[aprssig] Packet Radio Tone Generation
myetsko at insydesw.com
Sat Dec 18 20:41:09 CST 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
In addition, a lot of routines have a way to 'retime' on the edges. In a
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
tight looping to the end of the cell. If it finds a transition, it resets
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
> 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
> aprssig mailing list
> aprssig at lists.tapr.org
More information about the aprssig