Re: [rcpilot-devs] 9k6 packet modem

[Mark <ma...@cp...>]

	Hi Val,

	I designed a 9k6bps modem to a client using a PIC16F84 and modified
that design some years later to allow 19k2bps, using an AT90S2313 this
time. As they are commercial products, I can't speak to much about the
circuits and code I used.

	But I can sure help you on debugging your design.

	Well, for the start, the whole TX process is this:

	- Convert the TX bytes from NRZ to NRZI.
	- Pass them through a scrambler X0+X12+X17.
	- Get the analog samples from a mapped sin table or 
	- Calculate them from a FIR coefficient table.
	- Send the samples to a DAC (R2R).
	- Use a hardware anti-aliasing low-pass filter.

	Mark Jordan, PY3SS

On 8 Mar 2004 at 5:13, Val Petrov wrote:

> Hi Mark,
> Great to have you here. I've build AFSK modem on ATMega128 chip
> recently. The modem was tested to work up to 4800 bds using FlexNet
> soundcard modem on receiving end. AGWPE only allows 2400 bds in AFSK,
> so switching into FSK modulation may be an answer.
> But I ran into trouble when I switched to FSK. I used direct
> connection between output of ATMega chip and input of soundcard to do
> the tests. It seems that my data scrambling procedure is to blame. I
> wonder if you have a good description or perhaps a source code for
> data scrambler for 9600 bd FSK modem.
> Val.
> 
> 
> --- "Michael J. Pawlowsky" <mi...@vi...> wrote:
> > 
> > Hi Mark,
> > 
> > Glad to see you made it.
> > 
> > For the others, especially Val who is looking into the modem stuff,
> > I met Mark on the TAPR PIC-SIG.
> > Basically I was asking around to see if anyone had designed a 9k6
> > modem using a PIC.
> > 
> > Mark came back to me and said that he has in the past.
> > 
> > So I asked him if he could join us to help us come up with a
> > design.
> > 
> > As for the uP used, language etc., I'm not really too concerned
> > about that.
> > Mostly the schematic and code blocks needed.
> > 
> > We actually only need the TX part for now. I don't think we will
> > get to RX stuff this year.
> > On the receiving side we want to use AGWPE.
> > 
> > Also it would be nice to validate that we will be able to do this
> > using the audio sub-carrier of the video TX.
> > I've been doing it at 1200 baud and that works well, but not sure
> > about 9600.
> > 
> > Val was trying to get some code going from G3RUH but I'm not sure
> > where he is at with it now.
> > 
> > 
> > Val?
> > 
> > 
> > 
> > 
> > 
> > 
> > 
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