Hi Dave --
There is a lot of information about hearing available on the web. As
always, one should cross-check sources and exercise some skepticism.
Hearing is logarithmic. Turning down the gain makes it more likely
that signals will stay below the triggering threshold for the
attenuation reflex, so signals will remain more "different" in strength.
Frequency discrimination, in Hz, is better at lower frequencies than
at higher frequencies. The optimum point for greatest frequency
discrimination depends on the individual. Mine is around 400 Hz. Some
people are lower, some higher... but 400-500 Hz is a good starting
point. Hearing is less sensitive (and dynamic range is a little
smaller) at lower frequencies, so there are tradeoffs. There was an
on-line test for frequency discrimination available on the web... so try
a search.
73,
-- Eric K3NA
on 08 Mar 14 Fri 11:58 dave said the following:
Eric,
Where could one learn more about this very valuable, if arcane,
subject? Human hearing, that is, and how it relates to picking out
signals.
I've long turned my volume down to aid in picking out signals. They
seem to "pop" out of the noise much better at low volumes than at
high. I've attributed this to the greater sensitivity of the ear at
low volumes than at high, but that may not be correct. My
understanding is that he ear is logarithmic, the louder the sound, the
lower the ear response.
Also, are you suggesting that we'd help ourselves in picking out weak
ones by raising the pitch of our CW tone? I kinda prefer the lower
tones, but your comments seem to say that higher pitch, if we could
stand it, would be better.
Thanks for the very interesting comments. Postings such as your's are
why I continue to subscribe to the list - in spite of all the "noise"
here, hee hee!
73 de dave
ab9ca
Eric Scace K3NA wrote:
Igor --
Your question about DSP is an interesting one.
In my limited understanding of the K3 signal path, there is always
a DSP operation on the signal for filtering. While noise blanking,
noise reduction, and audio effects can all be switched off, I don't
know if it is possible to remove ALL digital signal processing from
the signal path; i.e., operate only with the crystal filters.
We certainly had plenty of marginal openings where we were required
to work a pileup of signals just at the level of the underlying
band/antenna noise. But... the antenna/band noise were a significant
step above the receiver's underlying noise floor (10 dB)... or one
turned on the pre-amp to bring the band/antenna noise above the
underlying receiver noise floor. In this case, the pileup was not at
the receiver's MDS level.
Furthermore, the operating positions were not equipped with a
second, different receiver and associated switching so that the
operator could do a real-time A/B comparison between the K3 and
"Brand X".
Yes, signal range could be from S1 (or less) to S9+40 dB, so more
than 90 dB. However, several other aspects of human hearing come
into play:
1. The most sensitive part of audio spectrum for typical hearing
is 2 to 5 kHz. If we take a K3 with very wide filters, and no
antenna, in an extremely quiet listening environment, and just
gradually advance the audio gain until we can just begin to hear the
receiver noise floor, we will be listening to a higher-pitch hiss in
this range of 2 to 5 kHz. White noise at lower frequencies won't be
perceptible yet until the receiver gain is advanced another 10 dB (at
which point frequencies down to 500 Hz are audible) or 20 dB (good
for frequencies down to 250 Hz).
If we narrow the receiver bandwidth so we are only listening to
100-700 Hz, for example, the receiver noise floor will appear about
10 dB louder (relative to the minimum threshold of hearing) at the
higher end.
2. Another frequency-sensitive aspect of human hearing is the
attenuation reflex. This reflex tightens two muscles in the ear, one
of which tightens the ear drum slightly and the other moves the three
bones of the middle ear to reduce the transmission to the cochlea
(inner ear). This is our own, human protective AGC.
The attenuation reflex begins to act at 65-70 dB above the
threshold of hearing at 200 Hz... but 80 dB above the threshold of
hearing at 700 Hz.
The "slope" of the attenuation reflex is about -0.6; i.e., a signal
that is 18 dB above the attenuation reflex threshold will be reduced
to just 6 dB above that threshold (i.e., 12 dB attenuation added) by
the time it reaches the inner ear.
Now let's look at an operator listening to a K3 in a perfectly
quiet listening environment (no other local sounds). If he adjusts
the receiver so that antenna/band noise is 5 to 10 dB above his
threshold of hearing at a pitch of 400 Hz, and then tunes across a CW
signal that is +95 dB above the band/antenna noise floor, that CW
signal will be about 100 to 105 dB above the threshold of hearing.
That signal will also be about 30 dB above the threshold for
triggering the attenuation reflex. At a slope of -0.6, the
attenuation reflex will cut that signal down by 20 db... so that it
is now 80-90 dB above the threshold of hearing. -20 dB of
attenuation is about the maximum the attenuation reflex can deliver
-- but that is only in children and teenagers. For adults, the
maximum attenuation level declines with age, so I (at age 55) can no
longer get -20 dB of protective attenuation. Maybe I get 10-15 dB of
attenuation, leaving the CW signal at something like 95 dB above the
threshold of hearing. Of course, once this attenuation reflex is
activated, that very weak CW signal down near the noise level will be
attenuated below the threshold of hearing, so no more copy.
