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".

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.

Have any of you ever done this and how did it work out for you?

- Jeff

wrote:
Have any of you ever done this and how did it work out for you?

- Jeff

Andy writes:
Jeff, I haven't tried this method, but one rule of thumb I have
always
believed in is:

"No matter how much you shift, limit, amplify or divide noise, it still
ends up as noise"

The only effective way I have ever found is to narrow the bandwidth
around the signal until the signal starts to get degraded. If done
digitally, it can be done by digital processing, but that changes only
the technique, not the principle...

So, while I would really like to try out some of these "improved
methods",
I am not confident enough in them to spend a weekend wiring together
some hardware.... In my younger years, I probly would've, tho....

Personally, as a CW operator of some 45 years, I have found that my
ears/brain does a lot better job of filtering than one would suppose,
especially
if I am copying some standard message where I sort of know the words
the other fellow will send. I only need 2 or 3 letters per word to
fill in
the pieces with devastating accuracy (grin).....

But, good luck on your efforts. If you do build up something, please

come back and post it here. I am sure that there are many
experimenters who try something like that if someone thinks it shows
promise..... But, please, take some actual measurements. And with
S/N ratios of around the 0 db level. My ear copy can still pick those

out, and many of the "processors" can't deal with noisy sigs in that
region....they tend to fall apart when anything below tangential
sensitivity is received....

Andy W4OAH in Eureka, Texas

Andy,

Thanks for your response. I guess I'm looking for a magic pill (though
I know better). I agree that the human brain/ear combination is
unsurpassed in digging out the really weak ones or the weak signal our
of the pile up. My unfortunate situation is my degenerating hearing. I
now wear hearing aids in both ears and have difficulty understanding
spoken conversation no matter what the volume level. I have always been
a CW only operator so the loss of SSB use doesn't seem so bad. I can
copy CW much better than listen to voices, but I can never be sure that
I can still dig the weak signals out of the mud and I'm pretty sure I
have lost some ability to deal with pile-up QRM. So I'm grabbing at
straws for gizmos to help me out as my hearing digresses.

73 - Jeff - KA9S

On Nov 3, 6:27 am, "AndyS" wrote:
wrote:
Have any of you ever done this and how did it work out for you?

- Jeff
Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have
always
believed in is:

"No matter how much you shift, limit, amplify or divide noise, it still
ends up as noise"

The only effective way I have ever found is to narrow the bandwidth
around the signal until the signal starts to get degraded. If done
digitally, it can be done by digital processing, but that changes only
the technique, not the principle...

So, while I would really like to try out some of these "improved
methods",
I am not confident enough in them to spend a weekend wiring together
some hardware.... In my younger years, I probly would've, tho....

Personally, as a CW operator of some 45 years, I have found that my
ears/brain does a lot better job of filtering than one would suppose,
especially
if I am copying some standard message where I sort of know the words
the other fellow will send. I only need 2 or 3 letters per word to
fill in
the pieces with devastating accuracy (grin).....

But, good luck on your efforts. If you do build up something, please

come back and post it here. I am sure that there are many
experimenters who try something like that if someone thinks it shows
promise..... But, please, take some actual measurements. And with
S/N ratios of around the 0 db level. My ear copy can still pick those

out, and many of the "processors" can't deal with noisy sigs in that
region....they tend to fall apart when anything below tangential
sensitivity is received....

Andy W4OAH in Eureka, Texas

wrote:
(top posting fixed)
On Nov 3, 6:27 am, "AndyS" wrote:

wrote:

Have any of you ever done this and how did it work out for you?

- Jeff

Andy writes: Jeff, I haven't tried this method, but one rule of thumb I have
always
believed in is:

"No matter how much you shift, limit, amplify or divide noise, it still
ends up as noise"


(snip)

My ear copy can still pick those
out, and many of the "processors" can't deal with noisy sigs in that
region....they tend to fall apart when anything below tangential
sensitivity is received....

Andy W4OAH in Eureka, Texas

Andy,

Thanks for your response. I guess I'm looking for a magic pill (though
I know better). I agree that the human brain/ear combination is
unsurpassed in digging out the really weak ones or the weak signal our
of the pile up. My unfortunate situation is my degenerating hearing. I
now wear hearing aids in both ears and have difficulty understanding
spoken conversation no matter what the volume level. I have always been
a CW only operator so the loss of SSB use doesn't seem so bad. I can
copy CW much better than listen to voices, but I can never be sure that
I can still dig the weak signals out of the mud and I'm pretty sure I
have lost some ability to deal with pile-up QRM. So I'm grabbing at
straws for gizmos to help me out as my hearing digresses.

73 - Jeff - KA9S

They're doing amazing things with PCs and soundcards, doing waterfall
plots to pick out weak signals. Perhaps this would work for CW as well
-- and if it does you can take a break from the hearing aids entirely,
and just do it all visually.

Alas, I don't know what the right software to use would be.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Thanks for your response. I guess I'm looking for a magic pill (though
I know better). I agree that the human brain/ear combination is
unsurpassed in digging out the really weak ones or the weak signal our
of the pile up.....

