Squelch And Tones
Squelch is the circuitry in FM radios that prevents the user from having
to listen to noise or unwanted signals by muting the radio's audio output unless
the proper type of signal is received. The radio's manual will
explain how to use a squelch level control knob (like the typical knob
in the illustration (see PDF file)) or menu setting, common on
pocket-sized handhelds.
These are the different types of squelch and associated features:
* Carrier squelch, the simplest and most common type of squelch, mutes
the radio when no signal stronger than a preset settable threshold is
present.
* Tone receive squelch requires a specific sub-audible tone for the
received audio to be heard, as when accessing a repeater. This keeps
you from hearing other users on the same frequency, just like
"privacy codes" on the popular handheld radios that use the FRS and
GMRS channels. (You still have to listen to the channel before
transmitting.)
* Digital Coded Squelch (DCS) - a short sequence of tones must be
received at the beginning of each transmission before turning on the
output audio. DCS is used by groups sharing a frequency so that they
only hear audio from other group members. (Like tone-receive squelch,
listen before transmitting.)
* Call sign squelch - digital systems, such as D-STAR, send the call
sign of the receiving station along with the transmitted signal. The
station called will then be alerted to the incoming call.
* Attenuation - some radios attenuate the received signal when the
squelch threshold is set to high levels. This reduces interference
from overload interference where strong paging and commercial signals
are present.
* Monitor - an FM radio's monitor button or key temporarily defeats
or "opens" the squelch so that you can hear any station using the
channel. This is used to listen for weak signals or for other stations
before transmitting.
* All new radios can generate sub-audible repeater access tones. (These
are also called PL (or CTCSS) tones). Some have a feature called tone
scan that enables the radio to determine what access tones a repeater
requires by listening to the stations using it. This is very useful
when traveling or accessing an unfamiliar repeater.
* DTMF or Touch Tone. dual-frequency tones are used to dial phone
numbers through a repeater's auto-patch or to enter IRLP and Echolink
access codes. A radio's ability to store and play back sequences of
DTMF tones saves a lot of time when using either service.
--- SBBSecho 3.01-Win32
* Origin: The Thunderbolt BBS - wx1der.dyndns.org (57:57/10)
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