--- [EMAIL PROTECTED] wrote: > Hi Joe, > > I'd like to look at the above statement for just a > minute. Since we're all in agreement that PM has a > 6db > per octave inherent "pre-emp" (that would be 20 dB > per > decade), let's look at the numbers in something > other > than a "theoretical" light- ie; real numbers. > > Real modulator voltages have to start somewhere as a > reference. If we take .1 volt P to P to deviate a PM > exciter to 5KHz of deviation at a 1KHz audio tone, > then it will take 1 volt P to P to drive the same > modulator to 5KHz deviation at 100 Hz of audio. I > think everyone will agree with this so far. > > It would therefore take 10 volts P to P to modulate > the 10Hz tone, and 100 volts P to P to modulate the > 1 > Hz tone. If we went down to .01 Hz (as mentioned > above), it would take 10,000 volts P to P to > modulate > the exciter! Personally, I don't want to be working > with such voltages > > We weren't going to delve into the topic that > follows at this time because > it'd be better to write a paper instead. But, since > we're talking theory here, > what the heck... > > Do you recall the relationship between acceleration > and velocity? > Acceleration is the derivitive of velocity. One way > of looking at it is you wouldn't use > the same kind of equipment to measure both > quantities. A velocity meter tells > you how fast the object is traveling. An > acceleration meter responds only to a > change in velocity. If the velocity is held > constant, the acceleration meter > reads zero. Yet, the object is moving, which makes > it seem strange that the > acceleration meter says zero. > > In radio, has your sense of symmetry ever made you > wonder why we phase > modulate and we frequency modulate, yet we don't > phase detect and frequency detect? > No? Well, okay, me neither - - until we noticed the > differentiation term in > the equation for the basic frequency demodulator. > That means that a frequency > discriminator is a differentiator! It doesn't > respond to a change in phase, it > responds to the rate of change in phase. It measures > the acceleration, if you > will, not the velocity. > > In the narrowband FM world, we always > frequency-demodulate and we never > phase-demodulate, right? That's because we can't > phase demodulate without a > reference phase for comparison, and the receiver has > no way of obtaining that > reference phase. So we frequency-demodulate instead. > If we used phase detectors > instead of discriminators, phase modulation would > work all the way down to DC. I > could shift the phase of my transmitted signal any > amount, and your receiver > would display exactly the amount of my phase change. > > Bottom line, and this'll be a bombshell for some: > The whole issue of > preemphasis and deemphasis is related to the fact > that we use frequency discriminators > to demodulate phase modulation! If we didn't > criss-cross FM and PM as we do, > we'd never talk about preemp and deemp. I don't know > about others, but from a > theoretical perspective, this is a whole new way of > looking at things for me. > > I'll just bet all of this stuff exists somewhere in > the form of dusty > engineering notes, and that early radio engineers > went through all of it before. It'd > be fun to find it. > > 73, > Bob, WA9FBO >
Again, I think you're off base with your supposition. You want us to believe that PM is why we Pre-emp FM. That's simply not the case. This is not supported by anything I have ever seen or read, only by you. Here is what I believe. Pre-emp is an easy way to get a few extra dB of SNR by "tilting" the audio response- thats it, plain and simple. Has nothing to do with PM, or the fact that early modulators were PM. It would be just as easy to "de-emp" the audio going into a PM exciter, and arrive at "FM" as it is to put a de-emp circuit in an FM RX. But, audio sounds better when the RX is de-empd. The squelch bursts aren't as bone rattling, since they consist of a large amount of higher frequency noise component. Since as FM gets weaker, the noise starts to increase at the higher end first- thats how a voter system works, or a noise squelch. You can often cause a radio to squelch when the voice audio still sounds pretty good, but there is enough noise at the higher freqs to activate the squelch. Reduction of annoying high freq noise on a demodulated FM signal is why we de-emphasize. Joe __________________________________ Do you Yahoo!? Yahoo! Tax Center - File online by April 15th http://taxes.yahoo.com/filing.html Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
