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Author Topic: human detection of weak CW signals  (Read 4245 times)
PA0WV
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Posts: 387




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« on: July 12, 2017, 12:12:41 PM »

In order not to hijack K1LEM's topic, I open a new one.

Interesting is to investigate how a minus 12 dB S/N ratio can be detected in noise with a bandwidth of 300 up to 2800 Hz
Lower pitch should have a better detection because the human mind has a built in filter about one octave wide.

For that reason I present a Morse wav file (NOT MP3 because these are compressed and mask weak signals)

The file is 8000 16_bit_samples/s and 320 second duration. so may take some time for downloading 4.6 MByte.
It starts at 350 Hz and after each wordspace the pitch is increased with 50 Hz.

The speed is 12 wpm, so that every  ham has the opportunity to report decoded text and it is easier decode slow code in noise.

Remember when it is hard to decode: play with the loudness in order to find the best result.

http://pa0wv.home.xs4all.nl/eham/MINUS12F.WAV

Andrew, Lou, Chuck, K0UA hw cpy?
« Last Edit: July 12, 2017, 12:21:23 PM by PA0WV » Logged

KA1VF
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Posts: 124




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« Reply #1 on: July 12, 2017, 12:41:39 PM »

 That CW recording at -12 dB was a challenge, but I managed to copy about 50% of it in my head.

             tnx and 73,
                              Bob
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NI0C
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« Reply #2 on: July 12, 2017, 12:58:09 PM »

Wim,

The easiest part was the first part at 350-400 Hz pitch: "Hallo-Lou and--the--group"
Past 400 Hz, each transition to a higher pitch really threw me for a while until I could make my brain shift.

Will keep working at it but don't want to give it all away and spoil others fun.  Your recording definitely verified my own choice of CW pitch for low band DX'ing-  330 to 360 Hz, attainable with the K3, but not with all transceivers.  Also very important (and you hinted at this) low volume levels help a lot.

Thanks & 73,
Chuck


 
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PA0WV
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Posts: 387




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« Reply #3 on: July 12, 2017, 01:30:39 PM »

Tks Chuck,

You are right; for reports of decoded text my email from QRZ page can be used.

73
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G4LNA
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Posts: 129




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« Reply #4 on: July 12, 2017, 02:02:48 PM »

That was interesting, I was having difficulty reading the code at the lower frequencies, I find my sweetspot is around 650Hz - 700Hz where it was quite easy to copy and above that the copy tailed off again. I always wonder when ops are copying at frequencies in their boots how they manage it, it beats me.

Thanks for posting that.
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VK6IS
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Posts: 299




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« Reply #5 on: July 12, 2017, 06:37:50 PM »

likewise, I've found that I'd prefer slightly a  higher tone.
so, when I'm listening to a 2 way qso, I'm tending to adjust the dial,
to get some mid point, that is like able.

that original wave at 5db was defiantly more readable.
but, these wav forms, are more like what is heard around these parts, today.
ie: generally a 429 type of reception.

sometimes you get lucky, and something that is more akin to that 5db example.
 Cool
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KE6EE
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« Reply #6 on: July 12, 2017, 09:00:38 PM »

Very interesting experience.

At first listening at a fairly high volume I could hear only noise. When I lowered the volume
suddenly the code became identifiable.

After the code became distinguishable I could copy it at any volume.

I found no difference in my ability to copy the code at various frequencies.

On the other hand at relatively high volume at any frequency the noise was very annoying and copying too unpleasant to continue.

I think we are dealing with many complex neurological processes including both auditory function and interpretation in the brain (or whatever fills the heads of hams).

Ordinarily when working CW I set my sidetone at a relatively low pitch (500 Hz or lower which is
approximately the C above Middle C on a piano). At this pitch matching pitches to zero beat another signal is easier than at a higher pitch.

After I make a contact I often turn on the RIT so I can increase the pitch of the received signal which I often find easier to copy than at my sidetone frequency. This may have to do with my receiver's audio qualities as much as anything else.

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GW3OQK
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« Reply #7 on: July 13, 2017, 02:39:28 AM »

Like Chuck I found the lower frequencies at the beginning easier to copy. When it jumped frequency I "lost it" as if it was another transmission nearby which I would tune out. Soon my brain would expect and adjust to the next. The smooth drift of a real signal is easier to cope with. I could not copy more than about 50% and I found no "sweet spot".

If we were trying for a weak signal QSO with repeats of callsign, names, RST 329 QRH, we would be successful.

73, Andrew
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VK5EEE
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Posts: 956




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« Reply #8 on: July 13, 2017, 10:54:04 AM »

What an interesting test. I found after listening several times, reducing the volume helped me get a few more letters. Seems to me it got harder as the pitch went higher. Not sure what % I managed, I guess 80%, sent to OM Wim via Email for analysis!

