Hola,
After reading about your software design difficulties for a
sound card millivoltmeter I have a different idea - so plan B.
No I have two modern laptops without serial or parallel
ports, so please put out of your mind all you mentioned
so far, but thanks for trying.
My new idea is to use the Baudline db scale on left hand side,
only as an indicator with correlation to absolute millivolts.
I have proved here that the db scale is linear so no problem.
The maximum audio level recommended for pc mic safety is
150mv RMS before squaring occurs, therefore I use here
100mv RMS as the highest working test point, which is -18db.
As mentioned previously, -20db equates to 77.5mv RMS which is
the most useful test point for pc audio level comparison purposes.
As you probably know, every 6db reduction is half voltage,
so for example -26db is half 77.5 or 38.75
and -32db is half 38.75 or 19.37 etc etc etc.
It's easy now to correlate the vertical scale to mv and a
two-column scale on paper can thus be made up by the user.
So no essential need in this case for a s/w millivoltmeter.
The next step is proving the correct signal level entering
the pc mic socket, with the mic slider gain setting at about
1/3rd up from bottom and pc mic software boost OFF.
Preparation for setting up and test cables required:
The user must make an audio patch cable from an external
generator to the mic socket.
Or a loop-back cable from pc speaker output socket to mic
input socket with say your internal siggen enabled at 1khz
and "gain" at about 0.39 for 77.5mv RMS sine output.
There should be an extra separate cable, with one end
parallel soldered to the mic jack plug connections and the
other end suitably terminated to plug into a good quality
general purpose multimeter capable of reading low level
AC volts.
I now draw your attention to my web article contribution
in CXI, for a detailed guidance explanation on this subject:
http://www.zen22142.zen.co.uk/Circuits/ ... estset.htm
It would be helpful to read the whole article on this small test set,
but please pay extra attention to the section Calibration using
the good multimeter.
As explained, the multimeter reading in average display,
is mv RMS x 0.9
So 77.5mv is .069v.
Finally when the correct reference mic input signal is proved
at .069v on the multimeter, Baudline input gain slider can be
set so the fundamental tone spike tip is at -20db and the rest
of the db scale is thus correctly referenced linearly.
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If a fluctuating audio signal is then applied to the pc, by looking at
Baudline, the whole dynamic range of audio can be assessed
both in db and absolute mv terms.
I hope all this information for my plan B is understood.
Best regards to all.
