Optical broadcasting

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Optical broadcasting

Postby Panrock » Sun Dec 07, 2014 12:07 am

Not exactly NBTV but this may be of interest. I know that some here have deep knowledge of light sources and sensors, not to be found elsewhere.

This is not about 'optical point-to-point', which of course has been well explored, but true optical broadcasting.

Steve O
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Re: Optical broadcasting

Postby DrZarkov » Sun Dec 07, 2014 5:57 pm

Of course that would also work with NBTV. Chris Long has made many experiments with transmitting by light: http://www.bluehaze.com.au/modlight/index.html
He reached 167 km (104 miles) with Luxeon LEDs! Of course in the empty space of Tasmania.
If you use Point-to Point transmitting or broadcasting (with a kind of LED lightbulb for example, as transmitter cirucit you can use one of our NBTV- video PCBs without any change) is just a question of how you build the transmitter.
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Re: Optical broadcasting

Postby Panrock » Sun Dec 07, 2014 7:51 pm

Hi Volker.

Someone over there also mentioned another instance of 'point-to-point'.

http://www.g0mrf.com/laser5.htm

It's amazing how they managed to line everything up. At least, with omni-directional 'broadcasting', you only have to line the receiver up! However, of course then you need much more power at the transmitting end. There are many more problems with this idea too, which I've listed over there.

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Re: Optical broadcasting

Postby gary » Sun Dec 07, 2014 9:00 pm

Panrock wrote:Hi Volker.

Someone over there also mentioned another instance of 'point-to-point'.

http://www.g0mrf.com/laser5.htm

It's amazing how they managed to line everything up. At least, with omni-directional 'broadcasting', you only have to line the receiver up! However, of course then you need much more power at the transmitting end. There are many more problems with this idea too, which I've listed over there.

Steve O


I am sorry - I know this is in very bad taste but I just can't help it - here is another example of cross channel communication - this is the result - to fully appreciate the (very clever) project you would need to backtrack from there.

https://www.youtube.com/watch?v=7Ydv9Ef-99I
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Re: Optical broadcasting

Postby DrZarkov » Sun Dec 07, 2014 10:31 pm

The problem with boradcasting very bright infrared is, that humans can't see it, but digital cameras can. (Try it with your smartphone/camera and an IR remote control.) A bright "NBTV-beacon" at Alexandra Palace could disturb many TV-sets and digital cameras. Not to mention some animals and airplanes...

But for the next NBTV convention it could be a nice project: Broadcasting inside the room. For reception an IR sensible photodiode and a simple pre-amplifier should be enough.
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Re: Optical broadcasting

Postby Steve Anderson » Mon Dec 08, 2014 11:25 am

The first item on the agenda for me would be to find out what the combined frequency limit is for the IR emitter and detector. As Volker suggests it should be a no-brainer for baseband NBTV and quite possibly baseband 405 (3MHz say). But 45MHz could be pushing it.

There are emitters that far exceed this requirement, well into the GHz region but they tend to on-off devices (digital) and of low power to couple into fibre circuits. But who's to say suitable devices may not be around or could be developed.

Steve O and I have chewed this over a few times in the past year or so, it's not beyond the realms of possibility but it does require a lot of effort and time.

The interference and safety issues have also been raised.

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Re: Optical broadcasting

Postby Harry Dalek » Mon Dec 08, 2014 5:53 pm

A little while back while testing head amps and transmitting to them via a white led ,i found just using a solar cell ( no head amp ) just the solar cell to the lap tops input as you see it has both enough band width and gives enough video levels receiving the led at that distance to produce a image .
I would bet on it using a laser you could increase this distance to across a room and who knows !
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Re: Optical broadcasting

Postby Panrock » Mon Dec 08, 2014 10:59 pm

That's interesting Harry. I had always assumed a white LED would be slower than 'normal' LEDs because of lag from the phosphor coating.

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Re: Optical broadcasting

Postby Harry Dalek » Tue Dec 09, 2014 3:14 pm

Panrock wrote:That's interesting Harry. I had always assumed a white LED would be slower than 'normal' LEDs because of lag from the phosphor coating.

