At 4/29/2004 10:05 AM, you wrote:
>Hi Bob,
>
>Fortunately, voice emission PSDs are spread fairly evenly across
>the entire 300 to 3000 Hz modulation band (thanks to pre-emphasis,
>otherwise the PSD would be sloped toward the low end, & yes we're talking
>FM not PM Bob don't go there!!)
>
>
>How could I turn down such a nice invitation? :-)
>
>Communication is difficult without definitions. PSD is some sort of
>distortion measurement?
Sorry: PSD="power spectral density": how the power is distributed as a
function of frequency. I should have spelled out the term first then used
the abbreviation.
>
>General comments: Carson's Rule holds true for all types of angle
>modulation, FM and PM included. And yes, the male voice peaks somewhere
>around 300-400 Hz, so preemphasis does spread the energy over a wdier
>spectrum. Beyond that, I'm not sure what "FM not PM" means.
I didn't want to confuse a highly technical topic with the FM/PM thing
again. Personally, I find it easier to think of angle modulation in terms
of frequency as opposed to phase. It's just easier for me to envision an
EM wave changing frequency & how that looks on a deviation scope as opposed
to changing phase. This is probably again due to the lack of practical
phase modulators (I've never seen a scope that directly displays phase vs.
time).
Don't think that I don't "like" PM, though. All my 440 MHz TXs are phase
modulated. The G.E. phase modulators seem to do fairly well at UHF. To
achieve "flat" audio, I simply de-emphasize all the way down to 60 Hz or
so. Applying this audio to a phase modulator makes for a very flat overall
system response. There may be a small amount of distortion at low
frequencies, but my ears can't hear it.
>
>Virgil and I have been trying for a long time to show that FM and PM
>aren't two entirely different modulation schemes; one is just the
>derivitive of the other. With that close of a relationship, it just seems
>counterproductive to keep talking about the two as if they were as
>distantly related as AM and FM.
>
Of course. Sorry if I made it sound otherwise, but I didn't want to
complicate the discussion of Carson's Rule & TX bandwidth by having to
explain how the numbers add up if using a phase modulated TX. For example,
how do we determine the modulation index from phase deviation for use in
calculating the sideband amplitudes from the Bessel function?
Bob
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