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How 405-line television could be broadcast licence-free

 
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Re: How 405-line television could be broadcast licence-free

Post by Michael Watterson » Fri Dec 19, 2014 3:20 pm

Dannk Doo Trebor

NCT (MOT) retest late tomorrow (20th), hopefully well enough to drive by then.

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Wed Jan 07, 2015 10:48 pm

Hi Trevor,

I too am still beavering away on this... it's proving difficult... teaching myself right now about 'transimpedance amplifiers' (tips welcome). The photodiode I have been using is a Hamamatsu S5973, which is very fast. I shall try to attach its data sheet here, but I am not sure if .pdf's are 'postable'. I am using Channel E2 in colour from my WC-01 as a source.

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Jan 08, 2015 12:24 pm

ppppenguin wrote:A long time since I've played with this sort of thing but nowadays a simple approach might be to connect it directly to the inverting input of a suitably wideband opamp.

This is exactly what I was doing last night. AFAICS though, the op-amp's feedback resistor - and thus the gain - has to be fairly low at these frequencies to cope with the stray capacity.

ppppenguin wrote:They certainly weren't around when I last used photodiodes. Transimpedance is simply a posh way of saying convert current to voltage with gain. The opposite of transconductance or gm.

So does that mean an emitter follower is a tranconductance amplifier? Sorry if I am making a boo-boo but I clearly have a lot to learn here.

Murphyv310 wrote:Hi.
I've reduced the operating voltage of the laser and an modulating the laser with 1v p-p signal, there are chokes and other filtering in the supply. Ill order a photo diode from Farnell.

Even though the capacitance of this photodiode is low, the operating load resistance also seems to need to be low to work successfully at this frequency, with a consequent penalty in voltage output. Thus the 'transimpedance amplifier' - an op-amp with the two input terminals representing a short circuit.

How are you modulating your laser in a linear fashion? If a laser diode is what you are using, these are delicate things and modulating circuits also need good soft-start protection (normally built in to their attached powering boards).

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Jan 08, 2015 12:54 pm

As an aside, if we can develop this technology, requiring a really linear method of modulating lasers or (fast) LEDs at high frequencies, I can't see any reason why the whole 'radio' band shouldn't be available on the light carrier from a given station simultaneously. 1500 metres anyone? Book your wavelength now! :aag

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Jan 08, 2015 1:19 pm

Mustn't keep replying to my own posts... but I've just seen the first inkling of a result!

The photodiode was connected straight across the input of the LM6171 op-amp with no other biasing.

Currently it's very insensitive - the photodiode had to be millimetres away from the LED and the 'picture' is a mere shadow, swamped by intereference from the RF leaking from the boards and cables. But it's a start... I suppose!

Steve

(Edit) I've increased the value of the op-amp feedback resistor and have now got a real picture, albeit at a range of three inches, and crawling with interference and hum. Placing a sheet of paper in the way cuts off the picture, demonstrating this is not due to stray RF pick-up.
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Jan 08, 2015 7:30 pm

An embarrassing update... I believe the results this morning were false! What seems to have happened is thiat the photo-diode amp was picking up rf leakage direct, but only passing it through when the photodiode was irradiated with light. Still checking up on this...

So there's still everything to play for. Let's see how Trevor gets on - and Michael too?
Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Fri Jan 23, 2015 6:34 pm

I have now improved the screening to prevent such 'false positives' in future. The limiting factor now seems to be the speed of the LED. Data on the speed of LEDs in general has been hard to come by, beyond the occasional mention of a rise-time of 20nS on some products. This is not fast enough.

However, I have found a resonant cavity LED intended for use with plastic optical fibre, which is fast enough. Five are now on their way to me from Japan.

http://www.hamamatsu.com/eu/en/product/ ... index.html

This LED is red. I am sticking to visible light rather than infra-red for this first experiment. A visible beam will be easier for setting up the optics and should give greater safety from accidental direct exposure into the eye.

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Refugee » Sat Jan 24, 2015 12:20 am

That is a very posh LED.
It looks expensive too.

 
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Re: How 405-line television could be broadcast licence-free

Post by Michael Watterson » Sat Jan 24, 2015 1:18 pm

Panrock wrote:I have now improved the screening to prevent such 'false positives' in future. The limiting factor now seems to be the speed of the LED. Data on the speed of LEDs in general has been hard to come by, beyond the occasional mention of a rise-time of 20nS on some products. This is not fast enough.

However, I have found a resonant cavity LED intended for use with plastic optical fibre, which is fast enough. Five are now on their way to me from Japan.

Excellent work.

