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option to disable the reverse burst or turn-off code on
transmit channels programmed with Call Guard
squelch. The reverse burst or turn-off code are always
detected on receive channels programmed with Call
Guard squelch (if it is sent).
The Call Guard squelch feature eliminates
distracting messages intended for others using the
channel. This is done by using a subaudible tone or
digital code to control the squelch. This tone or code is
unique to a user or a group on that channel. This tone
or code is transmitted with the voice signal but is not
heard because it is in the subaudible range and is atten-
uated by a filter. Call Guard squelch must be used in
both the transmitting and receiving transceiver to be
functional. When the channel is monitored before
transmitting, Call Guard squelch may be temporarily
disabled by the monitor mode described in Section
2.11.2.
Tone Call Guard Squelch
Tone-type Call Guard squelch utilizes subaudible
CTCSS tones from 67-254.1 Hz. Although there are
42 tones assigned, those above 33 are normally not
used because of their close proximity to the voice band
which starts at 300 Hz. In addition, tones 11, 39, 40,
41, and 42 are normally not used because they may
cause interference with adjacent tones (see Table 3-6).
A reverse burst (turn-off code) can be transmitted
when the push-to-talk switch is released to eliminate
the squelch tail (noise burst) in the receiving trans-
ceiver. However, both the transmitting and receiving
transceiver must be equipped with this feature for it to
be utilized. The reverse burst is a 180-degree phase
reversal for a period of time determined by the tone
frequency.
Digital Call Guard Squelch
Digital Call Guard squelch (CDCSS) uses digital
data instead of subaudible tones to control the squelch.
This data consists of continuous repetitions of 23-bit
words. No bit or word synchronization information is
used. When the push-to-talk switch is released, a turn-
off code is transmitted which eliminates the squelch
tail similar to the reverse burst.
Although there are thousands of possible code
combinations with 23 bits, only 83 are unique with the
data scheme used. The number specified when the
code is programmed is actually a seed for a special
algorithm used to generate the 23-bit data word. The
data is transmitted at a rate of 134.4 bits per second.
Therefore, approximately six words are transmitted
each second. When the data is decoded, 23-bit samples
are taken and then the bits are rotated to determine if a
valid code was received.
Digital Call Guard squelch can be programmed as
normal or inverted. The only difference is that the
waveform is inverted when "iDCG" is selected. The
inverted type may need to be programmed if the signal
is inverted by the repeater or another transceiver. If
digital Call Guard squelch does not function, try
changing to the other type.
2.11.7 EXTERNAL ENCODERS AND
DECODERS
External encoders and decoders are currently not
supported by the LTR-Net operating code.
2.12 SUPERVISORY TONES
2.12.1 GENERAL TONES
The following tones are heard at various times
when operating this transceiver. Some or all of these
tones can be disabled by the user if the menu
"TONES" parameter is available (see Section 2.4.12).
If the menu parameter is not available, the tones mode
cannot be selected by the user and is fixed by
programming.
Busy Tone - This tone is similar to the standard tele-
phone busy tone, and it indicates that the LTR-Net or
LTR radio system is currently busy. It is produced by
turning combined 480 and 620 Hz tones on and off at a
4 Hz rate. This tone sounds with all types of LTR-Net/
LTR calls (even if the proceed tone is enabled), and
"BUSY" is indicated in the lower part of the display
when it sounds. It does not sound on conventional
systems.
Continued access attempts are made while this
tone sounds. Therefore, it is possible to access the
system by holding the PTT switch down with this tone
sounding.
2-28
OPERATION
November 2001
Part No. 001-7240-001
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