I don't believe any definitions of PM and FM matter. PM is a Phase Modulator, and FM is a Frequency Modulator. What is important is what each does to the signal, or requires to maintain the 'status quo', or integrity, of the signal.
If you're going to limit your range to 300-3000 Hz, there is little point in listening to any reasons for one over the other. BTW, I don't think there is any such FCC rule for Part 97. However, if you want to keep all your options open: (and this all still applies to the 300-3000 Hz range, albeit not to as great a degree) Fact: PM does not reproduce DC well. Actually, it doesn't reproduce DC at all (beyond a spike) because in DC there is no change in the phase of the signal. The closer you get to DC, the worse PM will perform. FM, on the other hand, has no such limitation. See pic at <http://groups.yahoo.com/group/Repeater-Builder/files/fmpm1.jpg> . Yes, that is a poor drawing, and an extreme example, but it does illustrate the point. While this *may* not be a limitation of the modulation scheme itself, it is a result of either the scheme or the required associated circuitry. Fact: De-emphasis and pre-emphasis alter the audio (that's kinda their purpose in life). If you de-emph a signal, then pre-emph it, and the two 'curves' don't exactly compliment each other, what you end up with is not what you started out with, and the result will sound different from the original signal. You will most likely end up with a signal that, when compared to the original, is lossy on the bottom and top ends. See example at <http://groups.yahoo.com/group/Repeater-Builder/files/pix/coloraf.gif> . You can compensate for this to some degree, but it's far easier to compensate for it by eliminating both de-emphasis and pre-emphasis in repeaters (more on that later). Of course, it's impossible to eliminate pre-emphasis in a PM exciter. Not only that, to properly limit and clip the signal, you HAVE TO pre-emph it (or provide non-de-emphed audio from the discriminator), limit/clip it, then DE-emph it before sending it to the PM modulator (where it is pre-emphed yet again). This means that could be TWICE that you have to exactly match the 'curves'. Why do I keep putting that in quotes? Because it's really a fairly straight 'curve' of 6 dB/octave. I still call it a curve due to the ends trailing down (again, reference the pic). The biggest problem in matching the 'curves' is the knee of the 'curve', or where the circuit starts to de-emphasize or pre-emphasize. It's fairly easy to push out the 'upper knee' beyond 5000 Hz for pre-emphasis, but the higher you push out the knee, the lower your overall signal level will be - especially at lower frequencies. This one usually isn't a problem since you may only be pushing it out one octave to gain the desired effect to 5000 Hz. It's much harder to push the de-emphasis knee lower for the same reason. For every octave you push it down, your signal will be 6 dB lower in level. It doesn't take long until you run into level problems and need amplifiers (another source that will likely distort or color the signal). Now, the best way to match the 'curves' is to eliminate any changes. If you run the discriminator signal directly to the FM modulator (commonly called Flat Audio repeaters, or a term I coined... Unprocessed Audio repeaters), through the limiting/clipping circuitry, without any de-emphasis or pre-emphasis, you are transmitting a signal that is about as close to the original as you can possibly get. Since PM has inherent pre-emphasis, you must provide de-emphasis and exactly match the two of them. That is not necessary with FM. That's why FM is better than PM. Joe M. Coy Hilton wrote: > > First I would like to read each of your personal definitions of FM > and PM, then your dissertations on exactly why one is better than > the other, Complete. > > If I remember, The FCC requires 6 DB/octive preemphasis/ deemphasis > from 300 to 3000 hz. on PM or FM and that is pretty much the human > voice band width...I know that there is some that say that there is > a 7Hz component in human speech. I can't hear that low so it doesn't > matter. I know there is adifference but I would like to read each of > your explination with out interruption or additives from the other. > 73 and looking foward to it. <grin> > Coy 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/
