|
|
Posted by Dave Platt on 09/26/07 21:03
In article <46faa23a$0$7443$4c368faf@roadrunner.com>,
Green Xenon [Radium] <glucegen1@excite.com> wrote:
>I wish I could answer that question. Its a matter of personal
>preference.
When your preferences are at odds with physical reality, guess which
wins?
>QAM/multiple-level-QM/constellation-modulation are examples of exotic
>modulation techniques that allow you to transmit/receive the most amount
>of information while using the least amount of bandwidth. To further
>prevent excessive usage of bandwidth while maintaining signal
>efficiency, the baud should be no more than 1-symbol-per-second while
>the amounts of bits-per-symbol conveyed should be the most that is
>physically and mathematically-possible.
Mathematically, there's essentially no problem. All you need to do is
use a symbol representation which has enough discrete states to carry
the desired amount of information.
The problem here is "physically". Your proposal is very far out
beyond the point of physical realizability.
Video-grade luminance carries a lot of information. Just for grins,
let's say that it's the equivalent of a megabit per second - you can
get tolerable video at that bit rate using MPEG-2 or MPEG-4
compression of typical images. [An uncompressed/uncoded analog video
image requires a *lot* more bandwidth than this... maybe the
equivalent of 20-50 megabits per second?]
So, if you want only one symbol per second (one baud) to keep the
bandwidth down, you have to be able to transmit 1 megabit of
information in that one symbol. That requires that the symbol
representation include
1,000,000
2
different states. That is, the sender has to be able to choose from
among 2-to-the-millionth-power different voltages or RF levels (or QAM
or other constellation states), and the recipient needs to be able to
successfully distinguish between all of these states in order to
recover the megabit of information.
If you look at this in terms of the voltage being transmitted, it
means that the difference in voltage between any two adjacent states
must be large enough to be measured reliably. That would mean that
the _maximum_ voltage needs to be 2-to-the-millionth times higher.
If we want to ease the problem somewhat, we could use a QAM encoding,
with 2^1000 different amplitudes and 2^1000 different phases.
Even if you do that, the signal resolution you need is incredibly far
beyond what's physically possible. If you ran the entire system in a
bath of liquid helium (to keep the thermal noise level of all the
components down to a minimum), and you set the minimum voltage
difference between two states to something just barely above the
remaining thermal noise level ...
.... then the maximum voltage level produced by your transmitter will
still be billions of billions of billions of billions times (billions
*of* billions, not billions *and* billions) higher. It wouldn't just
burn out the transmitter... it would (if physically realizable) blow
everything in the solar system into incandescent plasma as soon as you
turned it on.
>If the original luma is transmitted on an FM carrier, the resulting
>deviations in the frequency of the FM wave will be excessive. If the
>QAM/multiple-level-QM/constellation-modulation equivalent of the luma is
>transmitted, the resulting frequency-deviations will be significantly
>smaller and hence use a lot less bandwidth.
Don't bet on it. Remember that the abrupt transitions between QAM
states create a sudden discontinuity in the signal... which introduces
higher-frequency components or sidebands in your encoded luma signal.
These luma sidebands, once FM-modulated onto the carrier, will cause
RF sidebands out quite some distance. That's one of the gotchas of
FM... it creates sidebands which appear *further* away from the
carrier frequency than the maximum carrier deviation!
In short, there's no free lunch. For *any* given baseband signal
(straight, QAM-encoded, or whatever), an FM-modulated carrier will
*always* produce sidebands which go out as broadly as an AM carrier or
further, and will *never* take up less spectrum than an AM carrier
with the same baseband information.
In other words, no matter what coding tricks you can think up to try
to make FM video narrower than AM, you'll lose, because you can apply
the same coding tricks to the input of an AM modulator.
--
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!
[Back to original message]
|