Optical broadcasting

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Re: Optical broadcasting

Postby samh » Wed Sep 23, 2015 2:02 am

Hi Steve,

As a thought what about trying a 5ghz video sender located on a convenient place some distance away? This could be your "receive end".

Also you can easily buy neutral density filters for optical light which are effectively an "optical attenuator", or even make ones using such things as sheets of paper, etc.

This would permit simulation of a longer path without the need for actually involving those distances, making your R&D easier.


When you are ready, we have a few sites which are some distance apart as you know. Some of them aren't that far from you. I would be interested to try some long-haul links and put some form of logger onto them to see what the availability actually is.

On some sites, I can tell you from experience, you will have problems with the "natives" if you start putting visual light on the masts, and most site owners would prefer not to have visual light on their masts for this reason, so it may be worth considering a mixture configuration for your development, perhaps a static red LED for alignment coupled with a high intensity IR LED.


This way you can start to actually develop the long-range IR solution, using "attenuators" that you put in line, and using IR LEDs.

It would also be interesting to see the difference between low IR and high IR, the lower and high wavelengths. I am told the very low IR works surprisingly well, often more than compensating for the lower powers available down there, but it would be interesting to see this in practice.


Hope this is of interest, keep up the good work.
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Re: Optical broadcasting

Postby Panrock » Thu Sep 24, 2015 7:48 pm

Hi Sam,

Thanks for the encouragement and offers of help, and nice to see you over here. :) As you'll have gathered, this is where the project is being discussed in the most detail. As ever, you make some useful points.

As for neutral density filters - quite correct. However, whenever possible I like to test over actual distance. This doesn't only give the performance at reduced flux density, it also can show up variations with atmospheric transparency and stability... and provide valuable experience of the actual difficulty in setting up optical receptors - of different types - in the field!

Yesterday I scouted out a line-of-sight route from an upstairs bedroom window to publically accessible land in my village - see the picture. I must emphasise I haven't - as yet - tried transmitting over this distance. I am still preparing the kit. So far I have 'portabilized' everything. The little receptor box is now self-contained with a built-in PP3 battery and the Sony TV has been fitted with a battery pack.

At the moment I am using 1 milliwatt of radiant flux at the transmitting end. This is set by the LED... not to be confused of course with the actual drive power. So far I have had a promising result when trying another 'normal' LED but it is by no means certain I'll also be able to drive the more powerful 'deep red' 230 milliwatt (flux) horticultural LEDs at these frequencies.

Steve O

[Map image removed after receiving advice]
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Re: Optical broadcasting

Postby Panrock » Tue Oct 06, 2015 7:06 pm

Between all the things going on re. work on my house, I've been continuing preparing for the next test.

Pictured is a 'mini' version of the receptor I have now made. One end pushes into the eyepiece hole of a binocular or telescope, the other connects to the set's aerial input. The tape shown wrapped round the (far) eyepiece end is a quick 'n dirty way to match it to the bore of the eyepiece hole. The circuitry and sensor are all contained within the screening of the 22mm copper pipe. The black plastic box houses the battery and switch.

The original receptor incorporated its own condenser-type light-gathering lens. This one relies on being pushed into the eyepiece end of a telescope or one side of a binocular, with additional light grasp thus achievable.

So far the new receptor has passed the 'kitchen' reception test. However the pick-up area of the low capacitance photo-diode is tiny indeed. This makes the pointing accuracy critical. Wobbly instrument mounts are therefore out!

I have purchased a cheap telescope for the forthcoming test. This will be easier to point accurately before inserting the receptor than a wide-field pair of binoculars would be.

Steve O
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Re: Optical broadcasting

Postby Panrock » Fri Oct 09, 2015 10:10 pm

Continuing to think out loud here. This helps me organise by thoughts and opens them up to correction from you lot, before they veer too far of course!

I have purchased a cheap telescope for the forthcoming test. This will be easier to point accurately before inserting the receptor than a wide-field pair of binoculars would be.

Well, err... maybe so, but the whole thing (as seen with this post) could be a red herring.

Using a short (condenser-style) lens of the same aperture as the telescope should provide an image on the photo sensor just as bright but because of its wider field of view it would not be necessary to point it so accurately.

Because the sweet spot on this low capacitance photodiode is so tiny, I already am finding it difficult to use the telescope as a light gathering device. This is working without its eyepiece, as a long focus lens, and the slightest jitter on the tripod mount means reception comes and goes.

In contrast, I was able to set up the short focus condenser-lens-based receptor box by hand and simply prop it on an All-Bran packet.

Possible disadvantages of the short focus unit could be greater difficulty in finding the transmission source while first pointing up, and the unknown way the photodiode will respond to a microscopic poInt image. However the rough optical figuring of typical condenser lenses could mitigate this latter effect.

The large field of view of the condenser lens admitting other, interfering, light sources is unlikely to be a problem, since the small sensitive area on the photodiode drastically cuts down the working field of view.

So - unless I've missed something - no need for fancy optical sighting instruments at receiving locations - just a standard condenser-lensed box.

