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INTRODUCTION
You and your friend have a pair of radios you use to talk to each other, but
did you notice that sometimes people listen in when you don't want them to?
How can you have a conversation that nobody can listen to but you and your
friend? Well you could encrypt the audio of the radio and decode it, but that
is an expensive option, and if someone has a decoder they can listen too.
But if we send the audio over a laser beam, the only people who can hear
your conversation are people who can see the laser beam itself. You don't
even have to encrypt the audio!
Ever wonder how sound is sent over a beam of light? There are several ways
to do it. One is to AM modulate the light beam. This means that we will vary
the brightness of the beam along with the audio, and the detector will give us
an output according to brightness. This is probably the simplest way, but
can't really be done very well since many lasers don't have a very broad
range of AM. This means that if the audio level is too low, the laser turns off!
This is also prone to interference from other sources that vary in brightness
like fluorescent lights which turn on and off at 60Hz.
A laser module is better used for turning completely on and off rather than
trying to vary its brightness slightly. Solid-state lasers perform better this way
than with AM modulation. The detector then will see the on and off state very
easily. If we turn a laser on and off at a known rate, usually very quickly, we
can ignore all other possible interfering signals on the receiver like
fluorescent lights. For example our LBC6K is turning the laser on and off at a
rate of 18kHz. If we just look at 18kHz, and filter out all other frequencies, we
filter out all other potential interfering light sources too.
So how do we get audio out of an 18kHz signal which is simply an on and off
signal? Well we could encode the data in a data stream, much like what a
modem does, meaning we would have to decode it digitally on the other end.
Otherwise we could simply pulse width modulate the 18kHz signal, and use a
low-pass filter to demodulate the audio.
Pulse width modulation (PWM); what the heck is that? Well, it is exactly what
it sounds like. If you have an 18kHz square wave you would normally have a
duty cycle of 50%. This means the on time is exactly that of the off time per
cycle. PWM means that you are varying this duty cycle according to the data
you wish to transmit; in our case it is audio. A high signal level would have a
longer on time vs. off time, and a quiet signal would have shorter on time vs.
off time. The total time per cycle always remains the same.
See the following diagram and you will see the waveforms in the LBC6K. The
top signal is the 18kHz clock. The middle signal is the clock converted to a
LBC6K • 4
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