[Laser] communication with CCD array receiver

[TWOSIG at aol.com]

In a separat post, the suggestion was made that CCD arrays could be used  for 
communications.  I forget who made the suggestion quite some time ago,  but 
there was an idea for using a webcam (or maybe it was a video camera) with  
software to allow you to mark an area on the display screen so that a computer  
would then search that area for flashing lights which could be decoded as Morse 
 code.
 
A Webcam running at 30 frames per second would need enough frames to  
identify dits and element spaces.  Assuming 4 frames to confirm a dit or  element 
space, that comes to 9 words per minute.  There might be some  camera shake, so 
the system would need to search the designated area instead of  locking on a 
single pixel.
 
It would be an interesting project.
 
Here is an oddball thought:  Suppose you had a four story building  that had 
four large windows on each floor.  A light communication  experimenter lives 
on a mountain top 100 miles away, and just by chance he can  see the building 
in a telescope.  He contacts you, and the two of you agree  to set up a set of 
signals using a light in each of the windows.  The  experimenter uses a webcam 
with a zoom lens through the telescope to record the  signals.  With enough 
optical resolution you could record images that would  show each of the sixteen 
windows, so there are 65,536 different patterns.   On a hazy day, the 
patterns blur so that some cannot be identified, but some  can.  On foggy days, the 
lights must be on in all of the windows to be seen  at all.  On windy days, the 
telescope shakes so badly that you cannot tell  where the building is in the 
frame, but you can see the patterns.
 
OK.  The story is dumb.  However it does present some interesting  problems 
to optical communication.  To get attention, you might use Morse  code flashing 
all the lights at once.  It is a slow data rate, but has good  ( or at least 
better ) signal to noise ratio for signal aquisition.   For ideal conditions, 
you would be able to use the 16 individual lights for  separate channels of 
data, or combine them for a faster overall data rate (  remember your data rate 
is limited by the frame rate of the webcam).  A  moving target ( like a 
satellite or a spinning balloon ) would present ambiguous  patterns some of the 
time.  Four vertical lights could be in any one of  four horizontally shifted 
positions.  However four diagonal lights form a  unique pattern even if shifted 
horizontally.  Degraded resolution or signal  attenuation might call for 
adjacent lights to be operated in unison,  reducing the number of patterns, but 
restoring signal to noise ratio.
 
What sort of equipment would be needed.  For a test setup and receive  
software testing, a grid of LEDs mounted on a board should work.  For field  
testing, the one or three watt LEDs should work to many miles.  Three watt  LEDs 
worked for AM voice over 100 miles.  The limitation will be the image  resolution 
more than transmit power.
 
A strange thought.  Perhaps worth sharing.
 
 
James
N5GUI