Optical broadcasting

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Re: Optical broadcasting

Postby Panrock » Wed Sep 02, 2015 4:09 am

The first verified results! The signal has not been down-converted but is at 'native' Channel 1: 41.5/45.0 MHz. Both vision and sound are being transmitted. The picture is still grainy at any distance over a few inches but remember this is on a strongly diverging LED beam picked up by a tiny photodiode without any concentrating optics at either end. Placing a piece of card in the beam completely removes the picture, as it should. The screening preventing an errant radio signal getting through is now very good.

Careful selection of the component types was essential.

These results have only been achieved after considerable trial and error! The uncensored chaos on my bench is shown.

Plenty more to do then, but at least it's a start.

Steve O
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Re: Optical broadcasting

Postby Steve Anderson » Wed Sep 02, 2015 1:27 pm

Well done Steve. It may only be a couple of inches, but as you say, it's a start. (No sniggers please).

You're wise to conduct these experiments in the visible spectra for safety reasons. Later with IR change to the S5971 photodiode which has more sensitivity peaking at around 900nm.

Not cheap op-amps, as all AD stuff seems to be, but really good.

Time to get some of those plastic Fresnel 'page magnifiers' and see how far they extend the range.

Chaos? I'm not going to even consider posting a picture of my workbench as it is. It's hard to actually see it with all the crap on top!

Steve A.
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Re: Optical broadcasting

Postby AncientBrit » Fri Sep 04, 2015 5:17 pm

Cracking stuff Steve. Well done.

Is this a first??

Cheers,

Graham
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Re: Optical broadcasting

Postby Panrock » Sat Sep 05, 2015 4:51 am

AncientBrit wrote:Is this a first??

Well, err... let's say that, as far as I know, no one has been mad enough before to try to modulate a light beam with an analogue, vision-and-sound, amplitude modulated, rf television channel. Stop that sniggering at the back!!! :lol:

Anyway, there are now more results. I've just added optics... the little box shown is the receptor, the big one the emitter. As expected, I am now getting a very strong signal strength from across the workshop. Banding on the picture is due to the camera shutter. Slight blurriness on the hf gratings - I shouldn't have hand held the camera.

In practice, with a setup of this sort, the ultimate range will be set by the compactness of the beam. I have just tested it over a 45-foot path, down the garden from the workshop to the house. It's weaker but still with noise free results.

Steve O
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Re: Optical broadcasting

Postby Panrock » Wed Sep 09, 2015 11:20 pm

Now I have a proven receiving box I can trust... this morning I have also managed to get results using a 'normal' hyperbright LED at the transmitting end. This was a surplus one left over from the red channel of my mirror screw project. Steve A originally chose it, and from its expected rise time, it really shouldn't work... but it does! The operating efficiency appears reasonable, though a direct comparison with the pucker Hamamatsu LED is difficult because it makes a differently shaped beam with the optics I am using.

The use of cheaper, standard LEDs would make upscaling the power (for eventual broadcast purposes) more achievable.

It's interesting that the beam doesn't flicker to the eye as a baseband-modulated one would, but looks steady. I guess flickering at 45 million times a second is just too hard for the eye to follow! :shock: ... even when the depth of that flickering varies with the video and audio signal... ?!?

Daylight flooding (at least through the open window of the workshop) doesn't seem to be a problem. I've also tried it with a weakened/occluded incoming signal. No difference. I suspect the immunity of this rf carrier based system to interfering (eg. street) light sources could be quite good.

Steve O
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Re: Optical broadcasting

Postby Harry Dalek » Thu Sep 10, 2015 1:55 pm

Very interesting Steve i have only tried this with nbtv where as i just used a solar cell as the reciever as posted a while back ..has me wondering if a solar cell if given enough transmitting light would also do away with any recieving electronics at these higher bandwidths also ? as in i mean no head amp electronics and well apart from my case the laptops mic amp and your case the tv monitor ...it is sort of the same idea but up scaled ...
I am not sure a IR transmitting led would be any good but a white led might give the same results as my little try ...
I don''t know if english people have solar cell panels on their roof as much as here in Australia but crazy as it sounds at night i wonder if they could be picking up tv signal if you used a light tv transmitter and pointed it at it ...as in an antenna to a crystal radio electric reception?
and BTW do you still have to pay for a tv licence fee in the uk ? outrageous ! i just came across 2 vintage aussie tv licence fee cards from the 60s but i think that ended in the 60s i can never recall it later than that .
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Re: Optical broadcasting

Postby Panrock » Thu Sep 10, 2015 9:57 pm

Hi Harry,

Harry Dalek wrote:Very interesting Steve i have only tried this with nbtv where as i just used a solar cell as the reciever as posted a while back ..has me wondering if a solar cell if given enough transmitting light would also do away with any recieving electronics at these higher bandwidths also ? as in i mean no head amp electronics and well apart from my case the laptops mic amp and your case the tv monitor ...it is sort of the same idea but up scaled ...

