) writes:
I have been looking into gizmos that improve CW copy. Most are audio
tone detectors that ignore short impulse noise bursts and then
regenerate the CW with a keyed tone oscillator. There are several of
these designs around and they are all well and good, but I stumbled
across something different and was wondering if any of you have had
personal experience with it?

An October 1971 article in Ham Radio magazine (pg 17) titled
"high-performance CW processor for communications receivers",
"Frequency modulating the telegraphy signals in your receiver provides
an interesting and profitable addition to conventional receiver
design".

I finally dug out the article. I haven't a clue to its worth, but
I don't recall that sort of scenario coming up in other places (while
the one about good filtering and using a detected voltage to key an
audio oscillator came up a number of times), which may mean nobody
found it useful, or nobody else could be bothered replicating the circuitry.
What you want to do is check a few issues later, to see if there were
any letters related to it in the "Comments" section.

It's interesting that the November 1971 issue of Ham Radio had an article
entitled "Weak Signal Reception in CW Receivers", which used nothing
cutting edge but was a summation of various things one could do to improve
reception.

Go back a few years, and you'd see an article or two about "under the noise"
CW reception, which of course amounted to PLLs driving some indicator, but
at the time were pretty out of the ordinary since IC phase locked loops
hadn't arrived.

I suspect to evaluate this, one really needs to dig through the magazines
and look at all the schemes. Ham Radio seemed to have a fair number in
the first decade or so. Something about that article you reference reminds
me of something in an article about a Hallicrafter's diversity receiver,
I forget the issue but it likely was in one of the annual October (or was
it November?) receiver issues. About '74 or '75, someone named Hilbert
had some scheme that involved active audio filters, but there was more
to it than I can think of at the moment. (I seem to recall there was
some "stereo" effect, in that different signals were fed to each ear,
which in itself may be worth pursuing. Use one of those schemes with
the detectors to key the audio oscillator, but also include some of
the signal from the receiver output, so you get the noise and the actual
signal in it.) Wait, I think it must have been "Hildreth", who also
wrote this article you reference. In which case, you can look up
what he did later; did he see some fault in this system, or did he
just realize it was easier to implement something at audio?

Someone mentioned in this thread something that hinted at Coherent
CW, which sync'd up the time and frequency at both ends to allow
for good filters and fairly deep in the noise CW reception. If you
know when and where to look, then it's easier to gather whether there's
a signal there or not.

By looking at the various schemes people have come up with, one
can get a better idea of each one's worth better than looking
at each one by itself.

Some of the schemes likely panned out to be duds. Others required
too much circuitry, at least at the time of the articles, so nobody
wanted to replicate them. And then likely they've been forgotten,
because otherwise more recent technology advances make the past easier
(look at how phasing SSB returned to some level of popularity when
ICs and digital audio came along). Others, like Coherent CW had
the disadvantage that they were a whole system, rather than a
processor, so you needed matching stations at both ends in order
for it all to work.

You can at least look over the cumulative index of Ham Radio magazine,
since someone has put it (or at least some version of it) online
at http://webhome.idirect.com/~griffith/hrindex.htm

Ham Radio seemed to be the place to look for that sort of out of
the ordinary schemes.

The idea is to sample the last IF of a receiver after as much IF
filtering as you can muster, and then using this as the RF input to a
FM modulator. The RF/IF is modulated at the audio frequency you like to
hear while copying CW. The next step is to frequency multiply the FM
modulated signal to increase the bandwidth and up the modulation index.
The following step is to treat it like any normal FM receiver IF and
run it through a limiter stripping off any amplitude information. The
last step is to put the signal into a normal FM discriminator to
recover the modulating tone you used.

What this is supposed to do is reduce or eliminate QRN (not QRM) from
the CW signal making a "quiet" background to copy the CW.

It gets the on/off of the keying, but yes it limits the signals. So
widely varying signals will be at about the same level (though that
may not always be a feature), and any QRN will be limited too.

In thinking about it, I'm not so sure it's all that distant from the
schemes that detect the CW and use that to key an audio oscillator. The
bulk of the circuitry is not there to improve the CW reception, but
to get that needed FM signal, with the incoming CW signal as
the "carrier".

Again, the more I think about it the more I think his later audio based
schemes may implement a similar concept.

Michael VE2BVW