Re: [Repeater-Builder] Audio Reproduction

[Kevin Custer]

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 the Yahoo! Terms of Service.