On Feb 14, 2011, at 2:48 PM, Jim Reisert AD1C wrote:
> Incidentally, this is why the space shuttle and other mission-critical
> systems have voting logic with at least three voters. This is my
> attempt to model this.
Triple Modular Redundancy (TMR) hardware was already in use back in 1969 by
Avizienis at JPL in spacecrafts when I first studied papers about it. It had
most certainly been used earlier than that.
The nice thing about TMR is that you don't need specialized hardware; you just
build three times the amount of hardware and a simple voting circuit. Later
work on codes for correcting arithmetic errors were more efficient; papers on
"fault tolerant computing" were in vogue in the 1970s. They were able to
correct CPU errors with some tens of percent extra hardware instead of three
times the amount of hardware.
The same is true with data. Except that in the case of data, good error
correcting codes had existed for a much longer time. Systematic codes for
correcting data transmission errors were introduced by Hamming in 1950.
In addition to error correcting codes, encoders used for modern HF modes often
include an interleaver to "spread the data out" so that consecutive bits don't
get smashed by deep QSB. Many DXpeditions already try to do something similar
to counter QSB by transmitting "AD1C 599 AD1C," which is obviously much better
in the presence of QSB than transmitting "AD1C AD1C 599."
Even Amtor (a.k.a. SITOR-B) repeats characters with a large time gap in between
the two copies. Back when W1AW transmitted Amtor bulletins right after their
RTTY bulletins, Amtor could provide perfect copy when the RTTY copy had already
turn into gibberish.
Even better than repeating would be to encode the callsign and an RST into
something like 4DF1GXZ8QP for transmission over the air, and let the decoder
expand that back to a pristine "AD1C 599."
Just because one uses RTTY already means that one is already accepting gross
inefficiencies (both in bandwidth and in transmission time). Baudot RTTY is
fun, but you can never accuse it of being efficient :-) :-).
73
Chen, W7AY
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