VP6DX -- the true story ?

[David Woolley (E.L)]

Eric Scace K3NA wrote:
>   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.
>
The product detector (or FM discriminator) is implemented in the DSP
firmware, so, if you could bypass the DSP, you would have a signal at
the final IF frequency (15kHz?), not a baseband one.

--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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[Eric Scace K3NA]

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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>

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[Marteinn Sverrisson]

I just finished assembling my new K3 s/n 1113. Now playing with it and trying
out some features.

The firmware is 2.02.

I tried CW text decode function, setting it up as shown om P. 31 in the
manual.

I get no decoded text on the display.

Also tried RTTY decode. AFSK A and also no decoded text.

What do I do wrong?

73, Matti

--
   Marteinn Sverrisson    TF3MA
          Langitangi 2    Internet: tf3ma [at] raunvis [dot] hi [dot] is
        270 Mosfellsbær   <http://www.raunvis.hi.is/~tf3ma>
               Iceland
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[Marteinn Sverrisson]

Sri, I forgot to tap the DISP, button. Now all is working correctly.
Very foolish of me...  :-(

73, Matti
Sri, abt the wasted BW

On Sat, Jun 28, 2008 at 04:18:48PM +0000, tf3ma wrote:

> I just finished assembling my new K3 s/n 1113. Now playing with it and trying
> out some features.
>
> The firmware is 2.02.
>
> I tried CW text decode function, setting it up as shown om P. 31 in the
> manual.
>
> I get no decoded text on the display.
>
> Also tried RTTY decode. AFSK A and also no decoded text.
>
> What do I do wrong?
>
> 73, Matti
>
> --
>    Marteinn Sverrisson    TF3MA
>           Langitangi 2    Internet: tf3ma [at] raunvis [dot] hi [dot] is
>         270 Mosfellsbær   <http://www.raunvis.hi.is/~tf3ma>
>                Iceland

--
   Marteinn Sverrisson    TF3MA
          Langitangi 2    Internet: tf3ma [at] raunvis [dot] hi [dot] is
        270 Mosfellsbær   <http://www.raunvis.hi.is/~tf3ma>
               Iceland
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