> [EMAIL PROTECTED] wrote:
>
> To further state the case, assume that there are two
> parallel universes.
Que "Star Trek" theme music... ;->
> In one universe, all hams use phase modulators and phase demodulators.
> The terms 'preemphasis' and 'deemphasis' are unheard of. The whole
> system has flat response. Since their phase demodulators track
> their phase modulators, the hams communicate via voice and data over
> their PM systems, all the way down to DC. They don't transmit
> "preemphasized audio" because they don't even know what that is.
> They've never seen it. You put flat audio in and you get flat audio
> out, including DC.
Actually, they would be transmitting pre-emphasized audio, as the audio
component would multiply every octave for the same input voltage. Yes,
it would be done transparent to them, but it would still exist. It has
to since the PM has natural emphasis. 1V input at 1000 Hz would result
in more deviation than 1V input at 500 Hz.
So, phase demodulators have natural deemphasis, right?
> In the other universe,
This would be our universe in the case of 'flat audio repeaters'. :-)
> all hams use frequency modulators and frequency
> demodulators. The terms 'preemphasis' and 'deemphasis' are unheard of.
> The whole system has flat response. The hams communicate via voice and
> data, all the way down to DC.
BUT, in this universe, the level of the audio is the same as long as the
voltage input remains the same. IOW: The frequency doesn't matter. 1V of
input at 100 Hz is the same level as 1V input at 1000 Hz Both will get
you 3 kHz of deviation. In this case, there really is no preemphasis.
Could you call this universe the universe of "True FM"??? (ducking)
> One day, a rip occurs in the space/time fabric and an FM-equipped ham
> hears a PM transmission from the other universe. It sounds tinny. Why?
> Because the FM guy's frequency demodulator (discriminator) is a
> differentiator. It pre-emphasizes the PM transmissions, so he hears
> attenuated lows and emphasized highs.
>
> So, which end is responsible for the tilt?
for the reasons I mentioned above, the PM guys are responsible. They
HAVE to be. If for no other reason, because PM is Bad. Bad bad bad. ;->
Seriously. It is the PM's fault due to the natural preemphasis. That
universe may not realize it, but it does exist. Ignorance of the
physical laws is no excuse.
> But the PM ham has never even heard of preemphasis, and has certainly
> never had to use it. He assumes the FM ham's receiver is responsible
> for the the tilting. After all, a frequency demodulator
> isn't sensitive to the phase of the incoming signal, it's sensitive to
> the rate of change of the phase, so the FM ham must be using a wierd
> receiver.
And to the PM guys, the FM users all sound bassey. That's because of the
(assumed) natural deemphasis of the phase demodulator.
> We live in both universes. We've decided that we will always transmit
> PM - - it doesn't matter whether it comes from a PM transmitter or an
> FM transmitter with preemphasis, it's PM mathematically. And, we've
> decided that we will always receive with a frequency demodulator.
True, except for 'flat audio repeaters'. They will work exactly the same
in either universe, as there is no pre-emphasis, de-emphasis, or
sub-space-emphasis in the repeater at all. The technology will seem
weird, but it will pass both equally well.
> Because we are crossing the two systems, we will always have to deal
> with the existance of preemphasis and deemphasis.
See last comments. :-) Lotsa repeaters don't have de-emph or pre-emph.
They just pass what is received. If it's a PM format signal, it comes
out a PM format signal. If FM, it's FM.
> It's neither good nor bad, it just is.
Naaa... PM is bad. Pure evil. ;-P
> But when you look at the picture as described above, it sort
> of nullifies what all of us have been saying for a long time - - that
> preemphasis is a natural result of phase modulation. No,
> preemphasis results from demodulating PM with a frequency demodulator.
An interesting theory which nullifies most of what I said above. Is that
really true, or is the pre-emphasis really existing in the PM, and it
would just be countered by a natural deemphasis in the Phase
Demodulator?
> It seems like preemphasis ought to be a natural result of PM, but
> that's because we only think in terms of discriminators. After all,
> all of our receivers and service monitors have them and no one has a
> phase detector. But it might be good to keep the big picture in mind
> during some of these theoretical discussions.
>
> 73,
> Bob
So, how is one to say that PM isn't naturally pre-emphasized and
naturally de-emphasized in a phase demodulator? How can you measure the
effects of each in the RF medium? After all, if you can't use a phase
demodulator or a frequency demodulator to test this, since each would be
'biased' (no pun intended).
OK. One last thing for this universe. Let's assume that it IS the
frequency demodulator that adds the pre-emphasis to the signal of a
phase modulator. Since it doesn't have the same effect on a FM signal,
doesn't that then imply that there is an inherrant difference between PM
and FM in how the signal is transmitted? In person, are you listening to
the frequency of my voice, or the phase of it? (most likely both!) What
does a sine wave input look like on FM VS PM?
A pleasure as always, Bob!
73,
Joe, KR3P
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