Re: [rcpilot-devs] 9k6 packet modem

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

	I have never used 9k6 in 20kHz BW channel. But I have seen
some 9k6 modems that generate a square wave signal and filter it
with an analog filter, and its throughput was lower than a similar 
one generating sinusoidal-like signals. The PK96 is an example.

	As it is very simple to add filtering to your modem, first 
try it without filtering. In case the results won't be good,
you can add the filtering steps later.

	Mark Jordan, PY3SS


On 8 Mar 2004 at 13:46, Val Petrov wrote:

> Thanks Mark,
> 
> How important is the filtering step?
> > > 	- Get the analog samples from a mapped sin table or 
> 
> My understanding is that it is done to reduce bandwidth of the modem
> output within narrow FM band spec. Do you think I can skip it? I am
> planning to operate the modem in 20 Khz wide bandwidth of audio
> subcarrier of video transmtiter.
> 
> Val.
> 
> 
> 
> --- Mark <ma...@cp...> wrote:
> > 
> > 	I forgot to mention the first step, that is the HDLC bit-stuffing.
> > 
> > 	Mark Jordan, PY3SS
> > 
> > On 8 Mar 2004 at 12:16, Mark wrote:
> > 
> > > 	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
> > > 
> > 
> > 
> > 
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