[aprssig] Packet Radio Tone Generation

[Ka2upw at aol.com]

Andrew asks:
>What happens at the rcver ?

The receiver breaks up the signal into 1200 chunks every second.
Each chunk is worth one bit.  The receiver looks at the signal 
during that bit's chunk and decides if the signal during that 
bit's chunk/time period looks more like a 1200 Hertz sine wave 
or looks more like a 2200 Hertz sine wave.  Typically you run
a separate "bit clock" that breaks the signal into these one bit
sections for you. Note that you have to have your bit clock in
sync with the transmitter's so that you will know where every
bit starts and ends.

Or you could use a phase locked loop, but that's a different
technique.

Once you decide if this bit is a zero bit or a one bit, then in AX25
AFSK you have to un-NRZI and un-bitstuff, frame the bits up into
characters, find the HDLC flags, get the packet, find the trailing 
HDLC flag, compute your received CRC for the packet and check 
the CRC in the packet to see if the CRC's match.  

But I've simplified this a bit. :-)

>Does it not check the distance between zero corssings 
>to determine if the tone is 1200 or 2200 ?

No. Usually not.  Zero crossings are not usually used in
1200 baud Bell 202 AFSK AX.25 packet radio.  In other
modulation schemes, yes, but not usually here.

Now, you *could* use the zero crossing to measure the frequency
of the sine wave, and use that to decide the bit, but it typically
doesn't work as well as other DSP techniques.  (You still
need a separate bit clock). The zero crossing technique was used 
years ago by some of the "op-amp connected to a serial port pin" 
programs which generates an interrupt to the PC which measure 
the time between interrupts to determine the frequency. It will 
work, but not very well.

Douglas KA2UPW

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