Right. Almost ten years ago, I used an audio FFT to measure the
harmonics in a Thinkpad's line level (also used for headphones) output.
The distortion decreased by 10 dB when the output was first set just
below clip and then reduced by 6 dB (half the output voltage). Just
below clip, the second harmonic was -30 dB; at half that voltage out, it
was -40 dB. To put this in perspective, Thinkpads are better than
average laptops.
This is the basis of advice I first published around that time to first
set the output level of sound cards 6-10 dB below full output and then
set input level in the rig. There are app notes on my website that
describe three different ways to do this, depending on your available
tools, one being our ears!
The "ears" method is this. Plug headphones into the computer output
jack, have the computer generate the tone(s) your going to transmit, and
increase the output level until you hear the tone(s) sound a bit
rougher, then back off the output of the computer until the tone(s)
sound half as loud. This works because our ear/brain perceives a change
in loudness of 6-10 dB as "half (or twice) as loud."
73, Jim K9YC
On 6/28/2019 2:29 PM, Joe Subich, W4TV wrote:
Among other issues, this article is incorrect when calling for the
"Device" (Master) volume level in Windows to be set to 100%. Nearly
every sound card for which I've seen test results has increased
distortion above the 70 - 80% level (-1 to -2 dB or -3 to -6 dB
depending on the driver calibration) and some "inexpensive" devices
have significant distortion above 50% (-3 or -10 dB depending on
driver calibration).
One will have a cleaner signal, all other things being the same, by
setting the Windows "Device" (Master) slider down a bit and increasing
the transceiver mic gain (Line In, DATA In, etc.) slightly. In most
cases, with the sound card "Master" at 70 - 80%, the transceiver mic
gain will still be relatively low.
73,
... Joe, W4TV
On 2019-06-28 10:18 AM, Andy Durbin wrote:
Does anyone know of any rig that uses "compression" rather than
linear gain control to implement ALC?
ref - https://www.dropbox.com/s/f4uaop5tqwzaweu/FT8Noise6.docx?dl=1
73,
Andy, k3wyc
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