On 9/8/2011 10:05 PM, Noskosteve wrote:
On Aug 31, 11:38 am, Jim wrote:
One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs. You could build a very nice full duplex repeater on a single
frequency that way. ...

No filtering, much less intermod issues in multi station at onee site
systems... all kinds of good comes of it.

Digital schemes on HF to replace SSB I can see having real trouble (the
biggest is the lack of a "party line" capability, the other is the long
propagation delay on HF paths), but on VHF and up FM, you already have a
"one person talks at a time" by virtue of the standard FM demodulator.

Uhhh. It's been a long time since I worked on such a system (1975 I
think),
but IIR the prop delay through space for moderate distances kills the
idea.
Rough calculations gives a round trip delay, at 10 miles from the
repeater,
of about 0.1 ms. For two stations at that distance that's 0.1 ms not
available for sampling, bit width and processing.

You don't need that big a guard time if you keep track of how far you
are from the repeater and adjust your timing appropriately (that's what
a lot of systems do, and it's what was used for coarse position finding
in the phase 1 E-911 systems). That was an ordeal in 1980s to
implement, but today, it's in the piece of cake area, at least from an
implementation complexity and hardware standpoint. There is probably off
the shelf IP for it, too.


Keeping the BW down
also needs
rise and fall time as well as guard times. It added up quiclky back
then.
The vocoder becomes very important to reduce the data rate.

Yes, but on the other hand, standard cellphones use 8kbps and while the
quality isn't great, it's good enough. Of course, that gets us into that
whole "any decent codec is tied up with licensing problems" rat's nest.