Sunday 24 July 2016

Weak Signal Volume Levels

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 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.


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.


PE4BAS, Bas said...

Thanks fro your post. It actually confirms what I always thought when lowering the volume. Btw, it takes a few minutes before your ears (brain) adapt. And of course you need complete silence around you. 73, Bas

Steve McDonald said...

Thanks Bas. I have noticed that as well and should have mentioned it. If my ears take a sudden loud-signal hit, it takes a few minutes to return to normal sensitivity levels again ... kind of like walking from a bright room into darkness and having to wait for your eyes to respond accordingly.

Anonymous said...

Good day Steve,

Interesting topic. Something I didn't see mentioned was that when straining to hear the very weak simply closing your eyes will also help you "hear".

Seems the brain spends a great deal of it's effort processing visual inputs, reducing that sensory input allows the brain to use more effort in processing it's other inputs. Kind of the like the comments you often hear about someone who has lost their sight suddenly has an improvement in one or more of their other senses.

I bet many do this (closing their eyes) when trying to hear to those weak signals but don't realize that they have or even that they did.

Works for me, I do it all the time. I also turn down the volume and the RF gain to hear better too.

cheers, Graham ve3gtc

Steve McDonald said...

Graham - you are spot-on and as you say, I didn't even realize that I often do this without thinking about it as well! Another great tip for those that have not tried this. I will often sit back, eyes closed ... with light turned off, and listen to a weak ndb CW ident as I try to pull it out of the din. Thanks for the reminder / pointer!

Vasily Ivanenko said...

Hi Steve.

I’m a speaker only listener – the speaker must be dead center for me and lowering the volume does not change my ability to copy CW tones in noise. Beat frequency and the bandwidth of the IF or AF filter do. I hear better with wider filter bandwidths as long as the receiver isn’t suffering IMD as a result of my filter selection and/or its strong signal capabilities or LO phase noise.

Hearing is ultra complex. I think of the apparatus of the inner ear much like an RF stage: affected by modulation of the anatomical structures; subject to phase response, resonance, dampening, signal-to-noise ratio, frequency response , bandwidth etc.

While much is hypothesized, spontaneous otoacoustic emissions , frequency discrimination, reduced modulation detection thresholds and frequency thresholds vary according to factors such as age and gender. Some research where subjects detected gaps in known frequency AF tones shows the type of background noise [ Gaussian versus white noise etc] and its bandwidth affected the ability to detect gaps in the audio tones. Sinusoid versus other distorted tomes may also be at play.

The effect of auditory load – how hard you are listening to some sound also affect some . For example, when concentrating on a radio show while driving, you may fail to hear a siren from an emergency vehicle.

Then, too, comes cognitive processing by the auditory cortex . What mysteries lie in how we process sounds inside the brain. Variations may occur with diseases and chronic conditions. Also, with training some subject can hear tones better -- this may be a cognitive factor.

Finally, the characteristics of the room may play into this, speaker versus headphones – how will changing the volume affect those characteristics?

It’s good to experiment to boost your copy, but hard to draw firm conclusions without solid data.

Thanks for your great blog!

Steve McDonald said...

Todd - Thank you for your kind comment. You do bring up many valid and interesting points. There is certainly much more to this than simply 'turning down the volume'! With the human-factor variation being so dominant, it would be a challenging task to gather data as well.
My comments pertain only to CW, which I probably didn't make overly clear. I think one thing for certain (and I say this with caution!) is that the difference in copiability between a speaker and a good set of headphones combined with low ear-impact volume levels, is the difference between night and day.