Well, I think the capacitance of solar cells would be a big problem a these high rf frequencies. The sensor I am having to use is really tiny and has a capacitance of only 3pF! Over anything but the smallest distances the received signal is really tiny too, so added electronics are a must.

While getting started in this field, it's easy to think you're getting real results from light when in fact what you're picking up is rf leakage - quite a bit of rf is needed to directly drive an LED. This happened to me during my first experiments. Good screening is essential.

Harry Dalek wrote:I am not sure a IR transmitting led would be any good but a white led might give the same results as my little try ...

I suspect a white LED wouldn't be much use above NBTV baseband frequencies, because (I think) they use laggy phosphor coatings. If you want to use an LED to transmit a signal from A to B, then a red one (or an infra-red one) will be best because it better matches the response of semiconductor sensors.

Harry Dalek wrote:I don''t know if english people have solar cell panels on their roof as much as here in Australia but crazy as it sounds at night i wonder if they could be picking up tv signal if you used a light tv transmitter and pointed it at it ...as in an antenna to a crystal radio electric reception?

What a nice idea! I don't think this would work though in this case, for reasons of capacitance mentioned above.

Harry Dalek wrote:and BTW do you still have to pay for a tv licence fee in the uk ? outrageous ! i just came across 2 vintage aussie tv licence fee cards from the 60s but i think that ended in the 60s i can never recall it later than that .

Yes, we still have licence fees here. But I'd better not get all political and on my hobby horse about it now! :lol:

Anyway, here are pics of the latest results from the sensor propped up on an All-Bran packet in my kitchen - I have to keep things 'regular'! (Do you have All-Bran in Australia too? :shock: ) This is receiving a signal from the workshop down the garden. I have confirmed it works fine at night and in daylight, but foggy windows are a problem...

There is plenty of signal being pulled in by the sensor in the kitchen. I found I could add 30dB of attenuation in the aerial lead and still get a reasonable picture. However, while (say) 1 millivolt might normally represent a cracking TV signal strength, the sensor and op amp don't seem to be quite so happy working at these low signal levels and VHF frequencies. Noise is just visible on the picture. I'll try playing with some circuit values to improve signal-to-noise, no doubt at the expense of gain. However, right now I do have gain to spare...

Steve O
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Re: Optical broadcasting

Postby Harry Dalek » Fri Sep 11, 2015 2:55 pm

Great pictures Steve really does show the results in the adverse to good transmitting situations..

BTW have you tried a night test by reflection off different surfaces ?

i suppose it would have to be close depending how much IR your led or leds can do ,i like the idea of light skip like Radio propagation .

As i remember head amps or LNBs on satellite dishes work better if you can cool them to stop thermal noise....I wonder if you did the misty light transmitting test and got some cold spray and cooled your detector would the noise decrease

IS IR light or heat ? or the same

i see red in the led transmitter but IR is invisible to the eye the camera should show it as blue ...is the tramsitting leds more towards the red spectrum i have reread your posts Steve sorry ...

UPdate
I reviewed the posts i see your using a Red Led from a past project ..
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Re: Optical broadcasting

Postby Steve Anderson » Fri Sep 11, 2015 3:29 pm

I have to agree with Harry, darn good results! I wonder, over longer distances of several km would atmospheric disturbances affect the signal? Chris long has done a lot of research on this and has achieved voice optical communication over hundreds of km. Dust, pollution or smoke may also be an issue. Well, there's only one way to find out I guess.

Chris had the advantage of a hilltop-to-hilltop path away and above the rubbish in the air that a large city creates. I also think these voice links were conducted at night when maybe the atmosphere is more settled.

Anyway, great results!

Steve A.
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Re: Optical broadcasting

Postby Klaas Robers » Fri Sep 11, 2015 6:59 pm

Steve, I can't find your circuit diagrams back, but you will know that the cumbersome capacity of a photo diode (of ALL diodes) decreases if you apply a voltage in the non conducting direction. This is the effect that is used in varicaps. In reality all diodes are rectifiers, varicaps, zeners and photocells, but the effect is only specified (measured) for diodes that are named in that way. A semiconductor diode is just a semiconductor diode.

