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Re: Digital Radios



>Digital radio will work very well until the FEC, etc, fails and the 
>communications goes from very good to unreadable with a small additional 
>reduction in signal level.  Unlike analog, digital is pretty much there and 
>very readable, or not there.  You don't get that extra range with digital 
>where the signal is very poor but readable.

You're quite correct that strongly coded digital communications have a
very sharp SNR threshold. In some real systems, <1 dB changes are
enough to go from "solid copy" to "no copy".

What you fail to point out is that this sharp digital threshold
typical occurs far below the point where an analog mode would lose all
usability.

This is especially true for slow speed textual communications, as
contrary to popular amateur folklore CW is actually *extremely*
power-inefficient for the data rate it supports.  SSB is somewhat more
competitive with good digital voice but is still inferior. How much
better digital voice is than SSB depends on the voice quality you
require. If you're willing to tolerate *very* poor voice quality
the two schemes are nearly equal, but if you want anything better,
digital quickly pulls out ahead.

Sharp thresholds are inherent in any efficient communication system,
as originally predicted by Shannon's theory. He showed that it was
possible to communicate with an arbitrarily low error rate on a noisy
channel as long as you stay below that channel's capacity, even by a
tiny amount. Try to exceed it, even by a tiny amount, and achieving
that arbitrarily low error rate suddenly becomes completely impossible.

So the only way to build a system that "degrades gracefully" with
decreasing SNR is to make it degrade that much sooner than an
efficient system that works perfectly until it collapses suddenly near
the Shannon limit.

Phil

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