How nice that the final few words were 599!
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Long Live Real Human CW and wishing you many happy CW QSO - 77 - CW Forever
PA0WV
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Posts: 387




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« Reply #9 on: July 13, 2017, 12:07:53 PM »

Hi Lou

Obviously the morse file was 30 second longer than the noise file, hi.  That is because I added some words to the morse.
I will correct that tonight.

No, I am absolutely sure the Morsesound is everywhere the same amplitude, a number of checks are built in, only the frequency is going up 50 Hz automatically at each word space, a reason to cluster words with = signs in order to obtain a sufficient  length per frequency. It is interesting to know what frequency is experienced to be best for copy.

I sent you some info in the email abt CW-filter 150 Hz wide.
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W3TTT
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Posts: 245




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« Reply #10 on: July 13, 2017, 01:32:24 PM »

Adding my thoughts -
I have not listened to this file yet.  But here is what I do to hear a weak CW signal.  I play with the tone and the IF shift.  By shifting the IF filter, sometimes I can change the tone of the background noise.  Then I play with the RIT and change the tone of the signal.  It can bring out a signal out of the noise.  Two nights ago, I picked up a station who was running 2 watts into an indoor antenna from Ohio.  And you know how bad the 40 meter band has been lately!

73, Joe W3TTT
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PA0WV
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Posts: 387




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« Reply #11 on: July 13, 2017, 01:51:45 PM »

Adding my thoughts -
I have not listened to this file yet.  But here is what I do to hear a weak CW signal.  I play with the tone and the IF shift.  By shifting the IF filter, sometimes I can change the tone of the background noise.  Then I play with the RIT and change the tone of the signal.  It can bring out a signal out of the noise.  Two nights ago, I picked up a station who was running 2 watts into an indoor antenna from Ohio.  And you know how bad the 40 meter band has been lately!

73, Joe W3TTT

What is the bandwith of your IF filter Joe?
I used 300-2800 Hz, but by shifting you can make it (300+n) till (2800+n) with n a positive or negative real number. and the CW carrier still within that band. Any idea about n and the tone pitch of the received code you experienced as optimal?

Wim
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VK5EEE
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Posts: 956




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« Reply #12 on: July 13, 2017, 06:56:13 PM »

Hi Wim,

You misunderstood me, I did not say the volume changed, but that *I* reduced the volume for the entire length and found that easier to copy than when at higher volume, as the noise perhaps is then more irritating when it is louder. After all, the S/N ratio is the same, irrespective of volume! So, herewith I learned a valuable lesson to remember when fishing out a weak signal, never to bother raising the volume.

W3TTT yes, I do the same, and that is very effective, changing the pitch of the background noise!

PA0WV, you said (by email) that you have a 150Hz filter you can add in that reduces the background noise but does not increase intelligibility of the CW if the S/N ratio is still -12dB? That's because the S/N ratio is still the same I imagine.

I find that a narrow filter in local noise improves the S/N significantly, so for your (next test?) you're going to adjust to S/N ratio is still -12dB and show that irrespective of the bandwidth of audio, the intelligibility does not improve, have I got that right?

I got 52 words out of 60 correct, which is 87% but not on the first try, so QSZ (repeats) were necessary. That shows that in a real-life message-handling situation using trained tried and tested QTC exchange techniques, 100% copy of the message would occur even with -12dB SNR.

I'm going to share this information with Radio Relay International and others, though I'm sure they already know it. Modern digital  modes can still go lower, but, they are dependent on modern technology and a computer. With CW and discreet home brew components, CW is the most efficient!

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Long Live Real Human CW and wishing you many happy CW QSO - 77 - CW Forever
PA0WV
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Posts: 387




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« Reply #13 on: July 14, 2017, 06:02:43 AM »

Hi Wim,

You misunderstood me, I did not say the volume changed, but that *I* reduced the volume for the entire length and found that easier to copy than when at higher volume, as the noise perhaps is then more irritating when it is louder. After all, the S/N ratio is the same, irrespective of volume! So, herewith I learned a valuable lesson to remember when fishing out a weak signal, never to bother raising the volume.

I experienced in the original soundfile clearly differences in loudness dependent of pitch, meant is the Morse file without noise. It must be my ears, the cheap speakers etc, because the amplitude in the soundfile was exactly the same for all frequencies.
Yes the biological human system is non linear. When you have a black-and-white grayscaled picture with extreme low contrast, it turns out the intensity of the on falling light is important to get the image.

The ears has the Fletcher curve, the sensitivity for different frequencies is dependent on the loudness. Philps made 60 years ago broadcast radio's with fysiological gain control. When you lowered the volume high and low pitches were extra gained compared with the middle range.

Quote
PA0WV, you said (by email) that you have a 150Hz filter you can add in that reduces the background noise but does not increase intelligibility of the CW if the S/N ratio is still -12dB? That's because the S/N ratio is still the same I imagine.

On
http://pa0wv.home.xs4all.nl/eham/CW2DB.WAV
you can find a short file (abt 44 s duration) with CW 12 wpm 712 Hz pitch in the middle of a filter of 150 Hz wide.

