[aprssig] Packet Radio Tone Generation

Mike Yetsko myetsko at insydesw.com
Sat Dec 18 21:41:09 EST 2004

Actually, most good routines will break a 1200bps signal up into 2400
so it can 'offset' into the middle of the first chunk.  Then use '2
chunks' per...

In addition, a lot of routines have a way to 'retime' on the edges.  In a
lot of
cases, that's at LEAST every character (bit size + start bits) but some do
every bit cell by looking at the middle of the cell time, detecting the
bit, then
tight looping to the end of the cell.  If it finds a transition, it resets
the time
reference, but if not, then it continues with the 2 cell time step.


----- Original Message ----- 
From: <Ka2upw at aol.com>
To: <vk4tec at tech-software.net>; <scott at opentrac.org>;
<aprssig at lists.tapr.org>
Sent: Saturday, December 18, 2004 9:18 PM
Subject: Re: [aprssig] Packet Radio Tone Generation

> 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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