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Posted by Dave Platt on 09/28/07 05:50
In article <46fc71d2$0$24285$4c368faf@roadrunner.com>,
Green Xenon [Radium] <glucegen1@excite.com> wrote:
>Would 300:1 work?
Not for video, no.
>The ULF is even lower-frequency than MW. If a baud of
>20-symbols-per-second but 300 bits-per-symbol is used, can the luminance
>signal have its temporal and spatial frequencies downshifted
>sufficiently so that it can be on a ULF-frequency FM carrier? Will this
>work even if the FM carriers center-frequency is 300 Hz?
No. No. And, no. You cannot possibly do this... for several reasons.
For one thing, trying to convey 300 bits of information per symbol is
still grossly optimistic. If you take a look at real-world modulation
systems (QAM64 and so forth) you'll find that they send much less
information per symbol... I don't have the best numbers available off
of the top of my head, but I'd be surprised if there's a system in
practical use which tries to send more than 16 bits per symbol. The
amount of noise on real-world communcation channels makes it
impractical-to-impossible to go much beyond that... and you are
proposing something *far* beyond that. Trust me - the laws of physics
in this universe won't permit you to do it.
For another thing, even if you *could* pack that much information into
a symbol, you'd still be capable of sending only 6000 bits per second.
That's far too little for anything resembling real video at real
resolutions... you couldn't even send one frame per second at that
rate.
For a third thing, the frequency content of a video luminance signal
starts just above DC, and goes up from there (to several MHz with
standard NTSC video). There really isn't any room for you to shift
the frequencies downwards by a useful amount... if you try, you end up
trying to shift real information down past DC. Either you lose it (if
you filter before shifting) or you *try* to shift it down past DC into
negative frequencies, and it aliases right back up into your passband
and distorts the rest of the signal.
To sum it up - what you're trying to do is fundamentally impossible.
It doesn't matter how you shift the luminance signal's frequencies
around. It doesn't matter how you modulate it. It doesn't matter
what tricks you play. You just don't have enough bandwidth available.
"You can't fool Mother Nature."
>So there is no way to decrease how far the sidebands will go?
With FM? Nope, not in the way you're hoping. Modulate a carrier Fc
with a frequency Fm, and the first sidebands will be at Fc+Fm and
Fc-Fm, just as would be true with AM. That's the narrowest you can
get.
--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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