Wouldn't it be an infinite negative level? It could also be the level of the
power coming in from the wall outlet it's plugged in to, which would be
dangerous to try and measure anyways.

----- Original Message -----
From: "Martin McCormick" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, April 06, 2004 11:19 AM
Subject: Re: voice modulated light?


>         In the interest of completeness, I'll share what little else I
> happened to think of after posting my first message.
>
>         I have also read about the experiments in "QST" and they do
> use AM CW, or at least they did for one test.  The light source was a
> laser which was pulsed on and off at some audio frequency.  All the
> detector had to do was, you guessed it, detect the light.  The signal
> could be fed in to a sensitive amplifier and then to a speaker or
> recording device.
>
>         Sony used to make a pair of infrared wireless headphones that
> worked great as long as you were within about 30 feet of the
> transmitter and had an unobstructed view between you and it.  The left
> and right channels were FM and were in the 2 megahertz region.  They
> sounded great unless you were almost out of range.  They also worked a
> lot better if there wasn't any direct Sunlight shining on the headphones.
>
>         The transmitter was a column of infrared LED's and a few red
> ones mixed in so that one could see the transmitter was on.
>
>         One thing to keep in mind is that LED's, lasers and gas
> discharge tubes can cycle on and off rather quickly so people have
> sent audio and data over them for years as more of an interesting
> experiment.
>
>         The trouble with LED's and to an even greater extent lasers,
> is that they aren't linear devices at all.  Lasers just quit or go in
> to a sort of LED mode if they aren't driven with enough current.
> LED's dim out very nicely, but they have the same non-linearity of any
> semiconductor device.
>
>         I sort of lied a bit when I said you couldn't modulate
> incandescent light bulbs.  While the filament does take enough time
> to heat and cool that it can't track a human voice very well, people
> have modulated the light from a bulb with devices that either wiggle
> back and forth or change the polarization of the light so as to act as
> a kind of shutter.  Texas Instruments has a chip with thousands of
> little mirrors on it that is the heart of their new DLP or digital
> Light Processing systems used in projection TV setups.
>
>         If you really want to mess with a cheap and dirty light
> modulator, shine a bright light through a spinning fan.  It'll
> certainly flicker off and on as each blade breaks the beam.
>
>         I used to work for our Audio Visual Department, fixing stuff.
>
>         Some of that stuff was 16-millimeter sound projectors.  It
> isn't hard for a person who is blind to fix a lot of the transport
> problems because the things that mess up the picture often also either
> mess up the sound or cause the machine to make bad sounds that
> certainly let you know that something is terribly wrong.
>
>         Sound movie projectors have a little stripe of black and clear
> wavy lines along one edge of the film.  On a 16-millimeter film, the
> audio goes down the edge of the film away from the sprockets.
>
>         The projector usually has a lamp called an exciter lamp which
> looks like a slightly-oversized pannel lamp.
>
>         That lamp is a regular incandescent bulb and some of the
> better projectors run it on either DC or high-frequency AC, maybe
> around 20 to 50 KHZ.  The idea is that light shines from the bulb
> through a little cylinder containing a couple of lenses to focus the
> light in to a slit-shaped beam that is extremely narrow.
>
>         The beam hits the edge of the film and tries to shine right
> through it to a detector which is mounted so that it can see the sound
> track, but hopefully, none of the pictures.
>
>         The film can actually have two different types of sound track,
> depending upon the equipment the movie company used to expose it.  One
> type is called variable density and makes the film darker or lighter
> according to the vibrations in the sound.
>
>         The other is called variable width and is completely black.
> As the sound varies, the black part of the stripe gets wider and
> narrower which lets more or less light through to the detector.
>
>         Either way, the detector sees a light that gets brighter or
> dimmer with variations in the sound.  The signal coming out of the
> detector is just like the signal coming from a microphone or tape head
> or phono pickup.  It contains a somewhat faithful representation of
> the sound of the actors voices when they spoke in to the microphone
> originally.
>
>         Now, does any of this relate to ham radio?  You bet.  That
> modulated light source is an AM signal pure and simple.  When nobody
> is talking and there is no sound, what do you think the signal level
> is on the film?
>
>         I'm going to let you guess.  Some of you will be surprised,
> but think about it.  What does silence actually mean?  Remember that
> this is an AC signal.
>
>         Oh yes, the reason that some projectors run the excitor lamp
> on DC or high-frequency AC is because an excitor lamp running on AC
> still gets a tiny bit brighter and dimmer with each half-cycle.  That
> causes a hum at 120 Hertz.  The fancier projectors take the trouble to
> make sure there is nothing audible on the power supply to the exciter
> lamp.
>
>         So now, let's see how many of you can answer the question as
> to what is silence when you have an AM signal?  Think carefully.
>
> Martin McCormick WB5AGZ  Stillwater, OK
> OSU Information Technology Division Network Operations Group
>