Eric, thanks for your posting. The description of human hearing mechanics is
very educational.
OK on DSP issue. Actually one other member of the dxpedition mentioned that
the effect of DSP that I have described is well known to him and was present
in K3 as well. He also mentioned that switching off AGC was one of the
methods to overcome this effect. That was my initial reason to ask you these
questions. I think you are right. DSP in K3 cannot be completely disabled
(as well as in most modern DSP based radios) Therefore it is hard to tell if
the effect should be attributed to DSP proper or DSP derived AGC. Anyway
several people told me that they have made comparisons between DSP based
radios and purely analog radios like IC781. In conditions of pile up made up
of weak signals (not more then S3) analog radios always performed noticeably
better.
73, Igor UA9CDC
73, Igor
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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