Optical broadcasting

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

Re: Optical broadcasting

Postby Panrock » Mon Feb 01, 2016 8:34 am

Hi Des,

Thanks for the kind comments and interesting references.

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

Yes, but it's been put on the back burner for the moment while I attend to more pressing projects. However, I have now obtained some high power deep red "horticultural" LEDs. The next step is to try these in order to raise the transmitted power by a factor of 250. In the meantime, I shall be bringing the standard-power rig along to the NBTVA Convention, but will probably demonstrate it at 120-lines rather than 405, since this is more in the spirit of "NBTV".

Monochrome wrote: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.

This was a standard "club" condenser lens, mounted in a tapered paper section cut from a soft drinks dispenser cup! Once painted it didn't look too bad. Obviously the positioning of everything is critical in order to get a good beam. A photo is attached. The pot shaft adjusts the DC bias to the LED.

Monochrome wrote: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.

Vision and sound are first amplitude modulated onto two separate RF carriers: 45.0 and 41.5MHz, which then I suppose attain the status of "subcarriers" once the resulting complete System A television channel is in turn amplitude modulated onto a beam of light.

One reason for this experiment was to see if I could provide the BHTG with an alternative way to broadcast vintage television, should licensing of the original Band I spectrum prove impossible. Using this optical method, out of the pick-up emerges a complete Channel B1, ready for plugging in to the aerial socket of a vintage receiver.

Steve O
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Re: Optical broadcasting

Postby Monochrome » Mon Feb 01, 2016 9:18 pm

Hi again Steve,

I was delighted to see you had responded to my posting and so quickly.

Thankyou for taking the time to respond and in such detail. I was quite surprised to see the size of the TX optics, I had expected something much larger and the relatively small size of the TX lens makes the achievent even more remarkable. I imagine it also makes aiming a little easier? The A4 Fresnel lenses which seem to be popular for optical comms' offer DX but at the expense of critical optical alignment.

For the first time in ages I am tempted to fire-up the soldering iron and have a go :-)

The link you provided to the British Heritage Television webpage also proved interesting and I should like to ask if you know what power level was used in the 2011 tests?

Thanks again for the detailed response and I wish you continued success with operation "deep-red" and optical video broadcasting.

73,s

Des (M0AYF)
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Re: Optical broadcasting

Postby Panrock » Mon Feb 01, 2016 10:34 pm

Hi Des,

Monochrome wrote: I was quite surprised to see the size of the TX optics, I had expected something much larger and the relatively small size of the TX lens makes the achievent even more remarkable. I imagine it also makes aiming a little easier?

At the transmitting end, as long as all the emerging fan of light from the LED is gathered, it doesn't matter how big or small the gathering lens is. All this affects is the initial diameter of the projected beam. Inevitably, the beam is never quite perfectly parallel and the spread of cross-sectional area that develops with distance dwarfs its initial size.

I'm happy to report the 2-inch condenser lens was able to gather virtually all the light and focus it into a roughly parallel beam - so long as it was placed at the correct distance from the LED.

Monochrome wrote:The A4 Fresnel lenses which seem to be popular for optical comms' offer DX but at the expense of critical optical alignment.

At the receiving end, it's another matter! There, you need to gather as much as possible of the so-called "parallel" beam, which soon becomes yards across. The Fresnel lens is one way of providing a big aperture for this job.

Monochrome wrote:The link you provided to the British Heritage Television webpage also proved interesting and I should like to ask if you know what power level was used in the 2011 tests?

The peak-white vision e.i.r.p of the low power tests was +23.3dBW. in plain English this is 214 watts, or an e.r.p (dipole equivalent) of 130 watts.

Monochrome wrote:Thanks again for the detailed response and I wish you continued success with operation "deep-red" and optical video broadcasting.

You're welcome, and thanks for your interest. :)

Steve O
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Re: Optical broadcasting

Postby Panrock » Fri Mar 25, 2016 11:57 pm

I'm now back on this, shortly to be full-time.

Steve O
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Re: Optical broadcasting

Postby Panrock » Tue Apr 12, 2016 12:23 am

Having now demonstrated the rig across the entrance room, on a 17-inch screen, at Saturday's convention, I am free to hack it about with regards to developing the technique further. I have added a low capacitance LED tapping point at the front of the transmitter and for now, removed the collimating lens.

The first task was to find out how critical the specs of the LEDs are for operation at 45 MHz when driven from a low impedance source. To my relief, it appears "not at all". For at this frequency, 6dB-down equates to a shunt capacitance of about 47pF at 75 ohms and 70pf at 50 ohms... not particularly onerous figures.

This morning I have had good results in varying degrees from red and green 5mm LEDs, and even also from white - though I expect this was from the blue component behind the fluorescent coating. However I had zero results from an old 10mm red LED. I have yet to try the deep red 'horticultural' LEDs, since these are surface mount devices.

At present I have no way to view the actual depth of the light modulation of the LEDs, beyond adjusting things for wnen they start to limit - then back off.

