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 --- In [email protected], Kevin Custer <[EMAIL PROTECTED]> wrote: > Hi Bob, > > [EMAIL PROTECTED] wrote: > > > But if I may critique your last sentence from a strict definition > > standpoint (and here's another place where Bob goes over the edge): > > Yes, you can install either an FM or a PM modulator. If you choose the > > FM, it must be accompanied by a preemphasis circuit that covers the > > entire voice audio spectrum. If you choose the PM, you won't need a > > preemphasis circuit. Such a system, by strict definition, is not an FM > > system, it's a PM system. If we had an FM system, we'd all be > > transmitting flat audio. Since we don't transmit flat audio and we do > > transmit preemphasized audio, an engineer outside of our circle would > > say we have a PM system. > > > While that engineer outside our circle say and be true with the > definition, the rest of the world will still use the term FM. > I have several customers that wouldn't even consider a PM modulator; > being shown the differences in yesterday and today's common modulator > technology used in equipment we commonly build repeaters from. If I > touted we have true PM modulation, I wouldn't sell a single repeater to > those folks striving for the best audio. > > > > > I can hear it coming: "Whoa! This here's a True FM transmitter, and no > > lilly-livered engineer is gonna tell me that this thing transmits > > anything but True FM!" Yes, but there's nothing wrong with a PM > > system. In fact, there doesn't have to be a single PM modulator in the > > whole system - - it can consist of 100% FM modulators and still be a > > PM system. The whole definition rests on whether or not the audio is > > transmitted preemphasized. See where I'm coming from? > > > Yes, we drive on a parkway and park in driveways. We also use FM > radios on VHF and UHF and manufacturers build new ones every day. > > > I will readily admit that available PM exciters can have all sorts of > > shortcomings. > > > The operative word is 'available' or readily available. While I'll > admit that there isn't anything inherently evil with the PM modulation > scheme, FM is easy.... We have manufacturers that have built channel > elements and ICOM's that have decent modulators in them as well as > synthesized radios that modulate FM easily and do a good job as well. > Again, in today's world, FM is easier than redesigning the whole radio > around a fancy new PM modulator that has little shortcomings. > > > They distort when you ask for a lot of deviation at very low > > frequencies like CTCSS. There are inherent nonlinearities in the > > varactor tank circuit that create a little distortion. Tuning the > > multipliers correctly is important for low distortion. And we've seen > > some sloppy engineering in the design of the audio and limiter circuits. > > > Yup, these are the things that result in the differences I have argued > over and over that I can hear. I still stand firm to say a common PM > modulator in radios we presently use to build repeaters from cannot > reproduce the user as accurately as those repeaters that employ an FM > modulator. Many of our customers have heard the difference and now > simply insist on us building their repeater with a FM modulator. > > > But if a guy wanted to, he could design one heck of a great PM exciter > > using, say, DSP, and it would work exactly as well as any FM modulator. > > > Yea, but it likely won't happen since it seems that FM modulators will > satisfy even the most picky listener (like me). Plus, FM is cheaper, or > at least right now it is. > > > Has this topic been bantered about for a long time? Yes! Will I > > change anyone's mind? No! > > > To a point I disagree. I think everyone has learned from these > discussions and ultimately we discover and use this technology to make > repeaters better. In time, someone will likely come up with a good PM > modulation system for use in our 2 way systems that will be better than > what we presently have, and we'll use it. > > > Is it time I finished this and did some real work? Yes! > > > It's good to see you are spending some hands on time with this stuff. I > would also like to thank you for taking time from your busy schedule to > teach us the correct definitions of many facets of this hobby and > technology; even if we don't use it in our circles. At any rate, when > the term True FM gets thrown around, you'll know we just mean plain old > FM. Like "Flat Audio" it isn't the best term, but neither are Driveway > and Parkway! > > > Am I starting to sound like Donald Rumsfeldt? Heavens, yes! > > > <grin> > > Thanks again, > Kevin Custer 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/
