[OZAPRS] Ozaprs Digest, Vol 21, Issue 1

Horst Leykam hhl at bigpond.net.au
Fri May 4 12:53:17 EST 2007

Good one, Kevin...
I still have my original Vancouver TNC as well as the one that goes in a
The XR chip is simply a phase lock loop demodulator and it knows nothing
about the data it receives. They do work surprisingly well. I ran my HF
gateway on an old MFJ1270 which uses those, and it performed quite well
until recently, when I switched to using the Packet Engine Pro with a
soundcard. Performance is similar. I often decode packets from the US
overnight both with the XR chip modem and the AGW packet engine. Note that
only the PE Pro can be set to decode 300 Baud HF.

> * Zero-Crossing reduces transients on the signal. This improves the 
> bit-error rate, as opposed to the 2206/2211

I don't remember what the EXAR chips do, but most IC modems are
phase-continuous at a data change.  It doesn't have to be at a zero
crossing.  Compare it with the TCM3105.

> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] 
> On Behalf Of Andrew Rich
> + I think from memory that packet does not simply use 1200 = high or 
> + 2200 =
> low.

They're the Bell 202 tone frequencies - 1200 bps packet.

> + Does it keep flicking between tones to "keep the energy up" in the
> detector ?


> + And a non flick is a state change ?

Other way around.

> + I have noticed an MX614 does tone changes at the zero point, what 
> + are the
> distanvantages then of using xr2206 xr2211 that do not do this ?

(I'm out of practice with this!)  By waiting until a zero crossing, you
also introduce some jitter.  It looks like modems can deal with it, but
don't take that as gospel...

> + Does packet use a CRC method ? is that what the passall command is 
> + in a
> tnc ? show now crc chucksum valid packets ?

PASSALL passes every frame that is decoded by the modem, rather than just
those which are addressed to this particular station.

Another general overview can be found on Wikipedia under Packet_radio.

(Packeteer since the Vancouver days)


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