That's why I plan to try miniature low power IR LEDs in a an array, each driven separately. Certainly the emitters used for optical digital links should be fast enough.

Screening and separate power and no earths is certainly needed for tests.

I've finished writing for now and health is a little better. Yesterday I got about 1/2 way through clearing up workshop.

 
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Re: How 405-line television could be broadcast licence-free

Post by Cathovisor » Sat Jan 24, 2015 1:28 pm

Michael Watterson wrote:Certainly the emitters used for optical digital links should be fast enough.

Standard method of moving HD digital video around 'long' (i.e around 100m) distances in television studios, at 1.5Gb/sec.

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Sun Jan 25, 2015 4:12 pm

Murphyv310 wrote:Hi.
No success here at all up at 45 mhz but at 1mhz it makes a cracking pantry TX even with an led from a GU10 lamp! Works from the shed to the house beautifully at night. I used the photodiode a lens and an op amp to feed to a radios aerial socket, a lash up but perfect quality once aligned.

Yes, one of the benefits of this idea is that the whole radio spectrum is up for grabs! Then over again and again - for each 'transmitter' and of course also for point-to-point links.

My tests so far, and yours too it seems, indicate that the average LED tails off in its response beyond a megahertz or two.

As for these special LEDs. I shall certainly make available any surplus to others. The more brainpower and enthusiasm we have behind this project - the better. The LEDs are a special import and I won't have them before early February. I have no prior experience of such 'resonant cavity' LEDs. It seems they are a halfway house between standard LEDs and Laser diodes.

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Till Eulenspiegel » Sun Feb 01, 2015 2:31 am

Transmission of audio by laser between the two cathedrals in Liverpool.

http://hopestreetproject.blogspot.co.uk ... oject.html

Till Eulenspiegel.

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Tue Feb 10, 2015 2:17 pm

Hi Trevor,

Progress at this end remains difficult. To date (according to the scope) I've obtained an adequate - if reduced - response with the emitter on Channel E2 (vision: 48.25MHz) but nothing over a few MHz at the receptor end.

I show the two circuits I am using for the emitter and the receptor. The transistors are shown by their RS part numbers. I have been careful to minimise capacitance in the construction. Electronics design has never been my strong point so corrections or suggestions for improvements are welcome! It would be helpful if others engaged in this project also would post their circuits here.

Trevor, I would also be happy to pop one of those 'fast' LEDs in the post to you - free - if you would like it. The same offer applies to Michael too, if he feels this part would be useful.

Cheers,

Steve
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Tue Feb 10, 2015 3:35 pm

Trevor, it's FOC only, on the understanding you'll be 'open-source' with your circuits like I have been. This is administratively simpler. PM me your address and it will be done. :qq1

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Tue Feb 10, 2015 8:44 pm

I wasn't able to send an attachment with the PM back to Trevor, so here is the pinout for the L10762 LED, bottom view. Pin 1 is positive, pin 2 is negative. Current must be limited to an average of about 20mA.

It's important that it's connected the right way round, since more than 3v in reverse bias could wreck it and I won't be sending out replacements! :aa

This diagram is also on the data sheet but is so tiny it's hard to see.

Steve

PS.
WARNING. THE LIGHT SOURCE ON THIS LED IS PINHOLE-SIZED AND VERY INTENSE. NEVER LOOK AT IT DIRECTLY OR YOU COULD RISK YOUR VISION.
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Feb 12, 2015 1:01 pm

Trevor, you should have received your LED this morning (or whenever the post arrives).

As to your questions.....

Sorry, what is "TOS"?

Yes a VHF germanium transistor could work if you can find one that can be stripped of its covering, but I would think it would be much more noisy than the Hamamatsu photo-fiode.

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Thu Feb 12, 2015 2:45 pm

Thanks Jeffrey.

A brief aside on another 'optical' topic, here's what I managed to lug downstairs into the workshop last night! It's my NBTV mechanical colour monitor (not the mirror screw) seen from the rear. The top black box gives aperture correction (for the Nipkow 'holes') and the chunky heat sink to the right keeps the many luxeon LEDs cool. This device is currently fed by a mechanical colour camera, but I'm going to adapt it to run from Darryl's WC-01 for better quality pictures, and there'll be a dem at the NBTVA Convention in April.

Steve
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Re: How 405-line television could be broadcast licence-free

Post by Terrykc » Mon Mar 02, 2015 4:15 pm

Murphyv310 wrote: After consideration and discussions today I think that this may not work.
Steve your LED is designed for pulsed operation, it is really too non linear for analogue applications.