When you use that filter you cut 12 dB of the noise of the original -12 dB S/N so that should make the new S/N after the filter 0 dB. Does that make the reliability of copy better? I doubt that, the presented file is even PLUS 2 dB S/N ratio.

Play with  the volume low to moderate.

What I possibly can do is present 150 Hz filtered morse files with identical  S/N ratio, I propose 0 dB,  but presented with different pitches. That is what IF bandshifting together with the RIT and a small ideal phase linear IF bandpassfilter of 150 Hz
could offer your ears.

I will not go that high but till max 650 Hz pitch The noisepower will be (and is in the presented file) the noisepower AFTER the filter, because the filter is possibly not 100% flat in the passband.
Quote
I find that a narrow filter in local noise improves the S/N significantly, so for your (next test?) you're going to adjust to S/N ratio is still -12dB and show that irrespective of the bandwidth of audio, the intelligibility does not improve, have I got that right?

As I expected the close to the pitch noise is most disturbing for copy reliability, the CW2DB.WAV file demonstrates that, so that I plan to make 0 dB S/N files with 150 Hz bandwidth and different pich.

Quote
I got 52 words out of 60 correct, which is 87% but not on the first try, so QSZ (repeats) were necessary. That shows that in a real-life message-handling situation using trained tried and tested QTC exchange techniques, 100% copy of the message would occur even with -12dB SNR.

Yes the repetition can not be an objection, because you repeat the same noisebursts that are linked with the Morse. So when you copy after a repetition, you copy it.

Quote
I'm going to share this information with Radio Relay International and others, though I'm sure they already know it. Modern digital  modes can still go lower, but, they are dependent on modern technology and a computer. With CW and discreet home brew components, CW is the most efficient!



Yes, but I am wondering what people do when they learn CW on the modern way with a keyboard needed to type in the decoded text. In emergency they need a keyboard. I think paper and pencil copy is essential, especially when you miss due to QRM, QRN or QSB some plain text you can fill it in afterwards.

I plan the same kind of test in an SSB forum chapter, with compressed and uncompressed nato spelling words (alfa bravo...) My objective is to compare CW with SSB.
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VK5EEE
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« Reply #14 on: July 14, 2017, 08:58:07 AM »

Dear Wim,

Very interesting! Well, yes, the file with 150Hz was VERY EASY to copy 100% especially at that slow 12WPM. So for me, at least, I'd say that did sound like 2db compared to -12db before in spite of the noise having a similar pitch. The CW signal, to me, was well above the noise level. I did not need to listen twice, the QRK was 4 to 5, the copy was solid.

Returning to your question about our experiences with the pitches in the -12dB 12WPM file. I found one or more frequencies most clear, I suppose I could listen again and work out what the pitches were. It started at 350Hz and went up 50Hz each time, so I could count that. But for me, there was no clear advantage of the low over the mid range pitches, though there was one or two of the pitches that I found easy compared to the rest, I think, but my failure of words near the end shows that the highest pitches were not OK for me.

A side note. As we get older, for me at least and I hear this is common in ageing, some sounds, even when our ears are half as good as they were, cause psychological pain, such as the clanking of dishes, loud noises on the other end of the phone when talking to someone on the phone, it seems we have less tolerance for noise that is not the sound we want to hear. But on CW at least, I don't have this problem, I'm happy with QRM.

On another side note. Am I alone in this: I love the sound of QRN -- not man made local noise, I HATE that but have to put up with it, but the sound of real QRN, static from storms. In fact, I rather like storms, not sure why, since I've been a kid. I found comfort in storms when tucked up in bed, but later I enjoyed being in the elements of a storm. Obviously not a direct lightning strike though! I used to listen to 500kc/s and sometimes 2182 and leave it on all night, the crashes of static I found, and still do find, comforting. So I actually PREFER a QSO through real static crashes.

QRK goes down a bit of course, but I just like the sound of those static crashes. Maybe its nostalgia or long time memory of a nice feeling with it. I am sure many sparks who had to listen to it a lot though did/do not like it. Especially if there is no AGC. AGC off, is not quite so nice a sound :-)

Looking forward to your future experiments, pitches and especially the SSB results. On a different topic RRI did tests with FEMA in USA (government emergency body) and CW proved the fastest end to end transfer of QTC, even faster than digital mode PACTOR but that is for different reasons... but to cut a longer story short, a human  being able to directly write/type (and fill in as you explained) and CW is easiest/faster than SSB when words/names need to be spelled out in NATO alphabet.

For me, as I don't listen to SSB much, I find it hard to copy SSB that is not having good SNR but those who regularly have SSB QSO find it much easier and can copy easily what is being said, when I cannot. But in CW, I have no problems! So surely it is a lot to do with experience, training!
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Long Live Real Human CW and wishing you many happy CW QSO - 77 - CW Forever
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