On 11/05/2012 07:03 AM, Thomas Kocourek wrote:
there are questions which can only be answered by folks using the
'product' in the real world.
Yes, many. But maybe not these. :-)
What happens when the sender and receiver equipment drift apart in
frequency?
Most softmodem implementations incorporate a phase-locked loop which can
correct this. When the frequency drift gets too large, you lose the
lock. Fading may cause a lost lock and failure to recapture.
This will be a problem for our implementation exactly as much as it is a
problem for existing digital softmodems, with which we have experience.
Our two present implementations use the same modulation as D*STAR and a
previous HF digital voice system.
Does using FM modulation vs SSB modulation give better stability to
the QSO?
You can consider all digital modems you are likely to run into as a
variation on FSK. Sometimes it's phase rather than frequency that is
being shifted, and there are sometimes multiple carriers running in
parallel, or multiple phase angles that each produce data symbols rather
than just two as with simple FSK.
So, our FM radios are usually running a single carrier FSK (because that
works with the more efficient nonlinear amplifiers) and our SSB radios
are translating an audio baseband with multiple FSK carriers into RF.
What comes out of the SSB radio in this case is not SSB modulation as it
would be used for voice.
Assuming FM modulation, is the resulting RF spectrum narrower or the
same as analog voice?
You can have it narrower, the same, or wider, depending on the voice
quality and the amount of error correction you want. What we are trying
now on SSB is a bit less than 1/2 the width of an SSB signal. What we
are trying on VHF is like D*STAR bandwidth right now, but I think we
will try narrower.
Are there plans to make the product self-tracking to mitigate question 1?
Yes. Ultimately, the limit in tracking is the bandwidth sent to the
digital modem, unless you have the modem tune the receiver to increase
the PLL range. Whether you build that into your implementation or not is
up to you, it doesn't change what we are doing over the air. Modern
radios don't drift badly so I'm not clear this is that large a problem.
Fading and noise are larger problems.
What hardware is supported by the product? In other words, minimum
computer specs, interface to radio, etc.\
For the HF implementation, two sound-cards using the same interface as
existing softmodem modes, or one sound card and a harness that uses the
L and R channels separately.
For the VHF implementation, the same connection as a 9600 TNC. Many
modern radios bring that to a connector.
Is there a feedback mechanism established yet for the general public
once the product is released?
I'd imagine a mailing list would be used.
Thanks
Bruce
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