Thank you Jim and Don,
I based my comments on the experiences I've had with the K3.
When I was running AFSK, I would use the standard tones 2125/2295, set
the levels on the K3 with 4 bars showing, the 5th flickering (no ALC).
But if I tapped the display (in HRD) to center on another station
(changing tones to say 1000/1170) I often noticed that the bar display
was often very different, either pegging the 5th bar or not even
reaching the 4th bar. Adjusting the Mic gain was the simple fix, but it
was something that had to be watched for each click of the mouse.
Based on the assumption that it wasn't the K3 as the cause, that leaves
the computer (no equalizer in play, so not likely) or the sound card (in
this case a SignaLink).
It's not scientific research but there you are. I don't have the
training or tools (scopes) to make refined repeatable lab quality
observations of audio. It also stops the K3 from power hunting if you
watch the audio levels (rarely saw that, even with 3 bars showing, even
the amp stayed at full power).
These days I use FSK and manually tune so this is a non-issue for RTTY
but having experienced this before, I watch carefully on FT8 (and it
doesn't seem to be an issue since I use 'fake it' which keeps the actual
tones between 1500-2000 Hz. and I'm unlikely to transmit harmonics of
the audio).
73,
Rick NHC
On 6/11/2019 11:07 PM, Jim Brown wrote:
On 6/11/2019 5:42 PM, Rick WA6NHC wrote:
A simple test, into a dummy load, set your audio tone for a lower
tone, watch the output, repeat with a mid-range tone and a higher
pitched tone. If you don't have to readjust the audio drive to the
radio (while keeping NO ALC), you're fine. If you do have change
levels, use the frequency shifting of WSJT-X to stay within the same
range.
Rick,
You (or at least your test) are (is) blaming the computer sound card
for what may also be the response of the TX bandpass filter. I would
be VERY surprised if the response of a sound card varied more than a
few dB from 200-3,000 Hz. If audio levels between the computer and the
radio are reasonably set, a few dB is not going push the system into
distortion.
It's common for RF, digital, and video engineers, whose signals are at
generally fixed levels, to fail to understand that analog audio is
NOTHING like that. Indeed, the level of ordinary voice signals varies
over a range of at least 30 dB. In analog systems, levels are set so
that peaks always stay comfortably below clip, where distortion rises
sharply.
My recommended methods of setting the output levels of sound cards all
boil down to one principle -- set the output of the sound card at
least 6dB below where it starts to sound distorted. Very easy to do,
at least three ways.
1) Use your ears -- plug headphones into the computer output, activate
"Tune," start with the Power slider fairly low and increase it until
the tone starts to sound harsh. THAT'S distortion. Now, back off the
Power slider until it sounds half as loud. That will bring it 7-10 dB
lower. Run the sound card there. (This works because humans perceive
7-10 dB "half as loud" or "twice as loud."
2) Use a scope to find clip by seeing flat-topping of the sinewave,
then back down to at least half the voltage, or maybe slightly more.
3) Use a voltmeter to find where the voltage stops increasing with the
Power slider, then it down so that the voltage is half or less of the
max value.
Once you've done this, the sound card should safely drive a line level
input without overload, and audio transmit level can be set just like
you would with speech. If the radio has no line level input, you
simply build a 20 dB voltage divider, (470 ohms in series between
computer and rig, 47 ohms in parallel with the mic input). or 1K and
100 ohms.
This is all very simple, it's stuff that we demonstrated that we
learned when we passed our license exams.
73, Jim K9YC
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