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Posted by Dave Platt on 09/28/07 01:13
In article <46fc4458$0$16496$4c368faf@roadrunner.com>,
Green Xenon [Radium] <glucegen1@excite.com> wrote:
>Can't the maximum voltage be set to be low enough not to cause any
>damage/injury to anyone/anything?
Not if you want to actually be able to use the signal. That was my
point about "liquid helium". If you reduce the upper voltage enough
to be physically realizable, then the smallest voltage difference
you'd have to be able to distinguish is *far* less than the
inescapable amount of noise in the system, even at extremely cold
temperatures where thermal noise is minimized.
>Also, couldnt the temporal and spatial frequencies of the luminance
>signal be downshifted prior to QM conversion so that not so many bits
>are required?
Not enough to matter, by a factor of zillions to one.
>Its possible to downshift the frequencies of an audio signal without
>low-pass-filtering or changing the speed at which it is played-back.
>Couldnt 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).
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.
>Is there any thing that could be done to the QM signal so that the
>resulting FM sidebands wont 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 carriers frequency increases.
>When the modulator voltage becomes negative, the carriers 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.
--
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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