Even worse, long exposure to signals above the attenuation reflex
threshold results in incremental and permanent hearing damage. So
that CW signal, at 95 dB above the threshold of hearing... and 20 dB
above the attenuation reflex threshold... represents an important
hazard. The USA National Institute for Occupational Safety and
Health has set a limit of about 1 hour per day at this level... and
that limit declines quickly at higher levels.
3. Fortunately, by setting the receiver gain at these low levels,
that loud CW signal is below the threshold of pain (about 110 dB
above the threshold of hearing at 400 Hz). The threshold of pain is
where the operator rips off the headphones and says "ouch"! We want
our receivers to limit signals (or static crashes) before they reach
this level!
So, we can't use a receiver that is perfectly linear over a 130 dB
range -- it would destroy our hearing! But we need to listen to
signals in a very quiet listening environment, as quiet as we can
get... and set the gain levels appropriately... and use some form of
signal limiting to keep signals well below the pain threshold.
And we should recognize that hearing varies from person to person.
As a result, one person with poor hearing range, listening in a
noisier environment and having his attenuation reflex triggered
often, will have receiver AGC and his own attenuation reflex
interacting to reduce signal strengths... eliminating weaker
signals... while another operator listening to the same radio with
good hearing (big dynamic range between his threshold of hearing and
attenuation reflex trigger point), with minimal receiver AGC, will
find a rich range of signals in the pileup.
The psycho-acoustic phenomenon of "masking" further complicates the
management of a pileup. But that's a subject for another time...
-- Eric K3NA
on 08 Mar 13 Thu 14:57 Igor Sokolov said the following:
Thanks Eric,
I understand you think AGC in high dynamic range radio like K3 is
not a necessity for the op with high dynamic range ears.
Actually the range should be close to 90-95 db on nowadays bands
where 59+40 signals co-exist with 1S unit signals very often. I also
figured from your post that other ops from VP6DX team choose not to
use AGC not because of some sort of problem existed in that version
of the firmware but purely because they prefer adjusting RF gain
manually to squeeze out the most from the pile up.
What about the DSP then. I experienced that myself operating from
8Q7 in the past and many others confirmed my impressions that modern
DSP based radios are no good when pulling call signs from the pile
up of many stations when the average level of that pileup is close
to the MDS of the radio. For example we could not use Orion in the
expedition on a weak pile up on 10 and 15 meters while the same
operators could easily pick out calls using IC775. Therefore I would
be happy to have a radio where DSP could be switched on or off
depending on the situation. Did you notice the above mentioned
effect while using K3?
73, Igor UA9CDC
Privet Igor --
As best I can remember, we used MCU v1.66 and DSP 1.52.
I didn't use the AGC because I did not need it. The point of
automatic gain control is to adjust the gain of the various stages
of the receiver in order to
a) avoid distortion/overdriving a stage, and
b) bring signals up/down to a comfortable listening level.
(b) is not relevant to an operator who is working a pileup. (b)
is relevant to an operator who is monitoring a frequency with one
station transmitting.
For operating a pileup, there are a variety of tools the brain
uses to distinguish the many signals:
-- pitch (CW)
-- style of speaking (speech) or keying (CW)
-- artifacts; e.g., auroral flutter, chirp, etc.
-- strength (all).
AGC tends to reduce the difference in signal strength, and so
removed valuable information.
In situations where static crashes interfere with reception, AGC
hang time on a loud static crash also increases the length of time
that a specific static crash interferes with reception.
I used headphones with good audio isolation between my ears and
the rest of the world around me. That allows me to set receiver
gain levels with the underlying antenna/band noise just above my
threshold of hearing... and to use at least 80 dB of my hearing
range for listening. In this quiet listening environment, I don't
need AGC.
Even in a less-than-quiet listening environment, if a band is
just open weakly (e.g., 12m to Europe), the range of signal
strengths in the pileup can be smaller: maybe less than 30 dB
between band noise and the strongest signal. So AGC isn't needed
here either.
My ideal AGC in these situations is one that only makes changes
in receiver gain when a stage in the receiver is about to be
over-driven (e.g., the A/D converter)... and removes those changes
relatively quickly. Even then, it might be fine to allow the
receiver to be over-driven (a static crash contains no
information). If a signal I want to copy is over-driving the
receiver, the best solution often is to reduce the RF gain manually
during the duration of the time when I want to copy that station.
If that station is just "interference" (e.g., a loud USA station on
80m CW calling VP6DX, when I want to work northern Scandinavia and
northwest Russia/western Asia during the brief opening), I have
other controls (filter bandwidth, notch) than might be better to
use that gain reduction (automatic or manual) that could suppress
the desired weak signals.
So, almost any AGC system is inappropriate for a DXpedition or
content environment... as long as the receiver and one's own ears
have enough dynamic range to handle all the signals presented to
it. The K3 has more dynamic range than other receivers.
73,
-- Eric K3NA
on 08 Mar 13 Thu 02:26 Igor Sokolov said the following:
Almost all of the operators ran the K3 with AGC off (all modes).
-- Eric
Eric, can you explain what did you not like about AGC in K3?
Did you notice any problems with DSP being permanently on in K3
when listening to heavy pile ups?
73, Igor UA9CDC
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