Service manual

OPERATION
3-29
April 2001
Part No. 001-9800-203
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.
3.9.6 PRIORITY GROUP SAMPLING
NOTE: Conventional priority groups are not sampled
while listening to an LTR call, when both system and
group scanning are off, or when transmitting.
The priority group sampling feature ensures that
messages on conventional priority groups are not
missed while listening to messages on conventional
non-priority groups. The sampling frequency of the
first priority group is programmable for 500 ms or 1 or
2 seconds. The second priority group is scanned at
least every 2 seconds. Call Guard squelch or external
squelch control is detected on each if it is
programmed. The first priority group is sampled while
listening to a message on a second priority group, but
not vice versa. When the message on the priority
group is complete, the transceiver returns to the
previous system/group, and if a message is still
present, it is received.
Priority groups can be programmed as follows:
• Fixed first and/or second priority system/groups can
be designated by programming.
• The first or second priority system/group can be
programmed to follow the selected system/group.
• No priority system/groups can be programmed. A
second priority group cannot be programmed if
there is no first priority group.
NOTE: The display messages and menu parameter
mentioned in the following information are not avail-
able with low tier models.
When a first priority group is selected or
displayed, is indicated in the display. Likewise,
when a second priority group is selected or displayed,
is indicated. When scanning, these symbols are
displayed only when a call is received on the particular
priority group. When a message is detected on the first
priority group, a tone sounds (if tones are enabled),
“PRIORTY1” is indicated on the bottom line for 2
seconds, and the priority message is received. Like-
wise, if a message is received on a priority 2 group, a
tone sounds and “PRIORTY2” is displayed.
If the menu PRIORITY parameter is available
(high tier models only) or an option switch is
programmed for the priority function, priority
sampling can be turned on and off by the user (see
Section 3.6.17). When priority sampling is enabled by
this switch, “PRI ON” is flashed in the display, and
when it is disabled, “PRI OFF” is flashed.
When a first priority system/group is sampled
while listening to a message on some other system/
group, a series of “ticks” may be heard. These ticks are
brief interruptions of the audio signal that occur when
sampling takes place. In some cases, the length of
these interruptions may increase to the point that
makes understanding a message difficult. This is espe-
cially true if Call Guard squelch is being detected. The
interruption could then be up to 420 ms. If a carrier
and incorrect Call Guard signal are detected on the
first priority group, the first priority sampling rate
changes to 2 seconds. This is called Adaptive Hole
Scan (AHS). Refer to the ATS description in Section
3.7.7 for more information.
3.9.7 EXTERNAL ENCODERS AND
DECODERS
General
Groups in conventional systems can be
programmed to operate with an external encoder/
decoder such as a two-tone or DTMF type. The