Interesting. I recall reading (in QST, no less) of PSK QSO's where
the human ear could NOT even tell a signal was being received, yet
the screen copy was "5x9" (if such a phrase even has meaning in
this context!-)

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

On 3 Nov 2006 08:00:47 -0800, wrote:

My unfortunate situation is my degenerating hearing. I
now wear hearing aids in both ears and have difficulty understanding
spoken conversation no matter what the volume level.

Sorry to hear about your hearing problem, apparently you have
problems of hearing some specific frequencies.

I don't know if this might help you, but one interesting observation
about a A1A beacon and FM receivers may be of interest.

We have a nearby microwave beacon sending out call sign and locator
using ordinary on-off keying. When receiving the signal on an FM
receiver with the squelch fully open, full quieting is achieved when
the carrier is on, but strong "FM-hiss" is audible, when there is no
signal.

After a while, it was not too hard to copy the beacon message in this
"negative-CW" format.

Since the FM-noise contains white noise, it should be detectable even
if some spot frequencies are undetectable due to selective hearing
loss.

One approach would be to run the CW signal through some kind of FM
receive, limiting the carrier, but producing white noise when no
signal is available (and you would have to learn to copy negative-CW)
or alternatively use some amplitude detector to control a noise gate,
i.e. when there is a carrier present, the white noise would get
through, with no signal, the headphones would be silent.

Just an idea.

Paul OH3LWR

Paul Keinanen wrote:
spot frequencies are undetectable due to selective hearing
loss.

One approach would be to run the CW signal through some kind of FM
receive, limiting the carrier, but producing white noise when no
signal is available (and you would have to learn to copy negative-CW)
or alternatively use some amplitude detector to control a noise gate,
i.e. when there is a carrier present, the white noise would get
through, with no signal, the headphones would be silent.

Just an idea.

Paul OH3LWR

Andy writes:

A hard limiter decreases the signal to noise by about 5.6 db, and
that's
a mathematical fact...

If we know the characteristics of the noise, and the
characteristics of
when it occurs and the distribution of the energy, then we have
"a priori" experience , and it really isn't "noise" anymore, and it can
be
dealt with --- in some cases rejected - and it's effects on the
intelligibiliy
of a desired signal made less...

There are two characteristics familiar to a radar engineer, which
deals with detection of a signal in the presence of noise::

Probability of detection -- Which is the probablility that a
signal
will be detected in the presence of noise without
an error.

Probability of false alarm -- This is the probability that
detection of
a vailid signal will occur when there is, in
fact, no signal
present.

Entire volumes have been written on Pd versus Pfa, since this
means
life or death to an aircraft (for instance) when a missle lock may
happen.....

If the other guy detects you and sends his missle before you
do, you will probly be dead.
If you fire your missles off at a ghost and have none left, you will
probly
be dead..

In the final analysis, CW is just On-Off signals, much like pulses.

The bottom line in all this is that if you KNOW what the signal is
going to
do you can increase the chances of detecting it properly.

For instance, if the signal is repetitive, it can be stored,
integrated,
differentiated, or accumulated with weighing functions to recover the
intelligence ---- "a priori" knowledge is necessary..

If you know what the noise is going to do -- impulse, popcorn, static,
broadband, random, etc --- you can use techniques to reduce it
hopefully without reducing the signal..

So fancy noise limiters, signal enhancers, and innovative detectors
will
work on some types of interference, and not on others. To be a
universal
S/N improvement, it has to work on "unknown" interference...

That's what the ham bands are like. It could be AM splatter, white
noise, a welder machine, the "woodpecker", car ignition.....
whatever....

That's what restricting the bandpass does, usually. Sometimes it makes
the S/N worse, but only with "special" types of interference.

There ain't no "magic bullet".....

The subject is a LOT more complicated than just any single simple
technique for recovering a signal...

However ,our ear/brain, with PRACTICE is an adaptive filter. It's
amazing
how well it works, after someone has been copying CW for a while.
Perhaps a microprocessor controlled adaptive filter can be made to
approach it, but ADAPTIVE filtering is the only hope that I can see,
given the different types of QRM and QRN that I have encountered.
My best bet is that someday an adaptive CW filter would be to do
as good as my own ear could do today.....

It's like looking at a noisy signal on a scope. Someone with a lot of
practice can see a valid signal several db lower in S/N than a novice
can do.... Sonar operators can do the same.....

"Waterfall" displays simply integrate the signal and noise over time,
which is similar mathematically to restricting the bandwidth as far as
the
S/N "enhancement" properties.... The characteristics of the signal
and the characteristics of the noise are known beforehand, and
that is used, via the display, to increase the Pd and decrease the
Pfa....

End of rant..... I need a beer.


Andy W4OAH

(retired communications and RADAR systems engineer and
ham for about 45 years, or so.... hell, I don't remember any more )

A hard limiter decreases the signal to noise by about 5.6 db, and
that's a mathematical fact...
___________________

Andy,

Can you point us to a reference document that explains this "mathematical
fact"?.

Joe
W3JDR

W3JDR wrote:


Can you point us to a reference document that explains this "mathematical
fact"?.

