> I think we're on two different wavelengths here. What
> I am talking about is practical application, not
> theoretical mumbo-jumbo.
OK, yes, I'm talking about the theoretical limitations. Your earlier post
said that "even theoretical PM falls apart at low frequencies", and that's
where I disagree.
> But compared to a volt P to P that is a small signal.
> We wouldn't run volts of audio down the same system,
> with no changes. You wouldn't put speaker level audio
> on a line designed for 10 uV, and expect everything to
> play fine. 100 dB of dynamic range is about the best
> we can expect out of a good CD changer.
That limit is imposed by quantization noise due to the 16 bit word length
for the samples; it's not an analog limitation. The theoretical limit for
16 bits is 98.08 dB. For 24 bits (common nowadays in audio work), it's
146.24 dB.
> Anyway, what I am talking about is real world
> limitations on the theoretical PM. Sure theoretically
> you could build a modulator that would do .01 to 3
> KHz. Would it be expensive? duh. Would it be complex?
> duh.
Considering that a PM Mastr II station originally cost more than what you
can get a digital broadcast exciter for nowadays, I don't consider the cost
to be the limiting factor. As far as complexity, it depends on how you
define the term. To some, digital logic and DSP is less "complex" than
analog circuit design. Let's face it, in either case you put audio and DC
into the box and you get modulated RF out of it. The "complexity" of the
circuitry that does that conversion is subjective.
Think about it - in a digital implementation you don't need analog circuitry
to high-pass filter, preemphasize, limit, deemphasize, low-pass filter,
gain-adjust, buffer, generate PL/DPL and sum it in, modulate, multiply, key
on and off, etc. - one DSP chip and maybe a few thousand lines of code would
replace most of the analog circuitry in a traditional PM (or FM) exciter,
and once written, it could be re-used for multiple bands in many models and
generations of radios. Cost effective to manufacture? Hell yeah!
I guess this begs the question - at that point, where you're doing
preemphasis and modulation via math versus analog circuitry and synthesizing
the modulated carrier, do you call it PM or preemphasized FM? I would argue
the latter since you could have response that includes DC.
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
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