On Wed, 31 Aug 2011 09:38:51 -0700, Jim Lux
wrote:

One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs.

That's easy. Because there's no emission designator for FCC approved
TDMA mobile/HT for ham radio. The ARRL is working on the problem.
http://www.arrl.org/news/arrl-files-em-petition-em-em-request-for-temporary-waiver-em-with-fcc-regarding-vhf-voice-and-data-e
Multiple time slot systems, such as DStar are currently approved. This
is nothing new. Just approval for P25 radios.

You could build a very nice full duplex repeater on a single
frequency that way.

Yes, except that the ARRL has decided to only petition for a waver for
single time slot TDMA, which can't be used for a repeater.

You're scheme would certainly work, and I too am wondering why nobody
has bothered to do it. Possibly because nobody really wants full
duplex (with echo, reverb, feedback, etc).

Sure it's totally incompatible with current FM
repeaters, but then, D-star isn't totally compatible either. D-star and
it's ilk are sort of half measures in that sense.

Dstar duz 5 independent simultaneous conversations through the
repeater. That's not what I would call a half measure.

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

Yep.

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.

Yep. We did that with Amtor (Sitor) for data. The big problem was
getting the switching time between T/R to less than the prop delay. If
you increase the time slicing to where it could handle voice (about
200Hz) but not be audible, the occupied bandwidth increases
unacceptably wide for HF. It's possible to decrease the switching
time, but then the latency (delay) increases to unacceptable levels. I
don't think anyone really wants a repeater with a 1 second audio delay
(even though they exist).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558