Steve O


Hi Steve
I had no problems at all ,lucky i didn't know that at the time or i might not have tried it .
Just wanted to convert that little book reading light to a light transmitter for light sensor testing
I never tried any great distance tests but i do see the interest in any one trying this ,lasers i think would be easy but as mentioned before Luxeons and may be a reflective parabolic dish for both head amp and light transmitter for some serious work.
Saucepan lids might have another use , Oh and that would be a shallow dish Parabolic .
Heres a parabolic calculator might come in handy to work out that focal point .
http://mscir.tripod.com/parabola/

OH i found this also on IR long distance transmitting may be useful if your going with a Ir led
http://privat.bahnhof.se/wb907234/remote.htm
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Re: Optical broadcasting

Postby Panrock » Thu Dec 11, 2014 9:20 am

Steve Anderson wrote: it's not beyond the realms of possibility but it does require a lot of effort and time.

Yes - designing a laser diode driver (say) that would work at this speed and with built-in 'protection' would be a difficult task.

I've done a little googling...

http://electronics.stackexchange.com/qu ... -to-20-mhz

http://electronics.stackexchange.com/qu ... rcuit?lq=1

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Re: Optical broadcasting

Postby Steve Anderson » Thu Dec 11, 2014 11:24 am

Musing on this further...you may recall in a thread 'somewhere' on this forum my efforts at producing a DIY opto-coupler as all of the examples I can obtain locally were too slow. The linear/analogue versions that is. I used some high-intensity red LEDs and a BPW34 photo-diode and it took quite a bit of effort to get a full bandwidth PAL 5MHz signal across 1cm!

Luxions and the like I presume are simply larger LEDs 'welded' to a heatsink tab, much in the manner of a TO220 package for example. If that's true I expect the junction capacitance to increase which will not help with the bandwidth. (Even forward-biased diodes still have capacitance, often lots of it).

At the quantum level where the action really occurs there should be little practical limit to the bandwidth - it's just that ruddy capacitance and inherent resistances within the junction that get in the way. If there were no resistance then the capacitance could be catered for, it's purely reactive and wouldn't dissipate any additional energy as heat within the device. But with the resistance you've got heat and phase angles to deal with and the frequency response suffers as a result.

There is no easy answer to this than to do some 'Googling' (I note you have already) and give it a go.

Steve A.
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Re: Optical broadcasting

Postby Panrock » Fri Dec 12, 2014 10:09 am

Thanks for the comments Steve. Any hunches as to the likely fastest superbright red LEDs out there (and sensors) would be gratefully received. I'll then have a play.

Steve O
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Re: Optical broadcasting

Postby Panrock » Wed Dec 17, 2014 8:34 am

Well the standard "rise time" of LEDS - when it is ever stated - seems to be 20nS. So 40MHz-odd could be a struggle. However, there are photodiodes out there that look easily fast enough. Though as Steve A says, it isn't as simple as that.

I'll mess around with this some more over Christmas...

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Re: Optical broadcasting

Postby DrZarkov » Tue Jan 06, 2015 11:30 pm

Unfortunally in German only, but this HAM ATV operator is quite succesful transmitting TV over (laser-)light: http://dj1wf.darc.de/

The english version of his homepage is unfortunally not identical and has less information about "laser-TV"...
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Re: Optical broadcasting

Postby Panrock » Wed Jan 14, 2015 5:57 am

I obtained a photodiode from Farnell (good for 'gigs'), and for a while obtained a 'false-positive' result whereby irradiating this with light affected the following circuitry in such a way that it would pass through stray RF...

Having now got my screening in order, It seems that the blocking factor is actually the LED. Indeed, my googling on the subject indicates that most/all common-or-garden LEDs are simply too slow for this application and run out of response in the low megahertz region.

I also tried modulating a laser diode I have to hand. This showed similar limitations to the LED.

I have found just one visible light LED (I want to stick to visible light in this experiment so I can see what's happening, and for greater safety) that specifically promises to be fast enough. I can't get this from Farnell, so I have approached Hamamatsu direct for help.

{EDIT} They have now agreed to supply me a minimum of five of the things - at a substantial price... still worth it I think.

Steve O
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