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From: jim@infoswx.UUCP
Newsgroups: net.ham-radio
Subject: Re: CTCSS, DTMF, and PL tones??
Message-ID: <3200024@infoswx.UUCP>
Date: Wed, 17-Oct-84 10:08:00 EDT
Article-I.D.: infoswx.3200024
Posted: Wed Oct 17 10:08:00 1984
Date-Received: Sun, 21-Oct-84 09:14:16 EDT
References: <122@sask.UUCP>
Lines: 38
Nf-ID: #R:sask:-12200:infoswx:3200024:000:1735
Nf-From: infoswx!jim    Oct 17 09:08:00 1984

Pete -- 

Here's a brief explanation of PL and CTCSS.

First of all, they are one in the same. CTCSS stands for 'Continuous Tone
Coded Squelch System'. PL stands for 'Private Line' and is a registered
trademark of Motorola, Inc. Another name commonly used is 'Channel Guard'
which is a General Electric trademark.

Now for the technical details (I'll use the term PL 'cause it's short!) --

Basically PL is nothing more than a sub-audible, low amplitude tone that
is transmitted along with the normal speech audio. When heard by a receiver
that is equipped with a PL decoder, the sub-audible tone will be detected
and the receivers' audio output stage will be activated. If the transmitted
signal does not contain the proper PL tone (there are 37 of them), the 
receiver squelch won't open, hence no audio thruogh the speaker. This 
scheme is primarily used by commercial two-way and public service radio
services, and in fact is required by the FCC in those applications. PL is in
use by amateurs in the US to a small degree, mostly on repeaters in the 440-
450 MHz band to alleviate intermodulation problems. In commercial applications
another advantage is that a large number of users, i.e., Joe's Plumbing,
Speedy Courier, and Pink & Green Taxicab, can all share the same frequency
(repeater) with each using a different PL tone. With this scheme, Joe's base
station and his fleet of plumbing trucks will only hear calls from each
other and not be bothered by the other users of the repeater.
There are some details of actual operation that I
have not gone into here, but they are not really necessary in this basic
outline.

Hope this has helped.

73's,
Jim Gerke, KC5OA
Infoswitch Corp.
Richardson, TX
allegra!convex!infoswx!jim