Re: [Repeater-Builder] Re: Audio Reproduction

[scomind]

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 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 the Yahoo! Terms of Service.