Wednesday, 7 August 2019

Weak Signal Volume Levels



The following blogspot was originally published in July, 2016. Hopefully some will find this of interest as the topic is still very relevant.

***************


A recent posting on Yahoo's Perseus SDR Group inquired about the use of external or PC-based DSP manipulation of signals partially masked by noise to improve readability. The most interesting part of this short discussion was the result of one response indicating:


"BTW one of the best and most simple noise reductions is to lower the volume."

to which the original inquirer responded:

"BTW, lower the volume to reduce noise ... ?? That was a joke, right ??"

Other comments soon followed, including my own, initially:

"Actually, for whatever reason, this works...at least when copying very very weak CW signals. I think it is more of an ear-brain thing where the noise
gets more focus than the signal when listening at moderate levels but
cranking everything down to a very low level has always improved copy for
me....not sure why this works as well as it does."

From Roelof Bakker, PA├śRDT:

"The ear brain system works much better at low volume as it is easily
overloaded by strong signals. Similar like too much direct light in 
your eyes will degrade contrast. I guess this is getting worse with
age, but I am not sure about that.

I have been watching many videos on YouTube which demonstrate ham
radio gear and most if not all use far to high volume settings,
which degrades readability. I believe it is a normal habit to raise
the volume for weak signals, but this is often contra productive.

When listening for weak signals at low volume settings, a quiet room
is mandatory. I have taken considerable effort in building a quiet
PC, that is aurally quiet.

What does wonders for copying weak signals with the PERSEUS is to
switch off the AGC."


"No it's not a joke and it's not the RF Gain. It's one of the capabilities of the human ear.

Of course qrm can be limited and reduced but noise is difficult. What you often see is that with all those noise reduction things is that the volume drops. Make an audio recording of a part with and without a (white) noise limiter switched on. Open it into an audio editor and you will see that the amplitude of the part where the noise reduction is on is lower. Now amplify that part to the same level as where the limiter is not active and play it back. You will be astonished how little the difference is.

It's probably also a thing that can differ from person to person but I've never seen tools that can make an unreadable signal readable. Most of the time they sound just different, not better."

Likely there is a ton of data showing how our ear / brain link deals with noise or tones buried in noise. With audio levels set to anything above bare minimum, I think it's very easy for your brain to react mainly to the noise and not to the tone. Reducing this level possibly puts the two back on even levels ... even though there really has been no change in signal-to-noise ratio.

When trying to copy very weak, difficult signals, I've always found that turning audio levels down to bare minimums helps me personally. As Roelof mentioned, the entire environment must be dead quiet as well so that there are no outside distractions. Even the sounds of the headphone cord, brushing against clothing or the table top, can make the difference between copy and no copy. Decades of copying very weak ndb CW idents buried in the noise as well as spending several years on 2m CW moonbounce, has taught me that my ear-brain connection works best when audio inputs are very, very low.

courtesy: http://justagwailo.com/

As an interesting aside, my years of copying weak CW tones, has shown itself in other ways as well. Before retirement as a high school tech ed teacher, staff were required to have their hearing checked annually, as part of the medical plan's requirement. Each year the mobile audio lab would roll-up for the tests. I would always make sure to sit perfectly still, with no headphone cord wires brushing against my clothing. The tones varied in frequency and intensity and were often extremely weak, not unlike the weak echoes I was used to copying from the lunar surface. The reaction from the examiner was always the same, every year ... complete astonishment when checking the results and usually a comment that I had the hearing of a teenager! Thankfully my hearing, which I've always been careful to protect, remains exceptionally good, for which I am truly grateful ... so often this is a genetic thing and there is little one can do about controlling hearing-loss as one ages.

I shudder anytime I see a young person with headphones or earbuds firmly in place and with the music volume cranked up to unbelievably high levels. Sadly, many of them will likely pay the price for this later in life as such hammering-away at the delicate auditory mechanism has a cumulative rather than a short-term effect.

So ... the next time you find yourself trying to copy that ultra-weak signal just riding along in the noise, try turning the audio way, way down. Take a deep breath and listen to the tone, not the noise. If you ask me, the best signal filter is still the one between our ears.

2 comments:

Markus and Gaye said...

Hi Steve. Glad to see this again. And I agree, too loud, no copy!!!! As you are well aware, I am hearing impaired . For me the combination of adjusting the volume very carefully so I can just hear what I am listening to plus the tone freq. which is usually around, 500 to 600 Hz is what works for me. Also , AGC off, RF level set so the noise is just barely heard and sometimes I find that my narrow 250 HZ IF Crystal filter and the very tight 170 HZ audio filter helps limit the level of extraneous QRN to make copy possible. In addition I have a audio limiter which I can set so all noise spikes are clipped often helps a lot too. You know the old noise limiters that boat anchors have. I built one into my home-brew transceiver and wouldn't be with out it. I tried a Digital Noise reducer and found that it wasn't any better then when I set my transceiver up at as outlined above. 73. Markus VE7CA

Photon said...

Very interesting post. I'm sure you are right, and will give this a lot more thought.