Since I've also verified this on an IC-7300 with it's own USB sound card seems
it's a Windows bug. I think this is a recent Windows bug since I used to see
linear behavior on the Pwr slider so that reducing to -3dB would cut the power
in half...but no more....
I've reported the problem to MS and tried to word it so they can see how easy
it is to duplicate the problem. But it will probably remain low in their queue
since it seems they tend to work on squeaky wheel things.
Even though the noise is low by the time you have 600 or so transmitters in a
2400Hz window it can add up. The limitation of FFT's to see the true
underlying signal strength is misleading...you need to integrate under the
peak's curve.
The real solution would be to not generate a full scale signal...I've also
verified that reducing the volume level to eliminate the effect does not reduce
the power out either. My LP-700 wattmeter shows the same wattage with 0 or
-3.0dB -- but the noise doesn't start until -0.1dB
It does also show up on an FT8 transmission.
Mike W9MDB
On Wednesday, February 17, 2021, 05:54:15 AM CST, Bill Somerville
<g4...@classdesign.com> wrote:
On 17/02/2021 11:08, Reino Talarmo wrote:
>From: Bill Somerville [mailto:g4...@classdesign.com]
Sent: 17. helmikuuta 2021 0:16
To: wsjt-devel@lists.sourceforge.net
Subject: Re: [wsjt-devel] Clipping
On 16/02/2021 22:10, Jim Brown wrote:
On 2/16/2021 2:01 PM, Bill Somerville wrote:
there are no analog audio stages involved. The trival distortion (lower than
-65dB) is probably the result of being passed through an unnecessary sample
rate converter with a small defect within the MS Windows audio sub-system. The
issue can be demonstrated without ever leaving the digital domain.
I understand that, but I understood Michael to be saying that he used one
computer to look at the audio output of the other. Yes, -65dB is pretty low,
trivial unless the guy transmitting it is 40 dB over S9 and you're trying to
work weak signals. :) Not an uncommon situation in Northern California on 160
and 6M.
73, Jim K9YC
Hi Jim,
I believe you have misread what Mike did, he used two instances of WSJT-X on
the *same* machine using a digital only loopback like VAC or VB Cable.
73
Bill
G4WJS.
Hi Jim and Bill,
There is still one possible mechanism to be checked. If I have correctly
understood what Mike did i.e. he used Tune to generate about 1500 Hz tone and
fed that to the WSJT-X receiver. All were almost perfect below a tone level and
above it bad looking “sidebands” at many frequencies was seen on waterfall. I
tried those files 210214_211315.wav and 21014_211345.wav and saw the same
strong looking artifacts. The essential issue was that those artifacts were
strong in the waterfall, but weak at spectrum display. More importantly the
artifacts were stong only, when I used “Low sidelopes” and “Flatten” activated.
Some minor ones are present (+/- 300 Hz), when I deactivate Flatten. With “Most
sensitive” I cannot see any artifacts.
How flattening is performed may affect to the result, but filtering seems to
affect more.
In any case my conclusion is that those artifacts are due to receiver signal
processing and so not really transmitted at all. I have not tried to analyze
those files using some independent spectrum analyzer.
73, Reino OH3mA
Hi Reino,
your conclusions are incorrect.
We have verified that WSJT-X produces clean audio when the Pwr slider is at
0dB. This has been done by analysing looped back audio on Linux and macOS as
well as by temporarily amending WSJT-X to write a PCM .WAV file with exactly
the samples that are sent to the audio output device. In all cases the data is
exactly as expected, i.e. single tone without harmonics, sidebands, or any
other distortion.
I have also verified that the same audio when recorded via a loopback like VAC
or VB Cable on Windows has the distortion added, the distortion is not
harmonics, it looks like close in sidebands but not a regular "comb"
appearance. I suspect it due to some internal audio processing within Windows
(perhaps a defective re-sampler algorithm that is always enabled), but it is
not possible to rule out VAC or VB Cable although the fact that they produce
identical results does tend to imply the issue may lie within the Windows audio
sub-system.
The distortion components are more that 65 dB down on the main tone so are
trivial and below the likely distortion output of almost any transmitter used
by Amateur Radio operators. If you are really concerned, although there is no
need to be, then reduce the Pwr slider by 0.5 dB and you will have no
distortion. Alternatively, do not use MS Windows ;)
73
Bill
G4WJS.
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