Hi Mike,
> Does the input gain slider actually affect anything other than the level
> readout? I recall Joe saying that the decoder always uses the raw PCM
> data. Though I don't know fortran very well skimming the symspec routine
> suggests that only the readout is affected.
The gain slider does not affect the decoding procedure.
> If it doesn't affect decodes then is there some other purpose for its
> existence?
See below.
> I've run into a great many operators who are confused or
> mistaken about it doing more than it actually does. Many see that the
> manual says to adjust the levels to 30dB and then do so by changing the
> slider rather than the RF gain or analog attenuation, which of course
> doesn't help if the audio is already clipping.
>
> It might make sense to give the input readout a makeover in 1.8 to
> resolve this confusion and make it intuitive where the "sweet spot" is.
In Section 5, Transceiver Setup, the WSJT-X User Guide says this:
"Use the receiver gain controls and/or the computer’s audio mixer
controls to set the background noise level (scale at lower left of main
window) to around 30 dB when no signals are present. It is usually best
to turn AGC off or reduce the RF gain control to minimize AGC action. If
necessary you can also adjust the slider next to the dB scale, but note
that the overall dynamic range will be best with this slider not too far
from its mid-point."
Seems pretty clear to me. Use controls ON THE TRANSCEIVER to set the
noise level to around 30 dB on the WSJT-X "thermometer" scale, with no
strong signals present and the slider at mid-scale.
If the background noise level is well above 40 dB with the digital
slider at mid-scale, you lose some "headroom" for handling strong
signals. If the background noise level is much lower than 20 dB, you
may lost a dB or more of sensitivity to the weakest detectable signals.
-- 73, Joe, K1JT
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