10 BK Radio
APCO PROJECT 25 SQUELCH CONTROL
Network Access Codes (NACs) provide the digital
equivalent of analog sub-audible signaling
(CTCSS/CDCSS) allowing a group of radios to be
selectively called within a system.
Users in the same area (using the same NAC) can be
further divided into Talk Groups, with each group
having its own Talk Group ID (TGID). Group Calls are
made by designating both the users’ NAC and TGID.
Each radio also has an individual P25 unit ID. A Unit-
to-Unit call contains the addressee’s NAC, and uses
the addressee’s P25 unit ID instead of the TGID.
When operating in Digital Mode, each channel can be
programmed to use either Normal squelch or Selective
squelch.
1. Normal Squelch
Normal squelch is used to mimic analog operation.
Signals are only qualified with the programmed
NAC. TGIDs and P25 Unit IDs are ignored. Each
digital channel is programmed with a receive NAC
and a transmit NAC. When an incoming signal’s
NAC matches the channel’s programmed receive
NAC, the radio unmutes. The default NAC is 659
($293 hex). The digital equivalent of carrier
squelch is achieved by programming the receive
NAC = 3966($F7E hex) the radio will unmute when
a digital signal with
any NAC
is detected. The
3966 ($F7E hex) NAC is reserved for receivers
and is not allowed as a transmit NAC.
2. Selective Squelch
Selective squelch is used for processing ‘Group
Calls’ and ‘Unit-to-Unit Calls’. TGIDs are assigned
on a per-channel basis. Users can be separated
into Talk Groups with each group having its own
TGID. Then, on channels programmed for
Selective squelch, the incoming signal’s NAC and
DMH Owner’s Manual 31
transmit on the knob-selected channel with the
emergency bit set, indicating an emergency condition.
If the Channel Selector is changed, the Emergency
Mode will follow to the newly selected channel. Cycle
power to return the radio to normal operation.
On channels programmed for analog transmissions,
and channels programmed for Mixed Mode
transmissions with the ‘TX Digital’ switch OFF,
pressing PTT in Emergency Mode will result in a
normal analog transmission.
USER SELECTED CHANNEL GUARD
User Selected Channel Guard is only available on
radios equipped with a keypad microphone. Not all
microphones support this function. Contact your
dealer to determine which features are available with
your microphone and have been enabled in the radio.
When the radio is being programmed with transmit
and receive frequencies for each channel, a receive
Channel Guard value and a transmit Channel Guard
value can also be assigned to each channel. If User
Channel Guard Selection is enabled, the Channel
Guard values for any channel can be copied to
another channel in the radio. On channels
programmed for Analog operation only, the CTCSS or
CDCSS guard values will be copied. On channels
programmed for Digital operation only, NAC’s only will
be copied unless the radio is programmed to also
copy the Talk Group ID (TGID), Mode (Digital, Analog
or Mixed) and Squelch setting (Normal or Selective).
On Mixed Mode channels, Analog or Digital values will
be copied as needed.
For example, to use the Channel Guard values of
Channel 9 with the frequencies of Channel 5:
1. Turn Scan and Priority Scan OFF.
2. Turn the Channel Selector knob to Channel 5.
3. Press
the
[9]
key on the microphone keypad. The
display shows
CG
.
The radio will now operate on the frequencies of
Channel 5 with Channel 9 Channel Guard values.
The display shows the Channel Guard channel (9),
and then the selected channel (5).
