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Posted by Green Xenon [Radium] on 09/28/07 03:25
On Sep 27, 6:13 pm, dpl...@radagast.org (Dave Platt) wrote in
http://groups.google.com/group/uk.tech.digital-tv/msg/118b30e3f36c8b05 :
> In article <46fc4458$0$16496$4c368...@roadrunner.com>,
> Green Xenon [Radium] <gluceg...@excite.com> wrote:
> >It’s possible to downshift the frequencies of an audio signal without
> >low-pass-filtering or changing the speed at which it is played-back.
> >Couldn’t the equivalent be done with the temporal and spatial
> >frequencies of a luminance signal?
>
> Once again, no, not enough to matter. If it was only a matter of 2:1
> or 10:1, you might be able to do it. The one-baud scheme you propose
> is off by a factor of 1,000,000,000,000,...,000 to 1 or so (*lots* of
> zeros in there).
Would 300:1 work?
300 Hz is the minimum frequency used by telephones and ULF
[Ultra-Low-Frequency] radios.
ULF radio = frequencies at least 300 Hz but no more than 3,000 Hz.
> Go do as I said, and google "Shannon Limit". To sum it up: there's a
> hard limit on the amount of information that you can send through a
> communication channel which has (a) a finite bandwidth, and (2) a
> finite signal-to-noise ratio.
>
> Your goal of trying to pack video-grade information into small gaps in
> the MW spectrum goes far beyond the Shannon limit.
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 carrier’s center-frequency is 300 Hz?
> >Is there any thing that could be done to the QM signal so that the
> >resulting FM sidebands won’t be excessive?
>
> >In an FM carrier, the neutral frequency results when there is no
> >modulation signal. The FM carrier changes
> >according to the modulator. If
> >the modulator voltage it positive,
> >the FM carrier’s frequency increases.
> >When the modulator voltage becomes negative, the carrier’s frequency
> >decreases. On a graph of the modulator signal,
> >the x-axis is where the
> >voltage is neutral. Above the x-axis,
> >the voltage is positive. Below the
> >x-axis, the voltage is negative.
> >The further from the x-axis the voltage
> >goes, the more broad the sidebands will be.
>
> >To prevent the sidebands from getting too broad,
> >the QM signal needs to
> >have all to voltages shifted closer to the x-axis.
> >This means the device
> >receiving the QM signals on the other end
> >needs to be more sensitive to
> >differences in voltages.
>
> >Thus it would help if the physical voltage difference between what is
> >interpreted as 1 or 0 be much smaller. The smaller the change in
> >voltage, the less extreme the resulting FM sidebands.
> Your understanding of FM modulation is incomplete, and it is leading
> you astray. You're only looking at half of the picture. You need to
> go back and hit the books again so that you understand the whole
> situation.
>
> Yes, it's true that the instantaneous carrier deviation depends only
> on the amplitude of the modulation signal. However, that's only part
> of the picture. Sidebands are created out *beyond* the maximum
> carrier deviation frequency... and they are created at frequencies
> which are at offset-multiples of the modulating frequency.
>
> You *cannot* narrow an FM signal down to a width less than the
> modulating frequency. If you want to carry (for example) an audio
> signal with 20,000 Hz of audio bandwidth, then the modulated FM signal
> will always have sidebands out as far as 20,000 Hz on either side of
> the nominal carrier frequency. This is true NO MATTER HOW SMALL the
> amplitude of the modulating signal, as long as there's any signal at
> all.
>
> If you FM-modulate a carrier with a very low-level 20 kHz tone, you
> might be causing only 1 kHz of carrier deviation... but there will
> still be sidebands 20 kHz above and below the nominal carrier
> frequency. That may seem strange and counterintuitive, but it's how
> the math works out, and how measurements confirm it works.
So there is no way to decrease how far the sidebands will go?
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