Anyway I had the telescope based outfit working in the garden yesterday and worked out that the minimum usable flux at the receiving end for a noise-free picture was about 5 microwatts. I did this as follows:

Beam contains (a bit less than) 1 milliwatt. Diameter of circular cross-section beam at end of garden = 16cm. Therefore area of beam (Πr²) = 201 sq cm. Receiving area with telescope stopped down to minimum usable = 1 sq cm. There proportion of total flux required = 1/201 of 1 milliwatt or 5 microwatts.

This sounds to me rather more than I had hoped.

Knowing this enables one to work out what the transmitting power of a future optical broadcasting station would need to be, based on say, a 5-mile (8km) service radius, 2-inch (50mm) condenser lenses on the receptors and current signal-to-noise performance. The final figure would be greatly affected by the compactness of the vertical radiation pattern of the omni-directional 'doughnut' h.r.p - if indeed this were to be used.

Steve O
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Re: Optical broadcasting

Postby Panrock » Mon Oct 12, 2015 8:18 pm

Some friends and I are to test the rig this afternoon over an unobstructed path at Didcot Farm, near Dumbleton. This will still be using the modest 1 milliwatt of power. All the units are now mounted on tripods and can be battery powered. Actually, the test won't tell me much apart from the dispersal of the present beam, but it'll be fun none the less.

At the receiving end, I shall be using the original 'wide angle' receptor unit. I believe the telescope based receptor unit would offer little advantage in light gathering and would be harder to point steadily.

(Added here later) the picture shows the test in progress. The receiver is seen on the road in the distance. I tried taking it out twice as far as this, at which point the picture became very noisy. My friend Mike looks cold as he sits on the bonnet.

I have done a tentative calculation to find out how close the system is to being affected by 'quantum noise' at its present level of sensitivity. This would set a soft but absolute limit to the minimum receivable signal level. It seems this limit is still about 20dB away. The calculation can be PM'd to anyone interested, on request.

Steve O
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Re: Optical broadcasting

Postby Steve Anderson » Tue Oct 13, 2015 11:23 am

The poor chap does look cold! As much as I enjoy visiting the UK occasionally, I prefer not at this time of year. I've just realised that the last time was over four years ago. Anyway...

It's hard to judge the distance(s) in the photo, what sort of distances were involved?

Even 100m on 1mW is a significant achievement.

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Re: Optical broadcasting

Postby Panrock » Tue Oct 13, 2015 6:41 pm

"Sixty paces" or metres seems to be about the present limit, beyond which the beam diverges too much for usable results. This agrees with my 'garden' test, where I stopped it down to simulate signal loss.

Not much is it! But at least it beats the 'inch' I originally started wth. :D

The next step is to see if I can fire up the 250mw LEDs at this frequency... :shock:

Steve O

PS. Possibly using a photomultiplier would get me closer to the quantum limit, but 1000v at the receptor(s) would be a definite no-no! And there's that danger of 'daylight flooding'' too...
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Re: Optical broadcasting

Postby Steve Anderson » Tue Oct 13, 2015 7:36 pm

Yes, plus there's the problem of obtaining red (and in time IR) sensitive PMTs, I know you have one or two, but it's not really practical as you hint at.

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Re: Optical broadcasting

Postby Panrock » Tue Oct 13, 2015 11:52 pm

Of course blue LEDs might be used with a PMT, but blue light gets more scattered in the atmosphere than red and is poorer at penetrating mist.

There are now two severe barriers to making any more progress.

The first one is POWER. Will it prove possible to efficiently modulate more powerful LEDs?

The second one is SAFETY. What will be the hazard to vision from sufficiently brilliant, largely invisible (so deceptive), deep red light sources? Could a transmitting array be made large enough so that even close up, the intensity would be safe? But then, a 1 kilowatt single bar electric fire is considered safe to look at... near or far. And a kilowatt of modulatable infrared would be a million times what we have now - seriously useful power.

Steve O
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Re: Optical broadcasting

Postby Klaas Robers » Wed Oct 14, 2015 2:21 am

Steve, the trick is the following. This is from the safety regulations on lasers:

- the amount of light power that can be focussed at the retina of ones eye should be less than 0.5 mW. (Class 1)
- This is the amount of light that a focussable IR laser may give.
- For visible light this might be 1 mW as your eye will close very soon if you see this strong light. (Class 2).

- For LEDs this is less a problem, as LEDs are difficult to focus in a narrow beam.
- As soon as you are slightly further away, most light is spilled outside the iris (black hole) of your eye,
- and that is not counted as light that can be focussed on the retina.
- If you come very close, less than 10 cm your eye cannot focus the light spot
- and light is spread over a larger part of the retina,
- which is not dangerous.

- So if you have a 1W laser, and the light is evenly spread over a circle of 32 x 32 "black holes",
- only 0.1 % of the light will get into your eye,
- which is 1 mW
- and for a visible laser this is seen as safe.

- The point is that lasers are so easily focussed into a narrow beam,
- that the light spot at longer distances still is just a few cm in diameter.
- For LEDs this is almost impossible.

- Realise yourself that someone that takes a telescope or binoculars to look at that distant light source, still can collect more than 1 mW into his eye. If you have more sources in an array, this counts for each separate light source (LED).

