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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