Re: [Repeater-Builder] Re: Audio Reproduction

13 Apr 2004 19:28:50 -0000

Hi Joe,
 
> 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.
 
Until a few days ago, I would have agreed.
 
Why do we all say that the response of a PM system rises 6 dB per octave? Because that's what we get out of our discriminators. But we're using a frequency demodulator to demodulate phase modulation, and that's where the rising response comes from. That's the whole point.
 
Frankly, we're all so used to FM demodulators that we don't even know what a PM demodulator looks like. It's called a phase detector, and while we can easily create one on the bench, we can't use it in radio systems. But the truth is, if we all used phase detectors, we'd never see preemphasized audio.
 
 
> 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. ;->
 
So I've heard...  :-)
 

> 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?
 
The math is on the side of the former. One of my old college texts is Communications Circuits: Analysis and Design by Clarke-Hess. On page 578 there is a block diagram of a basic frequency demodulator, and it shows a differentiation network ahead of an envelope demodulator. The d/dt term is definitely there, and that's where the preemphasis comes from.
 
I know all of this looks like pure heresy, but when you realize that 100% of all demodulators out there are frequency demodulators, it makes sense that we would think PM has "natural preemphasis" - - we see it 100 times out of 100 on our service monitors and receivers. But when we realize that we are always looking at both PM and FM with an FM detector, the puzzle seems to come together a bit more.
 
73,
Bob


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