Hi all Lowfers,
1. I'm happy to report that AC power came on at my home late yesterday.
Things here are slowly returning to normal -- but they still have a
long way to go. I know that many in this region are not so fortunate as
we have been.
2. Unless I hear of some serious problem, another release of WSJT-X is
unlikely for about 10 days. My available "radio time" this weekend will
be devoted to the ARRL International EME contest, and next week I am
busy with travel and QRL activities.
3. You may be interested in a full explanation of the audio signal level
issues we have been discussing. Some have suggested that with "normal"
audio levels from receiving equipment, the indicated WSJT-X input levels
are extremely high -- perhaps close to the full-scale "60 dB" mark on
the WSJT-X green-bar level meter.
The 16-bit digital samples from a soundcard fall in the range -32768 to
+32767. The reference level for the WSJT-X level meter is 1 bit, so
"full scale" or 60 dB means an rms level of 10^(0.05*60) = 10^3 = 1000.
This may be a "normal" level for the receiver and audio settings you
have used for other purposes. That fact might lead you to think that
the WSJT-X level meter reads "ridiculously high". In fact, though,
setting the WSJT-X leval meter to a much lower level of 20-30 dB (with
the recently added slider set to mid-scale) is still a good idea, and
here's why.
If the background noise level is set so that the rms output from the A/D
converter is 1000, you have only about 30 dB of headroom for strong
signals. That's not much dynamic range. Remember, we want to operate
with AGC off so that WSJT-X can find and work weak signals even when
your neighbor may be active the band.
A much better setup is to have the rms output from the A/D converter
more like 10-30 counts, or about 20-30 dB above 1 count. Yes, this
means that only a few bits of the A/D converter's range are being used
much of the time, when you are receiving weak signals plus background
noise. But almost nothing has been lost: A/D quantizing noise is still
negligible with respect to external background noise. Moreover, all
those higher-order bits are now available for the headroom needed to
give you 60-70 dB of dynamic range, rather than just 30 dB.
So: use the digital gain slider, if you must, to make the green bar
shorter. But recognize that if you choose to run with your 16-bit A/D
samples around 1000 or so, rms, you will have sacrificed 30-40 dB of
available dynamic range.
4. Another matter of interest: several *.wav files sent to me, and
several screen shots recently posted on the web, indicate that some
WSJT-X users are running with rather narrow filters (maybe 500 Hz or so)
in the Rx chain, ahead of the soundcard. In general this is not a good
idea.
Unless you need to get rid of some extremely strong QRM -- so strong
that WSJT-X would not be able to handle it -- wider bandwidths are
better, up to about 5 kHz. The normal bandwidth 2.5-3 kHz bandwidth of
an SSB transceiver is fine, and almost certainly is preferable to a 500
Hz CW filter.
Why? Because WSJT-X already does all necessary filtering, down to the
fraction-of-one-Hz tone spacings used by the various JT9 submodes. A
wide bandwidth coming into WSJT-X gives the software a *much* better
opportunity to do effective noise blanking. The noise blanker will be
highly effective, even if strong signals are present. (Note that I say
"will be". I have not yet optimized behavior of the NB in WSJT-X. This
is still to be done.)
When running in any of the JT9 submodes, the background noise level
shown on the WSJT-X waterfall should be nearly constant over the full
displayed frequency range. You should *not* see the peaked effect of
your IF filter(s) on the waterfall!
5. A brief comment for those experiencing sporadic decodes. With
steady, non-drifting signals that are time-aligned to within +/-2 s,
WSJT-X should decode JT9 signals very reliably down to levels within 1
or 2 dB of the thresholds listed in User's Guide table. If you are not
experiencing this level of reliability, there is surely a reason -- and
we'll help you find what that reason may be.
6. As already stated in the brief User's Guide: "A number of reported
parameters have not been properly calibrated." In particular, you
should take the displayed values of S/N as relative indicators, with
significant uncertainties -- especially if your passband is not very
flat, or if there are strong signals present, or ... I will turn some
attention to getting these things right, in due course.
7. Finally -- when I can again devote some time to WSJT-X, probably
starting in the week of November 12, I will be QRV myself. I'm ready to
put up a reasonably good Rx antenna for MF and LF, so I should be able
to listen for transatlantic signals. I am QRV on 160 m already, and
have made a number of JT9 QSOs there. (I note that a number of others
are playing with JT9 modes on 160 m and HF, even up to 50 MHz.)
That's it for now!
-- 73, Joe, K1JT
|
