[Laser] fundmentals

[James Whitfield]

Glenn,

We do not seem to be converging on a solution.  I will concede that in your
frame of reference "The only difference is...technology....Other than that,
at the most fundamental level, photons are photons, no matter the
wavelength."

That does not help me understand, or suggest improvement, on light
communication systems to be built by amateurs.  From where I look at the
situation, RF photons behave like waves, and have many off the shelf
practical systems.  Optical photons act enough like particles that Newtonian
optics are still useful.  There are differences between the behaviors of
optical photons and RF photons.  I think we should be looking at those,
still within the framework of fundamentals that you described much better
than my previous attempts.  For me the real issue is building a working
system, and maybe making it work better.

I enjoyed the comparison "Laser comms is just a little more advanced than
the spark gap radios of 100 years ago."  It makes me wonder what the path of
technology would have taken if the spark gap transmitter and galena crystal
set era had DSP and FEC tools.  (Maybe there were steam powered data
processors.)   I wonder if BPSK31 would have preceded voice signals?

OK, enough silly stuff.  It took me a while to process the probabilities.  I
think that you were assuming that the probability space was symetrical, but
I assumed bias that can be exploited.  For example, astronomers seldom look
for stars in the daytime.  To take pictures of dim stars, they use big
lenses, and they try to find them on Moonless nights.  That way something
that looks like a smudge on the picture is more likely to be something real
than if you took the picture in the daytime.  This poor example translates
to communication by suggesting that you need to look for and take advantage
of bias that helps you communicate.

As an example of bias, it would be easy to wire up a laser for simple Morse
code.  If I thought that I needed to send the message at 10 minutes per word
( that's right, I meant to say 0.1 words per minute ) I could not couple the
output of a photo sensor to a computer sound card to process the signal.  On
the other hand, I could send MCW at 300 Hz, even if I had to put an electric
fan in front of the laser beam to chop it up.  The sound card is biased for
"sound" input.  If I wanted to send 50 WPM, I might not even need the sound
carrier, but I am sure the sound spectrum would be odd.

Similarly I could setup a BPSK system.  The sound card might not care if I
choose a 300 Hz tone or an 1800 Hz tone as my audio carrier.  But if I use a
K3PGP front end, it certainly would make a difference to the overall system.
It might even suggest that I move the tone down to 100 Hz or less

I do not think that we have adequately identified the biases in the systems
that we are using.  There should be alternatives may work better or be
easier to use if look into the shortcomings of the systems.  At least there
should be some rationale to support standard practices.



James
N5GUI


The following is off the topic of the post:

Some time we can discuss voting schemes.  Your example  is that by voting
you reduce the probability of getting the correct answer.  I could not
follow that logic.  If it were true, there should be a lot of airplaned
falling out of the sky.

I also did not follow your comments about forward error correction.  It
seemed that you were suggesting that it is just redundancy ( repeating the
message).  I do not claim to understand it, but I would describe it as
mapping the message ( assumed to be N bits of information ) into a larger
space in such a way that the information may be fully recovered even if many
of the bits in the larger space get corrupted as it is transfered from the
sender to the receiver.

One more comment.  You used the term quantization noise in regard to
digitizing a sample.  I follow the idea of quantization noise if you are
trying to digitize a smooth function like a sine wave.  A while back I was
trying to imagine the wave form composed of 64 different frequency square
waves each having the value 0 or 1.  They would for a complex wave but their
sum could only have 65 discrete values ( 0 to 64 ).  The representation of
that wave form would be exact, so I am thinking that there is no quantizaton
noise.  Have you run across anything that would clarify that?