[aprssig] APRStt lives! (DCS=NOT)

[Stephen H. Smith]

Robert Bruninga wrote:
> OK, PL.  Should be easy to decode in a pic... Just count the
> frequency...
>
> Bob, WB4aPR
>
>
>   

If you are envisioning this as some sort of external add-on gadget 
(otherwise I assume you wouldn't be talking about programming PIC 
processors), this may not be as simple an undertaking as you assume.    
"PL" ("Private Line") is the Motorola trademark for the signaling system 
known generically as "continuous tone-coded subaudible squelch" 
(CTCSS).    The key is *SUBAUDIBLE*, more or less -- in the frequency 
range from about 60 Hz to around 180 Hz.     

Trying to take these tones off the speaker output (or inject them into 
the mic jack) of most radios will not work very well since the audio 
amps on both RX and TX intentionally roll off the low frequency response 
to keep these low tones out of the speaker.  [The RX audio channel 
typically starts with a 200-250 Hz audio high-pass filter.]

You will need a direct connection to the receiver discriminator, with 
it's  "down-to-DC" frequency response for these add-ons to work 
reliably.   [The connection point for OEM internal PL options is 
normally a separate audio channel with a 200-Hz cutoff audio low-pass 
filter tapped directly off the RX discriminator.]    If the radio has 
one of the 6-pin mini-DIN "data" or "packet" jacks, then no problem -- 
one can get at the raw discriminator audio easily. 


Otherwise you will encounter the same issue that prevented 9600 baud 
packet from being widely accepted by the average ham "end-user" -- the 
need to cut into the radio to tap the RX discriminator and TX 
modulator.  [And when was the last time you saw a hand-held with a 6-pin 
miniDIN data jack?]

------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
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Home Page:          http://wa8lmf.net

NEW!    *** HF APRS over PSK63 ***
   http://wa8lmf.net/APRS_PSK63/index.htm

Universal HF/VHF/UHF Antenna Mounting System
  http://wa8lmf.net/mobile/UniversalAntMountSystem.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths





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