Hi Dave,
What you're seeing is fairly normal for your setup. The 2.7 kHz 5-pole
filter is not as flat as the 2.8 kHz 7-pole filter, which I noticed on
RTTY. I subsequently switched to 7-pole filters (but not in the Sub-RX).
A JT65 signal is about 220 Hz wide, so when you get near the top of the
audio bandpass, you also run into the slope of that filter. As with all
other data modes transmitted by audio, 4-5 bars flashing is optimum.
There's another trick that the latest WSJT-X plays to minimize audio
harmonics that result from overdriving in the audio chain. If you allow
WSJT-X to do so, they will automatically put you in split mode, with an
offset that puts the second harmonic outside the passband of the TX filter.
My recommendations for setting audio levels is here. it follows the
discussion of USB interfaces. http://k9yc.com/USB_Interfaces.pdf The
reason I recommend setting computer sound cards 6-10 dB below audio clip
is that their percent distortion often rises by 10 dB in that last 6 dB
of output level. On a vintage Thinkpad, I measured -30 dB second
harmonic relative to fundamental just below clip and and -40 dB below
fundamental with the output reduced by 6 dB.
One update to my "rule of thumb" about whether you need a better
interface is that, thanks to decoding improvements in the latest version
of WSJT-X, you should occasionally see good decodes in the range of -26
to -28 for JT65. And a caution -- while second pass decoding often finds
a second station on nearly the same frequency as one decoded on the
first pass, roughly 5% of the time a second pass decode may be bogus.
73, Jim K9YC
On Sun,4/16/2017 2:29 PM, Dave Hachadorian wrote:
Transmitting on JT-65, I see a lot of variation in ALC and power as the various
tones change. I’m using Data A mode with the stock 2.7 filter. I just had a
QSO with tone frequencies of about 1800 – 1950 Hz, and ALC varied from zero to
five bars on the different tones. Sound card output is about 50%, and Line In
level is at 12. Is this variation normal, or am I doing something wrong, and
where should I set the ALC?
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