[aprssig] PL 100

David Rush david at davidarush.com
Sun Jul 18 13:21:36 EDT 2004

PL indicates the use of a "sub-audible" tone (also called CTCSS, tone 
squelch, and probably a few other names for the same thing).  It's not 
really "sub-audible" in many cases, I can hear it on my rigs in many cases.

Anyway, to use it you set up your receiver to only open the squelch when 
it "hears" the hum of the specific PL tone frequency.  Now any 
transmitter that wants you to hear it must transmit the correct PL tone, 
otherwise your radio stays silent.  The PL hum is quiet enough to not 
interfere with the normal voice communications (or packet data tones).  
Some radios filter it out so you never hear it, even though it's there.

In some cases different users all share a single radio channel, but each 
group is on a different PL tone so they don't hear the other groups.  Of 
course they can't all transmit at the same time.

So... for "voice alert", you set up a rig with a speaker, parked on 
144.390 MHz, with a PL tone of 100 Hz enabled on your receiver and 
transmitter.  When another "voice alert" user is within simplex range, 
you will hear his/her APRS packets, because they will be sent with a 100 
Hz PL tone.  You won't hear the other 99% of  the APRS packets.  Pick up 
the microphone and holler at the other APRS user (with PL100 Hz on, 
still), and quickly QSY to a suitable voice frequency.

Clear as mud?

David, ky7dr

Rodney Padmore wrote:

> Could someone please tell me what this means, especially the "PL" as 
> referred to by Bob B.'s development of Voice Alert. Sorry if the 
> answer is in plain sight but I just can't find it in the manuals. Tnx, 
> 73 de Rod,VE1BSK.
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