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'[PIC]: Scrambled radio'
2001\06\15@124415 by slide

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The company I work for just got a couple of the small 'Family Radio System'
(FRS) radios to complement our larger, Motorolas.

While using them, I had the thought about a small box that would plug in
between the radio and a headset/mic to accomplish scrambled comms.

My idea was to have an 8 bit DIP switch to set the 'scramble code.  The mic
would go to the ADC of a PIC, then I would take each successive byte and
either OR or AND it with the 'scramble byte'.

The output would go through a DAC and on into the radio for transmission.
Of course, at the other end, the process would come from the radio, through
an ADC, descramble, DAC to the speaker.

My questions are:

1.  Is this legal?  Can you scramble your private conversations?
2.  Is a PIC fast enough for this, or would I have to use something like an
SX chip running at a high clock speed to get anything useful?

I know this would only provide 255 codes, but it would be better than
nothing if competitors were listening.

Any thoughts would be appreciated.

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2001\06\15@141727 by Bob Ammerman

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This won't even come close to working: the radios do not have enough
bandwidth to send an arbitrary 8 bit analog sample and recover it properly.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2001\06\15@163618 by David VanHorn

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At 12:58 PM 6/15/01 -0400, Bob Ammerman wrote:
>This won't even come close to working: the radios do not have enough
>bandwidth to send an arbitrary 8 bit analog sample and recover it properly.

Good old frequency inversion might work..
Ring mixer and a constant tone in the 1khz range.
Same thing on the other side to decode.
Not very secure, but it's easy and cheap

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I would have a link to http://www.findu.com/cgi-bin/find.cgi?KC6ETE-9 here
in my signature line, but due to the inability of sysadmins at TELOCITY to
differentiate a signature line from the text of an email, I am forbidden to
have it.

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2001\06\15@180617 by Olin Lathrop

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> While using them, I had the thought about a small box that would plug in
> between the radio and a headset/mic to accomplish scrambled comms.

Check the FCC regs first.  On some bands at least, you are explicitly
forbidden (or used to be at least) forbidden to scramble your signal.

> My idea was to have an 8 bit DIP switch to set the 'scramble code.  The
mic
> would go to the ADC of a PIC, then I would take each successive byte and
> either OR or AND it with the 'scramble byte'.

This would lose information.  Imagine ORing with FFh or ANDing with 0.  You
need a different way to produce the scrambled stream.

> The output would go through a DAC and on into the radio for transmission.
> Of course, at the other end, the process would come from the radio,
through
> an ADC, descramble, DAC to the speaker.

This won't work because you won't get exactly the same bit pattern out of
the A/D at the receiver that you stuck in at the transmitter.  This is
unlikely to work even with a D/A and A/D tied together on the same
breadboard.  Now add the radio channel.  Due to the scrambling, a single
flipped bit would mean a totally different analog level.  Also, I don't know
if these radios transmit analog or digital.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2001\06\15@215454 by Steven J. Devine

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> The company I work for just got a couple of the small 'Family Radio
System'
> (FRS) radios to complement our larger, Motorolas.
>
> While using them, I had the thought about a small box that would plug in
> between the radio and a headset/mic to accomplish scrambled comms.
>
> My questions are:
>
> 1.  Is this legal?  Can you scramble your private conversations?

Its unclear, but from what I can tell it may not be.  Note that for CB
(27MHz AM variety) they are not allowed:

Sec. 95.412  (CB Rule 12) What communications may be transmitted?

   (a) You may use your CB station to transmit two-way plain language
communications. Two-way plain language communications are communications
without codes or coded messages. Operating signals such as ``ten codes''
are not considered codes or coded messages. You may transmit two-way
plain language communications only to other CB stations, to units of
your own CB station or to authorized government stations on CB
frequencies about--
   (1) Your personal or business activities or those of members of your
immediate family living in your household;
   (2) Emergencies (see CB Rule 18, Sec. 95.418);
   (3) Traveler assistance (see CB Rule 18, Sec. 95.418); or
   (4) Civil defense activities in connection with official tests or
drills conducted by, or actual emergencies announced by, the civil
defense agency with authority over the area in which your station is
located.


But Subpart B (FRS) states this:


Sec. 95.193  (FRS Rule 3) Types of communications.

   (a) You may use an FRS unit to conduct two-way voice communications
with another person. You may use the FRS unit to transmit one-way
communications only to establish communications with another person,
send an emergency message, provide traveler assistance, make a voice
page, or to conduct a brief test.


It's explicitly stated for some bands that encrypted communications are
illegal, such as amateur radio and CB, but for FRS it is unclear.  So as
long as its "voice", it appears to be OK...  YMMV.

Steve
------------------------------------------------------------------------
Steven J. Devine, President, Consultant, TZOGON Enterprises Incorporated
President, EAA Chapter 136 (Merrimac Valley)    http://eaa136.tzogon.com
EraseMEstevespam_OUTspamTakeThisOuTtzogon.com        HAM Tech lic: N1YZJ        http://www.tzogon.com
------------------------------------------------------------------------

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2001\06\17@021819 by Harold M Hallikainen

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On Fri, 15 Jun 2001 12:41:37 -0400 @spam@slideKILLspamspamCOTSE.COM writes:
{Quote hidden}

       Doesn't look legal. See
www.hallikainen.d2g.com/~harold/cgi-bin/section.pl?section=95.631
section
   (d) An FRS unit may transmit only emission type F3E. A non-voice
emission is limited to selective calling or tone-operated squelch tones
to establish or continue voice communications.

       See http://www.hallikainen.com/cgi-bin/section.pl?section=2.201 for
definitions of emission types. I suspect scrambled voice would be a
non-voice signal, but I don't really know...

       Reviewing some of the documents at
http://hallikainen.d2g.com/~harold/FCC/FccRules/CiteFind/095631.htm , the
FCC has granted a waiver to this requirement to allow the transmission of
GPS data when the user pushes a button on the FRS radio.

       This might be permissible on the new MURS frequencies in the Citizen's
Radio Service (the old "color dot frequencies"). See
http://www.fcc.gov/Bureaus/Wireless/Orders/2000/fcc00235.txt .


Harold





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