since 1973. A major player in the wireless industry worldwide, CES
Wireless products are
in use in nearly every country, and are depended upon for communications in a variety of
climates and environments by public safety officials, the military, and countless other
commercial and amateur users.
CES Wireless products are designed to be flexible, cost-effective, and reliable. We also provide
the industry's highest level of support after the sale.
The following information is designed to assist you in understanding the theory,
operation, and installation of telephone interconnects:
Q - What is a simplex radio radio system?
A - A simplex radio system is one in which only one frequency is used for
both transmit and receive. For example, a mobile radio transmits on 150 MHz, to a base
radio which receives on 150 MHz. When a conversation is ongoing, each party must make sure
that the other party has stopped, or neither of them will hear the conversation. A simplex
radio system presents several unique challenges to a telephone interconnect. For starters,
you must assume that if the general public is calling into your system, chances are they
are not familiar with talking on a radio system. To ensure that they know when to talk,
most interconnects provide a 'turnaround',
or a 'Over' beep which is transmitted into the telephone line when the mobile user has
stopped talking.
Q - How does the telephone interconnect know which party is talking in a
simplex conversation?
A - To address this issue, there are three forms of operation of CES
interconnects:
- VOX control - VOX, or Voice
Operated Transmitting, works on the theory that the telephone interconnect will respond to
the source of the audio present, be it on the telephone line or on the base radio's
receiver. This audio will then be routed to the other party. VOX operation makes for a
smooth conversation, but a problem can arise when a constant audio source, such as music
on hold, is present on the telephone line, which will lock out the radio. Since some FCC
regulations require that the radio (rather than the phone line) have control of the
channel, straight VOX operation is normally not the best choice.
- Sampling Operation - Sampling
operation works on the theory of the telephone interconnect constantly 'sampling' or
switching between the telephone line and the radio's receiver to determine who is talking.
Although this method results in control of the radio channel at all times by the radio
user, there is a somewhat annoying 'swishing' sound every half-second or so.
- Veratec™ Operation - Since
both of the above modes of operation have their advantages and disadvantages, CES offers a
third mode on many of our interconnects which combines 'the best of both worlds' of
operation. Veratec signaling on the Model 4700VP, for example works as follows:
- In the initial stages of
making a telephone call, the mobile operator requests a dial tone from the base. During
this initial time period the Veratec signaling operates in the sampling mode, transmitting
the dial tone to the radio, but looking for DTMF digits to be received and dialed into the
phone line.
- After the number has been
dialed and a conversation is ongoing, the 4700VP begins operating in the VOX mode, when
one party speaks, that audio is transmitted immediately to the other party.
- When the 4700VP's
operation changes to the VOX mode, a timer is initiated (and reset) every time the mobile
radio user stops talking. If this pre-programmed timer expires (such as when music on hold
goes on for an extended time) the 4700VP's operation mode returns to sampling, to give the
radio operator the opportunity to speak, or to disconnect the call.
Q - What
is half or semi-duplex mode of operation?
A - Half or semi-duplex operation involves a 'repeater' at the base,
which receives on one frequency, and simultaneously transmits this audio on a different
frequency. The purpose of a repeater is to greatly extend the range of a radio system
since its generally tall antenna receives signals from distant mobiles or
handhelds, and
repeats that signal out usually at a much higher power. Range of a 20 watt mobile, for
example, can often be quadrupled by using a repeater. The repeater is by nature full
duplex in that it is capable of transmitting and receiving at the same time. Mobiles and
handhelds in this system, however, still can transmit or receive only one at a time, but
on different frequencies.
Q - What is different for phone patch operation on a half-duplex system?
A - A telephone interconnect operates much better in a half-duplex
system, because in most cases, when a call is initiated, the interconnect holds the
repeater's transmitter on, constantly transmitting audio from the telephone line out to
the mobile. The only switching function, therefore, is for the interconnect to shuttle the
audio of the mobile operator in and out to the telephone line. Since most telephone lines
are full duplex, the person calling in from the phone line has little indication that they
are talking on a radio system. Although both parties cannot talk and listen at the same
time, the operation is much smoother, and the radio operator has constant control of the
conversation in spite of continuous VOX operation. The 4700VP can also work on a
half-duplex system.
Q - What CES Wireless products are suitable for full-duplex operation?
A - CES Wireless no longer manufactures full-duplex
products.
Q - What is Carrier
Operated Relay, or COR?
A - Carrier Operated Relay, or COR, is not a part or a device, but it is
a term used to describe a logic point in a radio's receiver which changes state (from high
to low or vice versa) when the radio begins to receive a signal.
Although the preferred method for
interfacing a simplex interconnect is using Discriminator or Detector Audio, where
internal COR mobile detection is used, it may not be possible in cases where this audio is
only available as squelched-controlled audio in the receiver.
If this is the case, then External COR must be used as an input to the CES Interconnect
you are installing. Using the receiver schematic diagram, determine the circuit that
derives the squelch logic and verify that the signal is a DC level change of 0 to 5 or
more volts from a no-signal to a received signal condition. This can also be simulated by
operating the squelch control. It is not important whether this active state is high or
low, since all CES simplex interconnects can be configured for either one. Note the logic
level (an active high or an active low) when the receiver squelch is 'open' (unsquelched
or receiving a carrier). This is the active logic level that will be used in configuring
or programming the interconnect. Note: This logic signal must change very quickly in
conjunction with receiving a carrier and when the carrier drops. In some receivers there
are similar functions that may be used as a signal strength indicator, and are DC levels
that change too slowly, or that change proportional to the received signal level, and are
not suitable to serve properly as COR logic. Do not use this type of logic. Often, these
outputs are labeled on the radio schematic as 'S-Meter', or similar descriptions.
If you are not able to determine a direct source for squelch logic, and if the radio has a
'busy' LED indicator, then the logic signal applied to the LED may be suitable for COR. In
some circuits, the LED may be turned on with an open collector transistor to ground. If
this is the case, the interconnects COR connection would be made to the collector and
would be an active 'LOW' logic state. In other cases, the LED may be illuminated by a
transistor switching a source current path to the LED. In this case, connect the COR lead
from the interconnect to the high side of the LED and is a 'HIGH' active state when
carrier is received.
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