User's Manual

19
RELM/BK Radio
Code Guard Operation
Analog Squelch Control
Sub-audible signaling (CTCSS/CDCSS) is used to allow a group of
radios to be selectively called in an analog system. Programming the
receive code guard equal to zero allows for Carrier Squelch operation,
where the radio will unmute whenever a carrier is detected regardless
of the transmitted Code Guard.
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.
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 0659 ($293 hex).
The digital equivalent of carrier squelch is achieved by programming
the receive NAC = 3966($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.
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 TGID must match the channels programmed