Bob Tinney writes:
>I guess that with modulation,the film is half darkened so that with 100%
>modulation the film would be either completely light or dark.
>
>How did I do?

        Excellent.  There's a man who knows his modulation.

        Here is the idea.  I wasn't looking for numbers or signal
levels because all of that would be different from one projector to
another.  We used to have a whole bin full of used exciter lamps that
we used for testing purposes and each one used a different voltage and
or shown in to a different type of photo detector, depending upon the
make, model and age of the projector.

        What I was looking for was an answer based upon relativity.
Not Einstine's theory of relativity or anything that sophisticated,
but how the signal levels related to being able to produce fairly
decent sound.

        Bob got it right in that silence, just like an un-modulated AM
carrier, is a reference point halfway between no carrier at all and
full power or full carrier.  In the case of the film projector, the
sound track is halfway between black and clear for the variable density
sound track and half the possible width of the variable-width sound
track.  The voice or music makes the full range of variability
possible when it is at 100% modulation.

        In this case, the carrier is a DC signal that is half of what
it could be with maximum light.  The other extreme is full darkness
which produces no signal at all.

        We used to do a crude test of the sound system by flipping a
piece of paper or film between the exciter lamp and the detector to
see if we could hear a loud thump from the speaker, kind of like
brushing a phonograph needle with your finger to make noise.

        Another interesting thing we'd notice on some projectors was
that the exciter lamp would provide some accidental modulation of its
own.  The filament in an incandescent lamp is a thin wire stretched
between a couple of stouter wires for support.  While this arrangement
can be made fairly robust, some of the filaments shook a tiny bit from
the vibration of the running motor in the projector.  This caused a
metallic whine to appear in the sound and a ping noise if you tapped on
the bulb.

        This is kind of like some of Alexander Graham Bell's
experiments in the late 1800's in which he sent the human voice over
light by bouncing it off of a mirror made of very thin metal so that
it would act like a diaphragm in a microphone.

        I once unscrewed the cover on a headphone and was able to
reflect the Sun off of the metal diaphragm and in to a photo cell.  If
you get everything lined up just right, it is not bad sound.
Now you know some of the things I have done in my spare time.

73


Martin McCormick WB5AGZ  Stillwater, OK
OSU Information Technology Division Network Operations Group