What I would do is making a tuned circuit on 44 MHz, or even better a loosely coupled tuned transformer (a band filter = two coupled tuned circuits), and place a positive voltage of say 12V, on the cathode of the photo diode and connect the anode to the top of the first tuned circuit. Then the diode is just a current source.

With a capacity of 3 pF of the photo diode this capacity can be taken in account in the first tuned circuit. You are optimal if this capacity is the only capacitor of the first LC resonator. You may even use the varicap properties of the diode to tune that circuit. But make the tuning voltage "hard" (emitter follower) as the current through the diode will vary linearly with the incident light. Otherwise the tuning will vary with the total amount of incident light.

Secondly try to find a coloured filter that is transparent only for the colour of light that you use. This will remove the noise introduced by all other colours of light that might hit the detector diode. For near infra red these filters exist in remote controls.

I have had an IR receiver for a TV which was designed in this way and I could control the TV over 200 metres! Of course this is of no practical use, so currently IR receivers are much less sensitve (and cheaper).
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Re: Optical broadcasting

Postby Panrock » Fri Sep 11, 2015 7:47 pm

Thanks for the nice comments chaps.

Harry Dalek wrote:BTW have you tried a night test by reflection off different surfaces ?

I have been using a small mirror in the workshop to fold the optical path so I can work with both the transmitter and receiver next to each other on the bench. Yes, it would be instructive to try bouncing it off windows etc. This should work just as well during the day as at night. The advantage at night is you can more easily point the beams by eye.

Harry Dalek wrote:i suppose it would have to be close depending how much IR your led or leds can do ,i like the idea of light skip like Radio propagation .

I've been steering clear of infra-red for now, because then you *can't* see the beam at all and judge it's of safe brightness. As it is, the deep red 660nM light I'm using has what's known as a 'luminous efficacy' of only 40 lumens per watt. This means the eye is only 1/16 as sensitive to it as peak yellow, which in turn means it's not a good idea to stare at it - there's more energy in the beam than it appears.

Harry Dalek wrote:As i remember head amps or LNBs on satellite dishes work better if you can cool them to stop thermal noise....I wonder if you did the misty light transmitting test and got some cold spray and cooled your detector would the noise decrease

It might do - slightly - I suppose, if it were really cold. I could try squirting on some freezer... in the meantime I have now improved things by tinkering with the circuit.

Harry Dalek wrote:IS IR light or heat ? or the same

It's all the same stuff, different wavelengths. The higher the temperature of the radiating body, the greater proportion of shorter wavelength infra-red, which then merges into light.

Steve Anderson wrote:I wonder, over longer distances of several km would atmospheric disturbances affect the signal?

Yes I believe they would. Based on data from how stars and various planets twinkle (or do not twinkle) when at low altitudes, I reckoned this effect would be partly determined by the angular size of the radiating array, and it would kick in only after several miles. I called this 'baseband flutter'.

Harry Dalek wrote:Chris had the advantage of a hilltop-to-hilltop path away and above the rubbish in the air that a large city creates. I also think these voice links were conducted at night when maybe the atmosphere is more settled.

In astronomy, the most stable 'seeing' is often accompanied by haze, while the most transparent weather conditions can be as unsteady as a jelly!

Klaas Robers wrote:Steve, I can't find your circuit diagrams back, but you will know that the cumbersome capacity of a photo diode (of ALL diodes) decreases if you apply a voltage in the non conducting direction. This is the effect that is used in varicaps. In reality all diodes are rectifiers, varicaps, zeners and photocells, but the effect is only specified (measured) for diodes that are named in that way. A semiconductor diode is just a semiconductor diode.

Yes I tried this, for the reason you mentioned. However in this case, and with this diode, it seemed to give no advantage over photo-voltaic mode, which - as I understand it - also offers the advantage of the best linearity. Since I'm amplitude modulating with an analogue signal, itself amplitude modulated, linearity is an important factor.

Klaas Robers wrote:Secondly try to find a coloured filter that is transparent only for the colour of light that you use. This will remove the noise introduced by all other colours of light that might hit the detector diode. For near infra red these filters exist in remote controls.

I'm currently using visible light of course. A gelatin filter would also introduce significant losses, even within its passband. An interference filter might be better but would have to be broad enough to encompass the (fairly narrow) bandwidth of the LED.

With high frequency modulated, weak light - 'quantum noise' can become a factor too.


