David,
Thanks so much for your input!
That's the idea a super cheap VHF repeater(no duplexers), also thinking of
doing something with and SDR to allow one repeater to manage like 10
channels in a 25khz of spectrum. But that's if I can get the timing figured
out.
Yes it does seem to be an issue on some radios...
If I can I'm trying to avoid noticeable delay, but it's an option if all
else fails.(I may well have to do that..)
One idea was to drop some of the carriers and run a higher symbol rate,
but for some reason I can't get the filtering to work right when I change
the sample rate above 16khz, and for anything higher than 100
samples/second I seem to need a higher sample rate to keep the
noise/harmonics down.
2. Great idea, I think I can get that to work! I doubt any clocks will
drift too much in that amount of time.
I guess the other thing is to have the first RX symbol after TX to know
what the last RX symbol was to compare that *differential QPSK.*
*Would it be possible to just pause the demod program some how every two
symbols and revive it just where it left off? *
3. Thanks I'll give that a try.
Thanks,
Daniel va7drm
Sent from Daniel's iPhone
On Apr 20, 2014, at 5:03 PM, David Rowe <[email protected]> wrote:
Hi Daniel,
That sounds very cool. Another nice application is single channel
repeater work for VHF. Need a radio that can tx/rx switch fast enough.
Couple of ideas:
1/ Have longer timeslots than 20ms, like say 200ms, estimate the rx
symbol timing over that window. The timing is probably constant for
200ms anyway, as it changes only slowly.
2/ Have a square wave shaped window for the timing estimation, that is
set to zero when you are transmitting, that way you get several of your
rx timeslots included in the timing estimate.
3/ Test any changes by measuring the BER at an Eb/No point, as per the
Octave simulations. It's really easy to introduce an error that will
degrade the demod performance.
Cheers,
David
On Sun, 2014-04-20 at 14:58 -0700, Daniel Mundall wrote:
Hello Guys,
I've been playing with the FDMDV modem to see if I can get it to
support Time Domain Duplexing(TDD), with the end of goal of making
full-duplex voice in around 2KHz.
Currently I have the modem configured at 100 symbols/second with 14
carriers spaced at 125Hz it works out to 2800bits/s. From my testing
that end of things seems to be working pretty well. I'm planning on
splitting this up into 20ms chunks (2 symbols at a time).
What I'm trying to figure out now is the best way to keep the timing
when I never have more than two consecutive symbols.
Does anyone have a good idea to keep the timing correct while having
20ms interruptions every 20ms?
Thanks,
Daniel Mundall VA7DRM
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