[hpsdr] The problem with digital modes

[John Laur]

The actual ETSI published DMR standard does not specify the codec (in fact
it goes to great effort not to do it), though AMBE by all practical
measures is the only real thing that can be put there right now. Although
it likewise does not provide any good mechanism for codec selection or
negotiation either. But it does allow some latitude for advertising feature
availability, so in theory it should probably be extensible to allowing
Codec2 to replace AMBE without causing lots of interop problems should
someone make the effort. It certainly seems to be a much better engineered
protocol than D*Star though.

In general, I think that TDMA schemes are a good way forward for amateur
radio on HF, VHF, UHF  primarily because they can support pseudo-duplex
operation without the large duplexers necessary to support
frequency-division schemes inside of our narrow amateur bands. I hope that
future SDR hardware will be better suited to support TDMA modes (faster T/R
switching, better clocks, etc.) I don't see CDMA really having anything
interesting to offer in the amateur radio sphere.

73, John KF5SAB


On Mon, May 12, 2014 at 1:24 PM, Shirley Márquez Dúlcey <mark at buttery.org>wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
> I agree that, in that case, we really could use better CODECs. Nearly
>> everything we do in ham radio is based on narrow-band frequency-division
>> multiplexing. We could certainly stand a bit more time-division
>> multiplexing or, better still, frequency-hopping or CDMA spread-spectrum
>> multiplexing. I'm not going to hold my breath tho'. That would ensure that
>> the spectrum was used more efficiently.
>>
>
> A problem with trying to make that switch on HF is that such modes would
> be incompatible with the existing users of the bands; you would have to set
> aside a band segment for TDMA or spread spectrum use. (Spread spectrum that
> used an entire HF band would raise the noise floor of the band and would be
> considered undesirable by anybody trying to do weak signal work. FCC rules
> don't allow spread spectrum on HF in any case, and nobody has yet proposed
> making it legal.) We are seeing some experiments with TDMA on VHF and UHF
> frequencies. DMR (and its gussied-up even more proprietary cousin,
> MOTOTRBO) uses TDMA to put two voice channels into the space of one. But it
> might be better to take an entire band segment and set it aside for
> something that worked more like GSM or CDMA rather than intermixing DMR and
> analog FM channels.
>
> CDMA historically had the problem of patent encumbrance. Some of the
> relevant patents expired recently and others will expire soon, so it may be
> time for some new ham work on that mode. One catch is that the spreading
> standard would have to be made widely available; otherwise it would
> constitute an unauthorized cipher. It's also not clear how the ID
> requirement could be met in a CDMA system.
>
> I am not pleased by the proliferation of ham radio modes that use
> proprietary technologies - things like PACTOR, or the various digital voice
> modes that use the proprietary AMBE codec. The need for that codec spoils
> the otherwise basically open DMR standard, and now that the free software
> Codec2 is available there is no good reason for hams to be putting their
> efforts there.
>
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