I'd like to be able to run the line
out audio from a scanner into the sound card and be able to display
the audio envelope. The amplitude scale wouldn't need to be calibrated
as long as I could count the number of divisions, I could compare the
peak deviation from the 252 to other signals on the air. A quick and
dirty deviation comparison checker.

Mike-

I have an old Heath deviation meter that uses a peak-reading meter to display
deviation. It was necessary to calibrate it using the "Bessell null method"
using a signal generator with sine-wave FM modulation. The signal is received
on an SSB receiver with a fairly sharp filter. Modulation is adjusted for a
carrier null, and the deviation is obtained from a table of bessell functions,
based on the ratio of modulating frequency to peak deviation. This method is
outlined in some ARRL publications that cover VHF-FM techniques.

What you describe is very close to how other deviation meters work. They
receive a signal, convert it to an intermediate frequency, FM detect and
display the detector output on an oscilloscope. It is a lot easier to
calibrate this kind of meter, since you only have to offset the transmitter's
frequency by plus and minus one KHz increments, and adjust the oscilloscope to
display the correct offset.

Using a deviation meter, it is normal to use some loud sound such as whistling
directly across the microphone, to adjust a radio's peak deviation control for
about 5 KHz. Then you adjust the radio's microphone amplifier so average
speech runs around 3 KHz.

I have a Turner desk mic connected to the 252 at the moment. I don't
really know if It's an improvement or not. I built a little adapter
box with 6 push button switches so I can control the radio,

The problem with the HTX-252, is there is no microphone amplifier adjustment,
and some people think it should work when speaking some distance away from the
microphone. Your Turner microphone may provide the extra amplification needed
to talk at a greater distance. While I won't tell someone they shouldn't do
that, I can at least partly justify keeping the adjustment set for talking very
close to the microphone. By close, I mean about a quarter inch or one
centimeter.

At home in the shack, you probably don't have sound proofed walls and ceiling,
but the ambient noise often consists of only an air conditioner blower or
Television sound from the next room. Under that condition, it wouldn't hurt to
have a little more gain so you keep the microphone at a comfortable distance.
However, many uses of the HTX-252 involve mobile operation where there are a
lot of loud noises from the engine, traffic and an even louder air conditioner
blower. In a heavy rainstorm you may not realize how loud it becomes. The
result is a decreased audio signal-to-noise ratio. The only way to overcome
that is by close talking, which increases the signal level with respect to the
background noise level. This condition is the basis of most Ham Radio and
commercial communications equipment designs.

Obviously many people don't subscribe to that philosophy, so there are a number
of amplified microphones on the market!

73, Fred, K4DII