Bob, et' al,
I would like to say that last years discussions of FM,PM, Exciter wars,
emphasis, Fourier mathematics, clipping and limiting was very educational.
Wouldn't hurt to have some of that stuff put together in a FAQ.
-- Terry
On Tuesday 06 April 2004 12:59, you wrote:
> > Hi Kevin,
>
> When we (Scott and I) go to build a repeater for someone or are talking
> about a radio set that has or needs an FM modulator, it gets referred to as
> true FM. This helps us to eliminate confusion about what type of modulator
> needs to be installed into a radio set that ultimately transmits FM in any
> event.
>
> Yup, I know what you're saying. The Handbook people mention 'direct FM' and
> 'indirect FM' as alternate terms, but I don't like those any better.
>
> 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.
>
> 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?
>
> I will readily admit that available PM exciters can have all sorts of
> 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. 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.
>
> Has this topic has been bantered about for a long time? Yes! Will I change
> anyone's mind? No! Is it time I finished this and did some real work? Yes!
> Am I starting to sound like Donald Rumsfeldt? Heavens, yes!
>
> Have a great day!
>
> 73,
> Bob
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