From: ornitz@kodak.kodak.com (Barry Ornitz)
Subject: Re: Continuous Tone Signaling on CB
Followup-To: rec.radio.cb
Summary: OK if done correctly
Keywords: CB, CTSS, selective calling, tone actuated squelch, Part 95
Organization: Eastman Chemicals Company
Distribution: usa
Date: Wed, 7 Oct 92 20:46:38 GMT

In article <laird.718485613@stable.ecn.purdue.edu> laird@stable.ecn.purdue.edu 
(Kyler Laird) writes:
>We're too cheap to buy a bunch of FM transceivers for our farm,
>so we're stuck with CB's.  I'm tired of listening to the crap
>that comes in off of I-65, so I thought I might try adding CTS
>to all of the units (~10).  A few questions come to mind...
>	1. Is CTS legal on CB frequencies?
>	2. What is the best CTS frequency to use?
>	3. Can I use a 555 alone to generate the signal, or will
>		I need filters.
>	4. How do I detect and filter out the CTS on the receiver?
>	5. Are there circuit board level devices available
>		(inexpensively) that will perform these functions?
 
This is covered in Part 95 Paragraph 412(b) of the FCC Rules and Regulations
which states:
       "You may use your CB station to transmit a tone signal only when the
	signal is used to make contact or to continue communications. (Examples
	of circuits using these signals are tone operated squench and selective
	calling circuits.)  If the signal is an audible tone, it must last no 
	longer than 15 seconds at one time.  If the signal is a subaudible
	tone, it may be transmitted continuously only as long as you are
	talking."

So, yes continuous tone squelch and tone burst type systems are legal.  CTSS
systems usually use tones below 250 Hz.  This is below the normal bandpass of
the CB transmitter audio and pumping the CTSS tone directly into the microphone 
jack will likely not work.  Addition of such a system to a CB would require
inserting the tone directly into the modulator circuit; this would require the
modifications to be performed by a FCC commercial class licensee.  It would be
easy to design a 555 circuit to generate the tones, but the detection and
audio squelch circuitry would be far more complex.  Sub-audible squelch systems
have never had a good reputation when used in AM modulation systems; they
work extremely well in FM systems however.

A much easier method would be to use a tone burst selective calling system.
A two-tone system would give you some flexibility and could be easily built.
All the audio tones would be within the normal voice frequency range.  The
tones could be inserted at the microphone jack and detected at the speaker.
No transmitter modifications would be required, hence you could do this work
yourself.

Commercial systems are readily available.  You can often find advertisements
in many amateur radio magazines.  You local two-way radio shop may also sell
aftermarket systems.  At one time Exar Semiconductor had some good application
notes on tone encoders and decoders.

A side note to an earlier question about "CB noisemakers":
In 95.413, it states that it is prohibited for a CB station...
       "(6) To transmit music, whistling, sound effects or any material to
	amuse or entertain;
	(7) To transmit any sound effect solely to attract attention."
					73, Barry
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|  ___  ________  |       Dr. Barry L. Ornitz          WA4VZQ
| |  / /        | |       Eastman Kodak Company
| | / /         | |       Eastman Chemical Company Research Laboratories
| |< < K O D A K| |       Process Instrumentation Research Laboratory
| | \ \         | |       P. O. Box 1972, Building 167B
| |__\ \________| |       Kingsport, TN  37662       615/229-4904
|                 |       INTERNET:   ornitz@kodak.com
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Newsgroups: sci.electronics