Steve O
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Re: Optical broadcasting

Postby Klaas Robers » Sun Sep 13, 2015 4:52 am

Panrock wrote:Yes I tried this, for the reason you mentioned. However in this case, and with this diode, it seemed to give no advantage over photo-voltaic mode, which - as I understand it - also offers the advantage of the best linearity. Since I'm amplitude modulating with an analogue signal, itself amplitude modulated, linearity is an important factor.
Steve O

In the so called photo-voltaic mode, to a virtual ground, the diode is biassed as well against the photo voltaic voltage. If you increase that voltage the only thing that happens is a lower parasitic capacity. May be even the quantum efficiency (one electron for one photon) is somewhat higher, that is: closer to 1. The linearity may also be better, however that is not so important as the remaining non linearity is mainly even (quadratic) and introduces only even harmonics, no intermodulation. These even harmonics (88 MHz) are filtered out by the TV-set.

Any way I would tune out the remaining capacity by an inductor to a resonance frequency of 44 MHz. That is much better than a wide band (0 - 50 MHz) front end.

Good luck
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Re: Optical broadcasting

Postby Panrock » Sun Sep 13, 2015 9:01 pm

Thanks for your comments Klaas. Presumably I could also fine-tune such a front end by varying the reverse bias voltage on the photodiode. The input tuned circuit would need to have quite a low Q since Channel B1 represents more than 10% of the carrier frequency.

Clearly, being vulnerable to incoming out-of-band signals is normally considered a bad idea. However the present arrangement does work very well. Any out-of-band signals will of course be rejected by the TV tuner. And in any case there will be far fewer (or zero) out of band signals contained in a light beam (with directional pick-up) compared to the standard radio environment. So maybe adding a tuned front end would be an unnecessary complication at this stage. Just my opinion.

Finally, it could actually be an advantage to have an untuned front end in the pick-up sensor, since this means users would then be able to receive other services in future, on other frequencies carried by the same beam, without changing their sensor box but merely connecting it to something else, like a radio. For example, one or more medium wave radio stations could be included on this 'optical gateway'.

In these days of digital television, data carriers, and OFDM etc.... and fog, none of this is likely to hold any commercial potential, but it could provide some fun for us hobbyists!

My attention now turns to scaling up the technique to permit some form of 'broadcasting'. It is possible that a mast in London might be made available for this. However this is definitely not 'in the bag' and further details will remain private.

First thought is to try some very-deep-red 'horticultural' 730nM LEDs to see if they can be modulated at VHF. Unlikely I know, but with a low impedance source this proved possible with another standard LED. Such horticultural LEDs are 250x the power of the one I'm currently using. The deep red should permit visual alignment (essential) but would be significantly better in cutting through mist.

The 'receiving aerials' might be modified/rigidly mounted 10x50 binoculars, with one side used for setting up and pointing at the source and the other taken up with the photodiode.

Alternatively it might be possible to use 'bullseye' condensor lenses. I am using two of these back-to-back for my existing sensor, which creates a very fast (c. f 0.1) lens of similar light-grasp to the binoculars. Far harder to align with a remote source though... and I am not sure how a photodiode would react to the absolutely microscopic pin-prick of light at the focus.

One issue is the total power needed at the transmitting end. This could be a deal breaker...

Another issue is safety. The radiating source on the mast must not be excessively intense as seen through such binoculars - or by nearby helicopters! Due to its directional properties, it wouldn't be visible at street level until very remote and thus harmless. The eye is very insensitive at this wavelength and the light would actually be far brighter than it appeared. I've done some swotting up online about this. Osram do a very useful document.

The system could be built up in stages around the mast, in modular fashion.

Really only thinking out loud at present! Feel free to shoot my more crazy ideas down, but when you do - try to suggest an alternative. :lol:

Assistance from the electronics big-hitters out there (you know who you are!) could become really valuable if ever this project develops. Many thanks to all so far.

Steve O
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Re: Optical broadcasting

Postby murphyv310 » Mon Sep 21, 2015 4:34 am

Hello Steve.
I'm glad I've caught up with this, the results have spurred me on to get back to this.
Have plenty of time to devote to it at the moment.
Cheers Trevor.
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Re: Optical broadcasting

Postby Panrock » Mon Sep 21, 2015 11:13 pm

Trevor! Good to see you over here. :)

It really is working surprisingly well. My next step is to attempt to beam it out of an upstairs window across the village to a neighbouring hill. I shall try using the current outfit first, but then plan to increase the power substantially (x250) - available LEDs permitting.

Good luck with the LED I sent you.

Cheers

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