OPERATION
2-26
April 1999
Part No. 001-9800-600
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
2.11.1.
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 3-2).
A reverse burst can be transmitted when the push-to-
talk switch is released to eliminate the squelch tail
(noise burst) in the receiving transceiver. 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.
2.11.6 EXTERNAL ENCODERS AND
DECODERS
External encoders and decoders are not currently
supported by the operating software.
2.12 SUPERVISORY TONES AND DISPLAY
MESSAGES
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 2.5.15).
If the menu parameter is not available, these tones can
be selected only by the programmer.
Busy Tone - This tone is similar to the standard tele-
phone busy tone, and it indicates that the 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 on the lower line of the display
when it sounds. It does not sound on conventional
systems.
No more access attempts are made once this tone
sounds. The PTT switch must be released and then
pressed again to make another attempt. Therefore, it is
not possible to access the system by holding the PTT
switch down with this tone sounding.
Intercept Tone - This is a siren-like tone (alternating
high and low tones) consisting of 700 Hz and 800 Hz
tones alternating at approximately a 2 Hz rate. This
tone indicates the following no access and error
conditions:
l
No Access - If this tone sounds 2-3 seconds after
pressing the PTT switch and “NO ACCES” is
displayed, the data handshake with the repeater
could not be completed. The usual cause is an out-
of-range condition. Six attempts are made before
this tone sounds. No more access attempts are then
made until the PTT switch is released and pressed
again.
