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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 4th Apr 2019, 9:05 am   #21
Electronpusher0
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Default Re: Television over a light beam

I also tried sending audio via light beam in my youth, I used an OCP71.
Actually I could not afford to buy an OCP71 so scraped the paint off a normal OC71. This was an accepted trick until Mullard got wise and filled OC71s with opaque goo to prevent this and maintain the price of OCP71s.

Peter
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Old 4th Apr 2019, 9:51 pm   #22
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Default Re: Television over a light beam

Yes, we also used an OC71 without it's black cloak. We used to look at the Mullard data books and wish for an ORP12 but funds back then were limited for a 13 year old.
We used to get our transistors out of Decca Radar's dustbins just off the Kingston-by Pass near the school. They were always OK but I suspect were 'blanket changed' when Radar units were being overhauled. Such fun! John.
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Old 5th Apr 2019, 12:20 am   #23
Graham G3ZVT
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Default Re: Television over a light beam

About the 7th image on this page: Radio Love, using a laser modulated with RF sub-carriers.

http://www.modulatedlight.org/Modula...Amateur79.html

I clearly remember that diagram and the accompanying story in the radio press at the time (probably PE or PW). I doubt if it amounted to more than a proof of concept.
The parent page is
http://www.modulatedlight.org/Modula..._LIGHT_DX.html
and I am sure somewhere in that lot are some recordings of Australian amateurs in cross-band RF<>light QSOs and one amateur that applied for, and received a letter of variation that left no doubt that his modulated light operation was legal.
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Old 5th Apr 2019, 3:22 am   #24
Panrock
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Default Re: Television over a light beam

Here's a little thought experiment. Take a RF modulated light system. Amplitude-modulate this with a signal containing very low audio frequencies. Can the light be seen to flicker?

Steve
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Old 5th Apr 2019, 4:28 am   #25
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Default Re: Television over a light beam

Nothing should be visible except as a strobing if another ac light source is present. The
problems that occur in a basic AM system mean that whether the modulating
signal is analog or digital, PWM is better, and it's the ability of the modulator to respond
to signals up to the upper video range that limit its usefulness.
Although IR is preferable to white light, I suggest that transmit/receive on RF say, 5.8GHz
is easier, remodulating back to VHF at the local set.
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Old 5th Apr 2019, 10:22 am   #26
Karen O
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Default Re: Television over a light beam

Quote:
Here's a little thought experiment. Take a RF modulated light system. Amplitude-modulate this with a signal containing very low audio frequencies. Can the light be seen to flicker?
If you modulate a light beam with a carrier that itself is modulated at low AF, then the mean intensity doesn't change but the power does. I don't know which the human eye responds too

See you tomorrow, Steve. It'll be good to see friends again.
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Old 5th Apr 2019, 1:19 pm   #27
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Default Re: Television over a light beam

Another way to look at it...

Take a System A 405-line television signal generator supplying baseband video. Connect the output to a suitably biased LED. On your generator, select a White Bar signal for a few seconds and then follow up with Black Level-and Syncs.

The DC content would vary dramatically between these two signals. The LED's average brightness would change noticeably between the two states.

But what if you were to repeat this, modulating your ready-illuminated LED with a 45 MHz carrier modulated in turn with the same two signals? (All AM and DC coupled).

Looking at the LED, what would you see? (Naturally I have tried this!)

Steve

Last edited by Panrock; 5th Apr 2019 at 1:30 pm. Reason: Clarity
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Old 5th Apr 2019, 4:15 pm   #28
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Default Re: Television over a light beam

I seem to recall that there was a commercial system using I.R.

It used FM modulation.
I worked in both A/V & broadcast technologies, & I think we used to hire out such links for short-range use. This was for 625-line analogue, together with an audio channel.

All your latest experiments sound most interesting, Good luck!

David.
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Old 5th Apr 2019, 5:04 pm   #29
Panrock
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Default Re: Television over a light beam

Time for the answer. Yes folks, I tried it.

In fact you can fill your light beam up with all manner of RF frequencies, bearing as many sudden DC modulation shifts as you like, and the appearance of the hosting beam will remain tranquil and unchanged.

After all, there's either 'more RF' or 'less RF' within it according to the amplitude modulation. In a way, the light beam plays a part just like 'space' does for the orthodox radio spectrum.

Steve

Last edited by Panrock; 5th Apr 2019 at 5:18 pm.
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