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feature, "BUSY" is displayed, a busy tone sounds, and
the transmitter is disabled.
The Busy Channel Lockout feature can be
programmed to operate as follows. Each conventional
channel can be programmed differently.
Off - Busy channel lockout is disabled and the trans-
mitter keys even if the channel is busy.
Noise - If a carrier is detected on the channel, the
transmitter is disabled when the PTT switch is pressed.
Tone (NAC) - If an incorrect Call Guard (CTCSS/
DCS) or NAC code (see Section 5.17) is detected, the
transmitter is disabled when the PTT switch is pressed.
An incorrect code is any code other than the one
programmed for the current channel.
If Busy Channel Override is permitted by
programming, it is possible to transmit even when the
transmitter is disabled by this feature. Simply release
the PTT switch and then quickly press it again.
5.5 CALL GUARD SQUELCH
5.5.1 INTRODUCTION
Tone or digital Call Guard squelch (also called
CTCSS/DCS signaling) can be programmed on each
conventional analog transmit and receive channel in
any order desired. The reverse burst and turn-off code
are always transmitted and also detected on channels
programmed with Call Guard squelch.
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 radio to be func-
tional.
5.5.2 CALL GUARD SQUELCH ENABLE/
DISABLE
The Normal/Selective option switch or menu
parameter (if programmed) can be used to disable
CONVENTIONAL MODE FEATURES
receive Call Guard squelch on analog channels or
group ID code detection on P25 channels. When selec-
tive squelch is disabled, "Sq Normal" is flashed in the
display, and when it is enabled, "Sq Select" is flashed.
When "Normal" is selected, the receiver
unsquelches only if a carrier is detected. Scanning and
Project 25 NAC detection are not disabled with this
mode selected. The selected mode is in effect until it is
manually changed again. Selecting another channel or
cycling power does not reselect a default condition.
There is a programmable option to display the monitor
icon when the "Normal" mode is selected.
5.5.3 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
radio. Both the transmitting and receiving radio must
be equipped with this feature for it to be utilized.
5.5.4 DIGITAL CALL GUARD SQUELCH
Digital Call Guard squelch (DCS) 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
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