Television over a light beam
 

This is really, I suppose, a plug for the forthcoming annual NBTVA Convention on Saturday April 6th from 10.00 am at the Plumptre Hall, Church Street, Eastwood, Nottingham NG16 3BP. An interesting day out for television enthusiasts. Mods please move if appropriate.

NBTV (narrow band television) is traditionally 'low definition'. So I shall be definitely pushing my luck this year by taking along a 625-line demo. This is to modulate a (fast) LED with a complete analogue Channel E4 (61 - 68 MHz) bearing a PAL colour picture and sound, then beam it across the hall, where it will be picked up by a photodiode + other bits 'n pieces and displayed on a set. The optics uses bullseye condensing lenses at each end.

This is an upgraded version of a b/w 405-line arrangement I demonstrated in 2016.

What's the point? Well, it's interesting to see how far you can go modulating an LED with a linear signal - and remember this event is about "television experimenting for fun".

KarenO on this forum is a prolific contributor there. It will be interesting to see what she brings along this year.

Maybe see some of you there. :)

Steve

Re: Television over a light beam
 

There was a Maplin project that sent a monochrome (but still 625 line) video signal over a light beam about 25 years ago. To be fair, the horizontal resolution was not great but it did work.

I suspect that with improvements in LEDs over the years that what you want to do is very possible, but not trivial. Good luck with it :)

Re: Television over a light beam
 

It certainly is possible. The results are good.

It's important to realise this in my case is not a baseband video signal being modulated onto an LED, as has been done many times before, but a set of modulated RF carriers comprising a complete channel... >10x faster. Out of the receiving photodiode pops a complete ready-to-go signal for the set's aerial socket.

More about the previous dem HERE.


Steve

Re: Television over a light beam
 

I would like to mention that Steve is a longstanding member of the NBTVA who has a long string of 'firsts' behind him. Steve has also been active in trying to resurrect the channel 1 405 service from/near Crystal Palace.

I look forward to seeing yet another of your firsts next Saturday, Steve.

Because Steve is modulating the LED with RF not baseband, there is the potential to convey other program material alongside the video channel in the HF and low VHF.

Re: Television over a light beam
 

Well... they do say "it takes one to know one" and I don't want this to sound like a mutual admiration fest, but the fact is that Karen's NBTV creativity as a member has been quite exceptional and never seen before, I think. And she combines it with digital skills far and way above my head.

Anyway... I look forward to seeing you on Saturday Karen, and hope all is fine. :wave:

Steve

Re: Television over a light beam
 

I can only say......WOW !!!

Re: Television over a light beam
 

Indeed!
When I was young my party-trick was sending/receiving baseband AUDIO across the street using torch bulbs and ORP12s

Re: Television over a light beam
 

You did well to get audio out of a sluggish CdS cell like the ORP12. One way to do this was to 'light-bias' it with an external light.

For me, it was the ORP71 photo-transistor. As a teenager, I remember writing in to Practical Wireless asking them if it could handle the audio frequencies needed for mechanical television. When they wrote back, they didn't seem sure but thought it should "handle 15Kc/s quite easily".

Steve

Re: Television over a light beam
 

I'm not far from Eastwood and went to the convention at the same venue last year. I'll do my best to go again although I'm rather unwell at the moment. Way back in the mid 70's I was interested in what was then known as LDTV. I knew Doug Pitt and Stan K (I won't attempt the surname spelling) I did build a mirror drum from meccano brackets but it was a failure being rather heavy, unbalanced and inaccurate. It was at one of the early meetings I suggested that low definition tv was a defeatist name as the goal is the best definition over a small or narrow bandwidth so it was decided to change the name to NBTV. My only success in the field.
Hopefully I'll witness your demo. I recall a school science project was sending audio via a torch bulb scraping the black paint from a transistor to receive. There was a project 'light beam telephone' in practical wireless or one of the hobby mags. The main problem for any meaningful distance is the optics used. We got several hundred yards on a good day.

Re: Television over a light beam
 

Way back in the 1990's I was involved with a project that evolved sending video via a infra-red link. When nobody was looking I opened the receiver box and found a IR photo diode typical of the type used in television remote control a few years earlier, and focused with a big Fresnel lens. The system worked Ok except at sunrise, when, because the receiver was looking to the east, sunlight saturated the diode.
Living in Nottingham, I'm going to pop along on Saturday.
Dave

Re: Television over a light beam
 

We could have done with that advice 50 years ago, we thought the secret was to keep the optics in the dark.

back on topic though, are semiconductor laser diode too slow compared to LEDs?

Re: Television over a light beam
 

I have limited knowledge of laser diodes. The answer all depends on the driver circuitry, which also should give protection - the bare diodes seem awfully fragile and easy to blow. Like LEDs, they should be current-fed to modulate them, but with L/Ds there are strict limits - step outside their parameters and you get no second chance. If you just take a laser pointer/pen you can modulate them to some extent by just varying the rail voltage, but yes... this will then be slow.

If I had been able to use a laser instead of a (rather special) LED for my 60MHz dem, the range would have been effectively unlimited. As it is, the range is set by beam spread from the transmitting condenser lens.

Steve

Re: Television over a light beam
 

A few years ago I read an article of somebody sending a 405 black and white line signal on ch B1, 41.5 Mhz sound, and 45Mhz vision, across his garden using either an LED or infra red LED. It worked well and only suffered a bit of loss during heavy rain or snow.
I would think that a 625 line PAL signal could be made to work across such a link. Obviously you would need a bit more bandwidth at both ends of the link. Good luck with
I can't make it to the event but looking forward to hearing how this goes.

