User's Manual

CONVENTIONAL FEATURES
21
Revised August 2002
Part No. 001-5300-007CD-NR
4.5.3 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 (210.7 Hz) are normally
not used because of their close proximity to the voice
band which starts at 300 Hz. In addition, tones 11 (97.4
Hz), 39 (69.3 Hz), 40 (206.5 Hz), 41 229.1 Hz), and 42
(254.1 Hz) are normally not used because they may
cause interference with adjacent tones.
A reverse burst is transmitted when the push-to-
talk switch is released and also detected when calls are
received. It is a 180-degree phase reversal for a period
of time determined by the tone frequency, and it elimi-
nates the squelch tail (noise burst) in the receiving
transceiver. Both the transmitting and receiving trans-
ceiver must be equipped with this feature for it to be
utilized.
4.5.4 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.
4.5.5 SELECTING CALL GUARD CODE
NOTE: Call Guard codes may be permanently repro-
grammed by keypad programming (see Section 4.14).
A different CTCSS/DCS/NAC squelch code can
be temporarily selected if the Squelch Code Select
option switch and a CTCSS/NAC code list have been
programmed. This feature allows the normal transmit
and receive Call Guard programming to be tempo-
rarily overridden with a code selected from this list.
The CTCSS/DCS/NAC list is programmed with
up to sixteen tone (CTCSS) or digital (DCS) Call
Guard codes. In addition, for operation on Project 25
channels, each position can be programmed with an
NAC code.
When the Call Guard code is changed using this
feature, it remains selected even if other channels are
selected. When scanning, the selected code also
applies to all scanned channels. If both analog and
digital (Project 25) channels can be selected or
scanned, the CTCSS/DCS code for the selected posi-
tion is used for analog channels and the NAC code for
the selected position is used for P25 channels. If a
channel is programmed for mixed mode operation, the
selective squelch type (analog or digital) programmed
for the transmit mode determines the selective squelch
type used.
Proceed as follows to select a preprogrammed
Call Guard code:
1. Press the Squelch Code Select option switch and
then rotate the Select switch to select the desired
position. The display indicates “SEL SQ xx” where,
“xx” is the selected code from 1-16.
2. To select the displayed code and return to the normal
display, press the Squelch Code Select switch again.
3. To check which code is selected, press the Squelch
Code Select switch once to display the current selec-
tion and then again to return to normal operation.
4. To return to the normal selective squelch codes,
select “DEFAULT” in this mode. The normal codes
are also automatically reselected whenever trans-
ceiver power is cycled or a talk-around channel is
selected.
4.6 PENALTY TIMER
A penalty timer may be programmed on conven-
tional channels to prevent transmissions for a short
period of time after the time-out timer disables the