I think you are really wasting your time with this. Laser diodes are widely used for fibre optic communication and, for digital links, the diodes are reasonably cheap and low powered yet can still be used with long fibres and low cost detector diodes.

Analogue is completely different. A special type of diode is required that can be modulated by an analogue signal. It also requires orders of magnitude more power - and 10dBm at 1310nm from a Fabry-Perot laser is a lot of power! - otherwise the noise generated by the detector diode is excessive.

I believe the cost of CATV laser transmitters are around the £10,000 mark (or used to be) although, in fairness, they can carry the full 40 - 862MHz spectrum.
.
Murphyv310 wrote:Of course there is the aspect of radiating Infra red or laser power at hundreds or thousands of watts if considering a multi directional source, even a beamed system would need many tens or hundreds watts of power for reliable operation over any great distance, my worry is what dangers are involved, great for experimentation but for a day to day form of communication we need to consider the risks ...

Various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm, children and young adults may perceive ultraviolet wavelengths down to about 310 to 313 nm.

The types of diode with which I'm familiar operate at either 1300nm of 1550nm, so well beyond the limits of visibility. With respect, what you appear to be planning is some form of death ray...!

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Wed Mar 04, 2015 8:57 pm

Thanks gents for your comments. A long post now... thank you for bearing with me.

Steve your LED is designed for pulsed operation, it is really too non linear for analogue applications.

I'm open to persuasion Trevor, but please direct me to your source for this information. Looking at the 'fiber end vs. forward current' on the data sheet, we see a gentle curve - almost a straight line except at the end' - pretty linear in fact. Does this characteristic change at high frequencies then? More info please.

I think it may work if its operated on a PWM basis but the device you have (and the one you sent me) would not be suitable even at the lowest Band 1 frequency that 405 lines uses (45Mhz & 41.5Mhz sound).

So what about the -3dB point at 70MHz, as stated (and shown on a graph) on the data sheet? Again, please clarify.

Where it may work is to use the lowest possible usable frequency down converted from band one, say 3.5Mhz and 7Mhz respectfully and then up converted at the receive end.
Sure.

Of course there is the aspect of radiating Infra red or laser power at hundreds or thousands of watts if considering a multi directional source, even a beamed system would need many tens or hundreds watts of power for reliable operation over any great distance, my worry is what dangers are involved, great for experimentation but for a day to day form of communication we need to consider the risks.
Earlier I estimated 20 watts should be sufficient, assuming a transmission and conversion loss of 10dB along the chain. But let's be a bit meaner and estimate only 1/100 of the energy will get through, end-to-end including conversion losses, on a clear day... a 20dB loss. We then start with 200 watts, radiated in a circular flat doughnut pattern.

Now my maths are somewhat pedestrian, as you will see. But consider:

200 watts would be radiated out over a circular area, of radius 10km and 100m thick (high) at its edge.

In reality of course I don't know what the total losses would be. But for the sake of this calculation let us lop off an arbitrary 20dB. So only 1/100 of the energy now gets to the other end to do useful work. This will be taken to account for loss through the atmosphere (under clear conditions) and conversion losses at the transmitter and receiver.

This is equivalent to a total flux of 2 watts spread round the edge of a radiated volume. This volume takes the form of a a flat cylinder 31.4 km in circumference and 0.1 km high. Its full circumferential area (the cylinder wall) is 3.14 sq km or 3.14 x 10 6 sq m.

Having already applied a 20dB loss (1/100 of the original signal), for the purposes of the remaining calculation we shall assume no further loss.

Now, the signal required by the TV tuner working out of the photo sensor is (say) 0.5mV into 75 ohms). This is 3.3 x 10 -9 watt.

So what *proportion* of the the full circumferential area (in total collecting 2 watts) contains 3.3 x 10 -9 watt ?

3.3 x 10 -9 / 2 is the answer. This can also be expressed as 1.65 x 10 -9. We multiply the full circumferential area by this factor to find out what collecting area will be necessary to gather the required signal.

Now the full circumferential area is 3.14 x 10 6 sq m.

So the required area is (1.65 x 10 -9) x (3.14 x 10 6) sq m.

= 5.18 x 10 -3 sq metres.

Let's obtain the radius 'R' of this area. Well, R squared will be 5.18 x 10-3 divided by Pi.

So R squared is 1.65 x 10 -3.

R will be the square root of 1.65 x 10 -3

= 4.06 x 10 -2 metres.

That's 4 cm.

