Hi Hasan and Rich,
a couple of points worth considering:
1) the "Flatten" filter has no impact on decoding whatsoever, so having
"Flatten" checked or not will not change SNR numbers reported,
2) many rigs have considerable roll-off in receiver gain towards the
bottom end of the expected receiver pass-band, the K3 is one of those
BTW. Consider the consequence of that if a reference spectrum correction
is applied to the pass-band? Note the reference spectrum does have an
impact on decoding as the correction *is* applied to the sample data
feeding the decoder.
In summary, the "Flatten" filter is great for improving the visibility
of signals on the waterfall and 2D spectrum, *if* the waterfall & 2D
spectrum frequency bounds are set to reasonable values. The reference
spectrum correction was introduced to help with the wider bandwidth
modes where an uneven receiver pass-band can cause different tone
frequencies of the same received signal to be artificially attenuated
and correcting that can aid decoding of weaker signals since the
decoders assume the whole signal is received at the same constant
amplitude that it was transmitted with.
In general with FT8 and other narrow band modes there is a *small*
chance that a reference spectrum correction may even out the tone
amplitudes of a signal received at the steep roll-off point of your
receiver, and help with decoding. But you must also understand the
consequences of the extra non-linear gain applied to that signal, and
those even further towards the edges of your receiver pass-band, by the
reference spectrum correction. Note that you can view the reference
spectrum captured curve in the 2D spectrum on the Wide Graph window, or
as a small insert graph at the bottom of the Equalization Tools dialog
("Menu->Tools->Equalization tools ...").
It is also worth noting that for MSK144 mode the phase accuracy of the
receiver is far more important than the amplitude accuracy. When using
MSK144 mode a phase correction should be considered rather than a
reference spectrum correction if you receiver performance warrants it.
Capturing a phase correction curve is somewhat more complex as it
requires a known good MSK144 received signal rather than background
noise as a reference baseline.
73
Bill
G4WJS.
On 16/10/2021 10:46, Hasan N0AN via wsjt-devel wrote:
The cause here for that was using Ref Spec instead of Flatten . I
chased it for weeks, it has no effect on decoding, but produces silly
high SNR if below 400 Hz or so. Simply uncheck Ref Spec in the
Waterfall controls, and go back to "Flatten" and I bet the problem
goes away.
73, N0AN
Hasan
On Sat, Oct 16, 2021 at 12:03 AM Black Michael via wsjt-devel
<wsjt-devel@lists.sourceforge.net> wrote:
I can see the opposite possibility in this data. Looks like below
300 the levels are low, high from about 300-400, then pretty
consistent
Red line here is 50-point moving average of 5000 points. Above
2200 we're getting into the region where bandpass limits might
have an effect of some sort (lack of noise or perhaps more likely
to have higher power transmitters?).
Inline image
Mike W9MDB
On Friday, October 15, 2021, 11:45:16 PM CDT, Rich - K1HTV via
wsjt-devel <wsjt-devel@lists.sourceforge.net> wrote:
Has anyone experienced exaggerated dB signal reports given by
WSJT-X to decoded FT8 signals of stations transmitting tone
frequencies below 500 Hz? I use a K3 with 2.8 KHz filter using
WSJT-X Version 2.5.0 . I have also seen this in earlier versions.
When stations using low audio tones are decoded, the signal levels
that WSJT-X gives them far exceeds the levels given to signals
with higher frequency tones of equal intensity level observed
visually on the wide graph.
I have run the “Measure Reference Spectrum” tool on a quiet
frequency and have the “Ref Spec” checkbox on the Wide Graph
checked. This didn’t resolve the noted anomaly. I first observed
this on Foxes running 3 or 4 streams using tones below 500 Hz with
the lower tones appearing a few DB stronger than the higher tones.
Any idea as to why this may be happening.
73,
Rich – K1HTV
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