The bottom line is: no more than 0.5 mW of focussable light from a point source into the iris of your eye. That is the reason that the laser in a CD-player emits 0.5 mW of light. I have looked to it, you can see it as very dark red light, but it is focussed 1 mm above the very small lens. You see that very tiny spot of light. I did not take a powerfull magnifying glass to look at it, as that is stupid.
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Re: Optical broadcasting

Postby Panrock » Thu Oct 15, 2015 2:52 pm

Yes, let's take Solar Irradiance for example. This varies with atmospheric conditions but we could say it is about 1Kw per square metre. The pupil of an adult's dark-adapted eye might have a diameter of 5mm. So its area {Πr²} would be 20 sq mm.

Divide one into the other and we find such a pupil admits 20 mW when looking directly at the Sun - enough to blind. In practice the pupil would rapidly contract, but even when around 1mm diameter it would still be admitting 1mW. So 0.5 mW seems a sensible limit. And even that would look bright.

Klaas, a question I have is does this limit also apply to a non-focussable into-a-point source, such as an extended array of LEDs viewed close up? This has critical ramifications for the design of a safe array. Also, does the 0.5mW limit include invisible radiation, such as infra-red? A practical system would use mainly infra-red, since the glare from a brilliant red source on a mast would soon attract complaints and could dazzle pilots.

As for the view through telescopes and binoculars, the total power into the eye could never exceed that gathered by the objective lens and this would rapidly diminish with distance {4Πr²}.

Steve O
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Re: Optical broadcasting

Postby Harry Dalek » Thu Oct 15, 2015 7:25 pm

HI Steve they are pretty strict down here with pointing lasers due to the fools in the past that pointed them at planes ,not sure of your laws in the uk .

I recall Chris long here

Went with luxeons over lasers for reasons explained and got some nice distances also recall others on the net doing the same ...if you are still going with laser as with lasers i try in my projects i always make it so i can ramp up the power from low to high so i don't blind my self by mistake ...so until you are sure its nice to have a choice with laser power settings .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Optical broadcasting

Postby Panrock » Thu Oct 15, 2015 7:54 pm

Yes Harry, lasers are not on at present - I can't find a way to modulate them fast enough. Remember this is modulation at VHF we want.

If lasers were ever to be used, it would only be behind a cylindrical blurry lens, to spread the beam into a 'fat horizontal fan' shape to cover a service area. This would make it a lot safer.

Steve O
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Re: Optical broadcasting

Postby Klaas Robers » Fri Oct 16, 2015 7:20 am

Steve, I see that you understood the message. I also did the calculations with the sun with 1 kW/m². However the sun is not a point source. So the sun will never be focussed on one point of the retina, it will always hit a certain area. That makes it less dangerous. However, our eyes are "designed" (evoluated) to withstand looking into it for a short time and no more.

This also applies for an array of LEDs. Each LED can be seen as a point source, so for each LED individually this 0.5 mW applies, if all light of it can be seen at the same time by one eye. If you have 100 LEDs, the light of the array will be scattered over the retina and will not accumulate. So an array behaves more or less like the sun. And the sun is safe, isn't it?

These rules apply also to IR light. Our lens is good enough for the near IR light (1 um wavelength) of IR LEDs. these are more dangerous, becaus you don't see them burning and your eyes will not close automaticly when you are looking at them. This lowers the power limit to 0.5 mW, while for visible light it is 1 mW (in some countries even 5 mW).

This of course is for the real light power, not the input power of the LEDs.
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Re: Optical broadcasting

Postby Monochrome » Sun Jan 31, 2016 10:51 pm

Hi Steve,

Perhaps a little late but "congratulations" on your success with transmitting a video signal via an LED light source. Its quite an achievement using such a high RF subcarrier frequency. I have only just caught up on this and other NBTV threads as I only seem to find the time to check the forum every few months these days :-(I am not sure why it did not catch my eye earlier but better late than never.

I have been interested in video via light for some time and have read about Clint (KA7OEI) and F1JWJ success using LED and laser repectively but I think your experiments have fired the imagination here more than any other so far. In recent years the bad-press and restrictions on laser experiments have made them less attractive for outdoor comms experiments. Also the experiments by Clint and others have clearly shown that over greater distances through the air LED light sources suffer less scintilation than that of coherent laser sources.

Perhaps the only other optical ATV experiments that come close to yours (that I am currently aware of) are those of M0DTS though he used a 20 MHz FM subcarrier and photomultiplier tube at the RX end of the link.

http://www.m0dts.co.uk/index.php?item=128

and the youtube video...

https://www.youtube.com/watch?v=oEuA351Hbdo

I should like to ask a couple of questions if I may. My first question, are you still experimenting with the optical link?

From links and details in this thread I note that some of the details are spread between different forums, so are your current experiments only reported on this forum or should I also be looking elswhere in order to catch-up?

Also, in your most recent long distance tests could you clarify what sort of optics you used at the transmitting end of the link please.

And finally, did you send sound as an intercarrier or are your experiments confined to video only at present. Very interesting stuff and I hope you are still experimenting with the optical video link.

Keep up the good work.

73,s

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