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Posted by Green Xenon [Radium] on 09/30/07 18:36
On Sep 30, 10:42 am, dpl...@radagast.org (Dave Platt) wrote in
http://groups.google.com/group/uk.tech.digital-tv/msg/82ff55b36ba97dbf :
> In article <46ff32e0$0$32538$4c368...@roadrunner.com>,
> Green Xenon [Radium] <gluceg...@excite.com> wrote:
> >Within physical-possibility, what is the largest amount of
> >bits-per-symbol [assuming a baud-rate of only 1-bit-per-symbol]
> You just made a meaningless statement. Baud rate is measured in
> symbols per second, not bits per symbol. I assume that you meant to
> say "assuming a baud-rate of one symbol per second."
Yes. I meant "assuming a baud-rate of one symbol per second."
Sorry.
F-------king typos!!!!!!!!!!!!!!!!!!!!!!!!!!
> > that can
> >be reached without the highest-voltage causing any clipping, generating
> >any temperatures above 70 Fahrenheit, resulting in any harm to
> >anyone/anything [including the equipment itself], or shortening the life
> >of the equipment and without the lowest-voltage being lost in the noise?
> >What is the maximum-possible amount of discrete levels between the
> >highest and lowest voltage in such a signal?
>
> And, another important constraint for actual usability is this: you
> have to make sure that the minimum-detectable difference between two
> different symbol levels can be detected reliably and accurately at
> _all_ levels. In the language of analog-to-digital converts this is
> referred to as "linearity" and "no missing codes".
>
> [As a counterexample: the human hearing system can hear down to 0 dBa
> or so... a sound level which is just barely above the noise created by
> the random collision of air molecules with the eardrum. And, we can
> hear sounds up to around 120 decibels above that, before damage starts
> to result. That's a pretty wide dynamic range. However, it's not
> linear... if we're listening to a loud sound (say, at 110 dB or so),
> small sounds are completely lost... you can't hear somebody whispering
> 10 feet away when you're listening to a rock concert.]
>
> >An 8-bit signal can have a maximum of 256 different voltage levels
> >between the highest and lowest voltage. Right? Go too high and the
> >signal clips, go too low and the signal will not be recognized.
>
> Right.
>
> At audio-quality sampling rates (say, 50,000 baud) you can buy
> converters that are linear down to around 22 bits, I think...
> marketers call them "24-bit" converters but they aren't actually
> linear down to those levels. At instrumentation rates like 1 baud (1
> sample per second), with filtering and averaging being applied to
> eliminate the noise, you can do rather better.
>
> I don't know quite what the state-of-the-art is for measuring signals
> at such low baud rates as you are referring to. I'd guess that it's
> somewhere in the range of 28-30 bits.
>
> 30 bits is roughly a billion-to-one ratio between the smallest signal
> and the largest. Crudely put, it would mean that you might have a
> circuit which has to handle voltages of up to 1000 volts, and has to
> be able to generate, and then measure voltage differences of a
> *millionth* of a volt, at all of these levels. That's going to be
> technologically difficult, to say the least.
>
> The *theoretical* limit is somewhat higher than this, but not enough
> to help you achieve what you wish. It'll be limited at the low end by
> the thermal noise level (a 50-ohm resistance at room temperature
> generates -174 dBm of noise over a 1 Hz bandwidth) and at the high end
> by whatever voltage you think your equipment can handle without
> damage.
>
> Even being extremely generous, and saying 32 bits of linear resolution
> (and thus reliable data) per symbol, you aren't going to get video
> across it. 32 bits per second is somewhere between "fast Morse code"
> and "old Teletype teleprinter or ticker-tape" bandwidth.
>
> Note that this is the generation and measurement limit and assumes an
> interference-free communication link (e.g. a well-shielded cable), and
> is *not* what you can get away with in a real-world radio
> transmission! The background noise level on LF radio frequencies is
> higher than this, due to both manmade and atmospheric electrical
> noise.
>
> As another poster has pointed out, there's a damned good reason why
> nobody uses very-low-baud-rate modulations to send large amounts of
> high-speed data over a narrow-bandwidth channel, despite a century or
> more of research and study and competition in the fields of radio and
> electronic communication. It just doesn't work, and the reasons why
> it doesn't are well understood by those who practice in the field.
If 400-nanometer-wavelength coherent light was used in place of electric
signals, would QM in this system be able to pack in more
bits-per-symbol. AFAIK, optical signals can have a greater dynamic range
than electric signals. That is one reason that optic fibers are
replacing coaxial cables. Less noise.
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