Re: [IRCA] Yes, SAH!

["Bob Foxworth" <rfoxwor1@xxxxxxxxxxxxxxx>]

>I believe Glenn Hauser thunk up SAH. 

>Glenn, if you see this, stand up and admit your perfidy!

>Perfesser Chuck 


It was Gordon P. Nelson, in about 1965 when he published
his paper in DX News which described sampling the AGC
voltage, buffering it, and driving a DC-coupled oscilloscope
to look for evidence of low frequency beats in the AGC
voltage (too weak to register on the S-meter). This technique
provided evidence of the presence of weak secondary signals
behind a strong dominant signal, that was inaudible to the ear.

He developed this primarily to hunt for weaker European
signals, and by use of the precision freq measurements from
the European Broadcast Union monitoring stations, which were
published monthly, the carrier offsets (which were derived
from comparison to known signals) gve a good indication
of the identity, though not absolute, of the interfering signals.

The technique involved looking for the tell-tale SAH on some
Euro channels that looked promising and then waiting for
a possible fade-up to audibility, and not wasting time on
freqs with poor prospects. The ultimate goal was always
to recover identifiable audio, this was just a way to make
the odds better of doing so.

Back in that day it was common to find signals several tens
of Hz off of the nominal channel, and this measurement
technique had a lot pf practical validity. Today, with greater
freq stability and accuracy of European TXs, and (I believe)
the lack of such EBU data nowadays, this is only academic
interest.

Some DXers sqwaked about this, as some DXers will do, but
in his defense, GPN never intended for this to be a way to
"log new stations" under conditions others could not duplicate,
giving a potential unfair advantage. His interest was in the
technology of the process, not in filling logbooks. There was
nothing particularly earth-shattering about the technology,
it was just that no one had ever applied it to MW DXing before.

There are still several here on this list who have been around
long enough to have been to the famous Watertown, MA
shack and seen the process in operation. In that day, it was
truly state of the art DXing.

One of his favorite demos was to hear the weak 640 day signal
from carrier current WTBS ("witty-bus") (before Ted Turner
bought those calls for his Atlanta TV) at MIT, a few miles away,
and show it beating with the even weaker CBN from Nfld.,
turning the loop, and looking at the (I'm guessing) 10 Hz beat
on the scope, and seeing it vary with the loop. This was mid-day.

Gordon's paper was never published outside of NRC AFAIK, and
so there are probably many DXers, who have not been associated
with NRC, who don't know any of the history.

Gordon was also first* to develop methodology of using fast
fourier-transform analysis of the DC waveform of the recovered
AGC voltage created by the presence of multiple carriers, to
count and sort the carriers. Today there are several software
programs that do this easily, back then it was a lot more
complicated.

*first to detail it in a non-classified forum such as the DX press,
though I believe he never went beyond describing it in very
general terms, and I don't remember how much success he
had, as it involved sampling a lot of data points at home and
then taking them to the lab where the mainframe was, to process
the data.

73, "professor Bob" (huh?)                           sent 1200 edt






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