[aprssig] APRStt Callsign and Checksum

[Robert Bruninga]

It would be nice if someone would write an APRStt CALLSIGN
GENERATOR routine for everyone to use.  They could just type in
their callsign, and out comes the DTMF keys to put into memory.
This automatically generates the 2-key conversion and checksum.
An ON_LINE generator would even be better!

> I have a question about the APRStt checksum.  
> What is it and what is it for?  

Since the callsign is only entered once into a DTMF memory for
easy transmission, we thought it should have a checksum to
greatly minimize the chance for errors.  You just add up all the
keys in the callsign, and keep only the units digit of the sum.
That then is the checksum digit.  If APRStt reecives a DTMF
string and the checksum does not match, then there is an error.

The APRStt DTMF string for callsigns is:

AcccccccccccVK#

Where V is a user selected overlay byte (or two byte letter)
Where K is the checksum (treat ABCD keys as HEX)
Where each letter of the callsign is encoded using the 2-key
text system

See www.aprs.org/aprstt/aprstt-user.txt

Bob, WB4aPR