[Lf] LF Remote Receiver

[Frank Gentges]

Vittorio,

Your numbers look attractive.  The filter seems to be growing here to 2
chips and 54 1% resistors.   If we can do it in the processor for free
that may be a key advantage.  You are suggesting an FFT, a removal of
unwanted spectral lines and then an inverse FFT if I understand. I am
note sure a 1000 Hz vs a 1002.2 Hz sample rate is an issue as long as
you use it on both ends.  Since we will use a sound card on both ends we
can stuff the sample into the D/A 11 times at the 11025 rate.

I am thinking about transmitting data both ways via a PPP I/P type
connection to make an internet connection simple.  We are seeing a lot
of DSL installations now with PPP over Ethernet so it looks like PPP
will be around a while.  

We can also use a direct modem connection and PPP I/P may be the best
way to go there also.  We need to carry the RX320 control commands from
the base station to the remote. That is a 1200 bit per second link and
usually is idle unless the receiver is being tuned or adjusted. We have
a small amount of signal strength data to come from the remote along
with the digitized sound.

The third method would use an RF link using VHF FM in place of the
modem.  The remote transmitter would come up on the air in response to a
command from the base station.

In addition to the digitized narrowband signal, we could digitize a
block of wideband stuff and transfer it in non real time for playback. 
This could let us listen to a sample of the European LF broadcast
signals.  We have had armchair copy at times at Nags Head on them in the
winter and spring.

I want to redirect the RX320 data port at the base station so the RX320
control may be via any of the RX320 control programs.  The base station
will most likely be running a flavor of the Windows  operating system in
order to run these programs.  

The remote station will run Linux so we don't have to drive down and
clear blue screens of death and reboot every few days.  The remote
should run for months without attention once we get it debugged.  My
local linux machine has now been up for 217 day without a reboot.

With a standard internet connection we can place these remotes anywhere
in the world where we can get a decent internet connection.  A 28.8
connection should do fine.

There is nothing keeping us from using this on MW or HF either.  N2JEU
has an RX320 web radio but it uses audio compression and control is
limited although many can listen at the same time. We are looking for
something a bit different.

Your comments are providing some good insight and are most helpful.

Frank



Vittorio De Tomasi wrote:
> 
> Frank,
> 
> I think that 14 USD are too much for a bandpass filter: a DSP solution
> is really a free lunch!
>  You need about 2N(log2(2N) + 1) operations to do the whole filtering
> process with FFT. Let us say you sample at 11,025 samples/s, and you use
> a buffer of 1024 samples. It follows you need about 25,000 operations
> for each buffer to do the filtering, i.e. you can go real time with a
> system that is able to deliver 250 kFLOPs (if I'm not wrong).
> 
> Since 1002.2 Hz is a bit odd sampling rate, you can interpolate down to
> 1000 Hz using nearest-neighbour interpolation (this usually results in
> some THD, but it needs only a look-up operation) or linear interpolation
> spending about 3 FLOP for each emitted sample.
> 
> However I have a question: how do you think to transmit data ? Remember
> that if you send 16 bit samples, you need to find a way to synch them,
> otherwise you could put together the wrong sequence on the other side...
> probably it is better to use some transport layer like TCP/IP: anybody
> there willing to hack KA9Q code ?!?
> 
> Vittorio
> --
> *************************************************************************
> Vittorio De Tomasi         ik2czl at amsat.org
> Home page:                 http://space.tin.it/scienza/vdetomas
> My DSP page:               http://www.freeyellow.com/members/padan
> 
> "Wir muessen wissen; wir werden wissen" (David Hilbert)

-- 
Frank Gentges 
K0BRA, ex AK4R, W3FGL
Check out our LF web page at <http://amrad.org/projects/lf>