Double this to get the lens 'aperture'. Thats 8 cm. So assuming 200 watts at the transmitter, a 10km/6.2 mile 'fringe' distance, and 99/100 of the energy getting lost by the time it reaches the receiver input, a 3½ inch aperture collecting lens would be required... and of course a smaller lens closer in.

but we must think of the safety aspect if using high power.
With respect, what you appear to be planning is some form of death ray...!
Ha ha! You've rumbled what this is about at last! :aah But more seriously, of course we must be aware of the safety issues. However, as an example, take a 1 Kw single bar electric element radiating infra-red + a little light in the corner of your room. Should this be classified as a 'death ray'? Remember we are not talking here about penetrating millimetre waves or microwaves - just heat (or light), which barely penetrates the skin. And even IF we were to radiate kilowatts of this, remember overhead sunlight already exposes us to 1.37 kilowatts per square metre (less atmospheric absorption). We'd have to standard pretty close to our radiating array to catch even a tenth of that!

I think you are really wasting your time with this. Laser diodes are widely used for fibre optic communication and, for digital links, the diodes are reasonably cheap and low powered yet can still be used with long fibres and low cost detector diodes. Analogue is completely different. A special type of diode is required that can be modulated by an analogue signal. It also requires orders of magnitude more power - and 10dBm at 1310nm from a Fabry-Perot laser is a lot of power! - otherwise the noise generated by the detector diode is excessive.
I believe the cost of CATV laser transmitters are around the £10,000 mark (or used to be) although, in fairness, they can carry the full 40 - 862MHz spectrum.

Waste of time? As a practicall proposition for us, yes absolutely. Such a project would require big bucks funding and some serious research effort, and even then there is the insuperable obstacle of 'the weather'. I think I stated near the start of this thread this was a flight of fancy. But it's interesting to explore none the less.

If suitably analogue modulated laser diodes could be found (and afforded!), these would be ideal; especially if used with a rough-surfaced cylindrical lens, which could then concentrate the radiation into a blurry horizontal 'fan' line aimed at the horizon.

I still remain to be convinced special HF LED's will not work, at least in principle. Nobody's saying it wouldn't be difficult though.

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by valvekits » Sun Mar 08, 2015 10:19 pm

Panrock wrote:
If suitably analogue modulated laser diodes could be found (and afforded!), these would be ideal; especially if used with a rough-surfaced cylindrical lens, which could then concentrate the radiation into a blurry horizontal 'fan' line aimed at the horizon.
Steve


I wondered about the cylindrical lens bit though.
I remember playing with a laser pointer which produced a spot of around 2mm. Twenty feet away the spot had diverged to to about 15mm. On a dark evening at the maximum distance where I could still see it, the spot was the size of a football.

Eddie

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Sun Mar 08, 2015 11:35 pm

Hi Eddie,

A cylindrical lens translates a spot into a line perpendicular to the cylinder - eg. a vertical cylinder will give a horizontal line. In our case, this could concentrate the energy toward the horizon.

Your laser pointer probably needed its focus adjusting! And the system being proposed uses several thousand times more power than the 1 to 3 milliwatts probably coming from your laser pointer.

Steve

P.S. I've just made a concave plano-cylindrical lens as part of an NBTV project. It's function is to convert a square picture from a Nipkow Disc to the 'tall' aspect ratio (3:7) of the original Baird system. The lens is filled with water! It is seen here, along with the 'squeezed' 30-line picture resulting.
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Re: How 405-line television could be broadcast licence-free

Post by valvekits » Mon Mar 09, 2015 2:53 pm

It’s an interesting topic Steve, I was just trying to visualise what your signal might look like after it has passed through the cylinder lens. Most likely diverging, astigmatic and probably picking up some spherical aberration on the way.
I like the simplicity of your water filled lens, maybe we can have a chat if there is a NBTV convention at Loughborough this year.

Eddie

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Mon Mar 09, 2015 3:08 pm

Hi Eddie,

As I understand it, aberrations only are a problem when you have stream of light containing image information. A single line, like that from a laser, simply goes where it will. The cylinder, in a sense, creates a severe aberration but it's one that's wanted.

I have already demonstrated this 'point-into-line' effect for myself by shining a laser module through an acrylic rod.

Yes, I shall be at the Loughborough convention. See you there on April 11th!

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Panrock » Tue Sep 01, 2015 6:18 pm

The first genuine results have been obtained with this at last.... after a lot of hard graft!

More at http://www.taswegian.com/NBTV/forum/vie ... 590#p17590

Steve

 
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Re: How 405-line television could be broadcast licence-free

Post by Doz » Wed Sep 02, 2015 10:34 am

Fantastic !

Dare I ask how much the diodes were?

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