Re: Television over a light beam
 

That was me! See pics. Note that I've now moved on from using an All-Bran packet to a proper tripod....

Steve

Re: Television over a light beam
 

Semiconductor lasers are used for transmitting signals through glass fiber. That would put the usable switching speed in the GHz range.

Re: Television over a light beam
 

Yes, and megs at least are presumably needed for internet fibre. Switching is one thing though; fast linear light modulation (for analogue) quite another. What are the devices that do this? Or perhaps there's no call for such a thing.

Re: Television over a light beam
 

In the early 1990s I was part of a team from the Optoelectronics Research Centre at the University of Southampton who did a demonstration at (I think) the Royal College of Physicians in London when we won the Rank prize for our Optical Amplifier which directly amplified optical signals in a length of fluorescent fibre where the power was provided by a fat-ish infra-red laser, so no need to detect to electrical signals, amplify, and re-modulate. We had a source laser diode set up in a "bias tee" network sending a beam down a few kilometres of fibre, wound up on drums, through an Optical Amplifier, a few more kilometres of fibre, and all detected on a simple but quite fast photodiode with a simple resistive load (we had bags of light signal after all). We hooked up a pile of then-current VCRs, all tuned to put their RF carriers at different frequencies, and for good measure used a set-top indoor aerial to add the terrestrial (then analogue) channels into the mix of one of the VCR's outputs, and fed the whole "signal soup" to the laser diode, each signal source having its own feed resistor into the bias tee. At the receive end we had several TVs with at least twelve channels of push-buttons on the tuner, and every one was tuned up for all 12 different signals. We fed the recovered signal soup to the aerial socket on the TVs through a little aerial distributor box and we could flick channels beautifully. Four different channels, eight different videos, receive-end quality easily good enough to clearly see the limitations of a VHS tape compared to a proper TV channel. I seem to remember we had Top Gun on the TVs while the boss-man was talking to the visiting VIP - one Margaret Thatcher. You may remember her.

I have no doubt that a modern fast LED will do what you want it to over a reasonable distance. The physical vibrations are a problem in line-of-sight communication across a room, but even so, you should be OK. You can modulate an LED with a bias tee just like a laser. You run the laser or LED from the current-limited supply through a resistor in the positive feed (as long as the current source doesn't panic about having to shove out a volt more than it's expecting) and add one or more other resistors feeding to the anode of the diode, where it joins the feed resistor. You only have to couple in the signal(s) to the remote end of the resistor(s) and the small fluctuations of current caused by the signal through the feed resistor modulate the brightness of laser or LED because the current source will be stabilising current at low audio frequencies and it won't have anything to say about the sort of frequencies you'll be modulating on there. You want to be pretty sure you're only going to get signals of the amplitude you're expecting, though. A big transient will pop the laser. Worn that tee-shirt. At the time, a decent laser diode packaged up pre-aligned to an optical fibre was about £1500. Er, oops. It came off in my hand, officer.

(On the way home, all the kit was piled into the back of a heavily loaded Peugeot 405 estate. As Janet spotted the amber traffic light make an appearance beneath the red, she gave the right pedal a press and the left pedal some upward movement, the boot popped open and half the contents of the boot emptied themselves right across the junction. I was in my car behind. All I could do was sit tight at the stop line, switch on the hazard lights and try not to laugh enough to actually burst a blood vessel. Dancing around in the junction, piling things back into the boot, were Janet, who did most of the work on the fluorescent glass fibre, and Richard, who had done most of the rest of the work. A unique pair of academic stars trying to avoid the oncoming busses.)

Re: Television over a light beam
 

That's most interesting Rob. I am using a simple feed arrangement that sounds very similar to what you recommend, whilst trying to ensure the RF amp module sees something like the 75 ohms it's expecting.

However, I did find that results with everything 'on the bench' gave a false positive with some signal reaching the receiver from inevitable RF leakage. For conclusive results, it was essential to have a good physical separation between transmitter and receiver and the ability to put your hand in the beam to 'cut it off'.

Steve

Re: Television over a light beam
 

Quite right about the RF leakage. It can travel across a bench at surprisingly low power, but of course it only happens to signals you don't actually want to cross the bench. Try and couple your signal generator in to the frame aerial of an old valve radio so you can re-align it and no stray coupling will ensue.

Of course if your receiver is getting loads of signal and doesn't need a pile of gain it's much easier to stop the stray coupling.

If you want your LED driver to present a 75 ohm load to the source, a first approximation is obviously to make the feed-in resistor 75 ohms. The capacitance of the LED junction will make it more-or-less a short circuit at a decent RF frequency. The tricky bit is to get the amplitude of the drive signal right, so that you're getting the desired level of modulation. It can work without too much trouble. A 100mV RMS signal over a 75 ohm resistor modulates the diode with 1.3 mA RMS, which is the right order of magnitude at least, if your diode current is a regular sort of current. When I did it, I got very lucky and didn't need any fancy attenuators or worse still, amplifiers. The outputs from the VCRs did fine on their own. I'm not sure how that happened; I'm no great favourite of Lady Luck's!

Re: Television over a light beam
 

I did exactly the same thing in between my mates house in the next road to my upstairs bedroom. It worked but the sound gave the impression that the audio coupling capacitor in the receiving amp was leaky! That would have been back around 1961. John.