Hi Joe,
Bob (S-Com Bob), would it be a fair statement to say that
traditionally,
transmitters have generally been PM (either true PM or
emphasized FM),
but receivers have been FM all along? I base this on your
last post.
Yes, absolutely!
To further state the case, assume that there are two
parallel universes.
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.
In the other universe, 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.
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?
Is the FM ham correct in saying that the PM guy is transmitting a
'nonstandard' signal with preemphasis? In an FM universe, PM seems strange
because it has a tilt to its response.
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.
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.
Because we are crossing the two systems, we will always have to deal with
the existance of preemphasis and deemphasis.
It's neither good nor bad, it just is.
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. 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
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