If the LEDs are not going to be critical or unaffordable after all, this opens the door for the experiments at higher power.

I have been taking a first look at top-of-mast obstruction beacons, which are omnidirectional and have a vertical spread angle of 3 degrees. Maybe something could eventually be built into this.

Steve O

Edit: I have now ordered some infra-red power LEDs for the next stage; eventually an array of them is to be driven at 1.6 watts. I believe these will be powerful enough to present a serious invisible radiation eye hazard. I am finding the regulations: IEC62471 abstruse and unintelligible. I welcome all advice on this subject!
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Re: Optical broadcasting

Postby Panrock » Wed Apr 13, 2016 11:09 pm

I feel a bit bad about going on and on about this project of mine, while making minimal contributions to others' threads!

This will have to change. Thank you for your patience.

Anyway I have created a new web page, where all news in future about the 'opto' project can be brought together in one place. It's HERE.. Hope to see you there. The mods may now wish to close this thread, according to what they decide.

All the best,

Steve O
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Re: Optical broadcasting

Postby AncientBrit » Thu Apr 14, 2016 12:18 am

Hi Steve,

I for one would hope you continue to post here.
I find the posts very informative.

Kind regards,

Graham
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Re: Optical broadcasting

Postby DrZarkov » Thu Apr 14, 2016 4:29 am

Hi Steve,

indeed! Please continue to post here. It is very interesting and relevant.
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Re: Optical broadcasting

Postby Steve Anderson » Thu Apr 14, 2016 12:30 pm

I think we all would like to be kept updated on developments here - and the occasional reminder to visit the newly created web-site.

This does have an application for NBTV too. As in modulating the optical source at (say) 1.00MHz and the modulation being an NBTV waveform, just as it would have been done a long time ago. A second carrier would be needed for sound I guess, was that done originally?

Steve A.
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Re: Optical broadcasting

Postby DrZarkov » Thu Apr 14, 2016 3:23 pm

With Steve's system you could transmit NBTV in the original 160 meter band, modulated on light. For example 1700 kHz for the picture and 1800 kHz for sound. The optical receiver would be connected to the two receivers of the Baird televisor. That would be as original as possible.
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Re: Optical broadcasting

Postby Panrock » Thu Apr 14, 2016 9:21 pm

OK chaps - thanks for the feedback. I've now changed the new page so that this NBTV forum thread is recommended as the place to come for anyone who wants to discuss the project with others.

Absolutely correct, Volker (DrZarkov) and Steve A. Trevor (MurphyV310) has already done this with audio and a medium wave radio frequency and reports good results. As mentioned before, given a sufficiently linear amplifier (a tall order?), you could have your own complete long, medium and short radio spectrum on one of these light beams.

I'm very pleased that the infrared LEDs are working so well. These are cheap, fast, and a better spectral match to the photodiode. They also have a built-in lens that is near ideal for the purpose.

Steve O
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Re: Optical broadcasting

Postby Klaas Robers » Fri Apr 15, 2016 5:49 pm

Yes originally two BBC medium wave transmitters were used, one for the video, the other for the sound. I also would recommend to use two separate transmitter diodes, one for the video, the other for the sound. I fear that the nonlinearity of laser diodes is too severe to combine both carriers onto one diode. This will introduce too much distortion.
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Re: Optical broadcasting

Postby Panrock » Fri Apr 15, 2016 9:57 pm

Hi Klaas, we missed you at Convention this year. :)

Not using the laser diode you refer to of course, and I have experienced no problem whatsoever with non-linearity or cross-modulation. The am vision and sound signals are absolutely normal, don't interfere with each other, and seem linear.

However the linearity of the actual carrier (sine) waveforms is anybody's guess. Again though, there are no obvious tunable harmonics present, which would point to problems in this regard. I have simply clicked round the turret tuner looking for these. A Spectrum Analyser would be better of course.

Steve O
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Re: Optical broadcasting

Postby AncientBrit » Tue Apr 19, 2016 5:33 pm

Hi Steve,

Have you thought of publishing an article beyond the Newsletter and the forum?
What you have achieved is truly remarkable and I think deserves wider publicity.
Practical Wireless changed editors around 2 years ago and now embraces a much wider range of articles including digital HF communication and even embedded logic.
Might be worth a punt in approaching them if you do decide to broaden your publicity.

Kind regards,

Graham
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Re: Optical broadcasting

Postby Panrock » Tue Apr 19, 2016 8:33 pm

Hi Graham,

Thank you for your comments, supportive as always. Much appreciated though not really earned. And thanks for the tips.

Well, there's now the page at http://www.radiocraft.co.uk/opto.htm of course, though it's a bit chaotic. Also Andrew Emmerson was on the phone last night. He's going to mention it in his regular column in 'Practical Electronics'.

Other than that I'm basically a shy soul and not into 'publicity' as such. I prefer the actual inventing process.

Cheers,

Steve O
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