Joe
W3JDR

Andy writes:

No. It was about 25 years ago when I was designing the TI2100 FM
Marine
Transceiver for Texas Intruments, which was my last commercial Fm
unit. ( Some two meter stuff since then as home projects, tho )

It was info gleaned from several technical papers and I don't for
the
life of me remember which ones. I got a limiter and noise source
and checked it in the lab, at the time, and it seemed consistent.
I don't remember exactly, but I think I combined noise with a
signal and amplified the hell out of it, and then put in an
attenuator to get it back down and measured the S/n in a receiver.
Then I put a limiter in between the amp and the attenuator, and
decreased the atten to get the same level into the receiver, and
measured the S/N again. While I didn't get exactly 5.6 db, I
remember it was close enough to believe that the mathematical
derivation was confirmed ( in my mind ) and that my measurement
error was probly due to my own imprecision in the experiment..
Anyway, I moved on..... and it settled the question on whether
hard limiting "improves" things.....



Sorry, but that's just one of the numbers that stay with a guy, like
-174 dbm (God's noise) , and 8.5 db ( tangential sensitivity), and
10Log(bw), and 3.14..... Heck, I forget my phone number from time
to time, but numbers that I have used for most of my life stay with
me.....

And, being in the profession, I have, at some time or another,
verified
them myself in the lab when the opportunity permitted.. I take that
back.... I have never verified PI....... I hope I haven't been too
gullible..... :)))

So, I regret not having the mental acuity any more to jot down some
derivations for you. But , if they are not correct, there's a lot of
products
on the market which I built whose development was a wild fluke....

If you want to pursue it yourself, I would suggest a few texts that
have guided me... Skolnik's Radar Handbook ( the smal one, not
the BIG one --- I call it " small Skolnik" ) has a LOT of tech info
that
is presented in a level only slightly greater than the ARRL handbook.
Also "Principles of FM" -- damn, I don't remember the author.....but
how many could there be ? :)))) I might have it in my workshop.
If I run across it I'll post it here.

Well, good luck. Some knowledge can be passed on as a proven
fact and one needs look no farther..... like PI, for instance... Other
is in conflict with what someone thinks to be "how things work", and
doubt
is in the air.... No matter -- I was the same way, when I had the
energy to pursue it.....

Good on ya' , mate,

Andy W4OAH in Eureka, Texas

So it's not a "mathematical fact" in the sense that any of us can look it up
and see how it was derived, it's your recollection of something you heard
and vaguely remember convincing yourself it could be true, right?

While we're at it, what's the significance of your reference to "8.5 db (
tangential sensitivity)"?

Joe
W3JDR





"AndyS" wrote in message
oups.com...

W3JDR wrote:


Can you point us to a reference document that explains this "mathematical
fact"?.

Joe
W3JDR

Andy writes:

No. It was about 25 years ago when I was designing the TI2100 FM
Marine
Transceiver for Texas Intruments, which was my last commercial Fm
unit. ( Some two meter stuff since then as home projects, tho )

It was info gleaned from several technical papers and I don't for
the
life of me remember which ones. I got a limiter and noise source
and checked it in the lab, at the time, and it seemed consistent.
I don't remember exactly, but I think I combined noise with a
signal and amplified the hell out of it, and then put in an
attenuator to get it back down and measured the S/n in a receiver.
Then I put a limiter in between the amp and the attenuator, and
decreased the atten to get the same level into the receiver, and
measured the S/N again. While I didn't get exactly 5.6 db, I
remember it was close enough to believe that the mathematical
derivation was confirmed ( in my mind ) and that my measurement
error was probly due to my own imprecision in the experiment..
Anyway, I moved on..... and it settled the question on whether
hard limiting "improves" things.....



Sorry, but that's just one of the numbers that stay with a guy, like
-174 dbm (God's noise) , and 8.5 db ( tangential sensitivity), and
10Log(bw), and 3.14..... Heck, I forget my phone number from time
to time, but numbers that I have used for most of my life stay with
me.....

And, being in the profession, I have, at some time or another,
verified
them myself in the lab when the opportunity permitted.. I take that
back.... I have never verified PI....... I hope I haven't been too
gullible..... :)))

So, I regret not having the mental acuity any more to jot down some
derivations for you. But , if they are not correct, there's a lot of
products
on the market which I built whose development was a wild fluke....

If you want to pursue it yourself, I would suggest a few texts that
have guided me... Skolnik's Radar Handbook ( the smal one, not
the BIG one --- I call it " small Skolnik" ) has a LOT of tech info
that
is presented in a level only slightly greater than the ARRL handbook.
Also "Principles of FM" -- damn, I don't remember the author.....but
how many could there be ? :)))) I might have it in my workshop.
If I run across it I'll post it here.

Well, good luck. Some knowledge can be passed on as a proven
fact and one needs look no farther..... like PI, for instance... Other
is in conflict with what someone thinks to be "how things work", and
doubt
is in the air.... No matter -- I was the same way, when I had the
energy to pursue it.....

Good on ya' , mate,

Andy W4OAH in Eureka, Texas