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How 405-line television could be broadcast licence-free

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Panrock
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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

 
Posted : 12/02/2015 1:01 pm
Panrock
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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

 
Posted : 12/02/2015 2:45 pm
Terrykc
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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.
.

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...!

When all else fails, read the instructions

 
Posted : 02/03/2015 4:15 pm
Panrock
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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

 
Posted : 04/03/2015 8:57 pm
valvekits
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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

 
Posted : 08/03/2015 10:19 pm
Panrock
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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.

 
Posted : 08/03/2015 11:35 pm
valvekits
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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

 
Posted : 09/03/2015 2:53 pm
Panrock
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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

 
Posted : 09/03/2015 3:08 pm
Panrock
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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

 
Posted : 01/09/2015 7:18 pm
Doz
 Doz
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Fantastic !

Dare I ask how much the diodes were?

 
Posted : 02/09/2015 11:34 am
Panrock
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Hi Andy,

The paperwork with the details has been put away now but it was about sixty quid for the five, since I had to get 'em direct from Hamamatsu and 'minimum charge' etc kicked in...

The opamp used (connected to the photodiode) was the fastest one I could find at RS also available in a DIP package... I'm not upto the tiny surface mount stuff.

First trials with lenses look promising... I should be able to beam this across the 'laboratory' at good signal strength. More anon. :)

Steve

Edit.. I'm getting confused. It was the LEDs that were sixty quid (but not each!) and the photodiodes came from Farnell : stock no. 1495575... again fairly pricey.

 
Posted : 03/09/2015 11:43 am
Panrock
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A further update. I am now getting transmission on a light beam across the workshop with very strong signal strength. Pictures of the units and more at http://www.taswegian.com/NBTV/forum/vie ... 593#p17593

Steve

 
Posted : 04/09/2015 7:57 pm
Panrock
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For anyone here interested, I shall be demonstrating my system at the NBTV Convention on April 9th. This is held at the School of Electronic, Electrical & Systems Engineering at Loughborough University. I'll be aiming to transmit the signal across the hall. The receiver will be a Philips 17TG100-U tuned to Channel 1.

I am pleased with the results so far. I am now looking at practical ways this could be taken further toward the goal of true 'broadcasting'.

Steve

 
Posted : 23/03/2016 9:08 pm
Katie Bush
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Hi Steve,

It's good to hear that your project is still moving ahead.. Sadly, I can't be there to see your demonstration, but I wish you well on that.

Do you suppose there could ever be an omnidirectional version? Or is that something that would never be possible? I'm just thinking how nice, and realistic, it would be to point your "aerial" at a transmitter, from anywhere in the surrounding area and be able to find a signal.

Marion

 
Posted : 24/03/2016 12:23 am
Panrock
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Do you suppose there could ever be an omnidirectional version? Or is that something that would never be possible? I'm just thinking how nice, and realistic, it would be to point your "aerial" at a transmitter, from anywhere in the surrounding area and be able to find a signal.

Yes Marion, that's the 'Holy Grail' and I'd like to see this too. Calculations based on the actual performance achieved so far (rather than my earlier theoretical ruminations) and making certain other assumptions mean we are looking at an input power of over a kilowatt to achieve this... always assuming the capacitance can be kept low enough to support the modulation frequency.

I have been considering LED-based tower-top 'obstruction' lamps for the hardware this sort of system could be built around. The original LEDs might not be used, but the housing and built-in reflector would provide a near-ideal vertical polar diagram (v.r.p) alongside the 'omni' h.r.p.`

At the receiving end I have been looking at Fresnel Lenses. The largest I have obtained is 14 inches across. This gives enormous light gain and also appears to offer sufficiently good optical quality. 'Windage' could be a problem here though. Another way forward might be to adapt porous satellite TV dish hardware, but chrome plate the dish! This would depend on how good the figure of these dIshes actually is. It might be also necessary to 'flat' the mesh. Fortunately we don't need 'telescope' quality in this application.

The first higher power experiments are planned for after Loughborough. I'll keep on the case!

Steve

 
Posted : 24/03/2016 2:27 pm
Katie Bush
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Hi Steve,

That raises an interesting thought - How far into the infra-red do you have to go, for the system to work? Or alternatively, would the same system work in optical, or near infra-red? I'm thinking in terms of a "What if" scenario in which my thoughts lead me to wonder if existing optical red obstruction lamps could be modulated, and still perform their intended warning function.. In my area alone, there are dozens of power station, hill top pylons, telephone relay masts, etc. etc. All with red lamps - large and bright ones.. Now if they could be modulated, we could be swimming in usable signals!

Obviously, issues like fog, heat haze, rain and low cloud would be a problem, but..........?!

Marion

 
Posted : 24/03/2016 9:53 pm
Panrock
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Hi Marion,

That raises an interesting thought - How far into the infra-red do you have to go, for the system to work? Or alternatively, would the same system work in optical, or near infra-red?

The advantage of using the optical or near-infrared bands are that the emitters and sensors are relatively simple and low-tech. Things get much more difficult as you go to the far infra-red and millimeter wave spectrum.

For interest I also attach a chart of infrared transmission in mist. A wavelength of 2 microns seems about optimal. The situation gets worse in heavy fog, when infrared gives little advantage over light.

I'm thinking in terms of a "What if" scenario in which my thoughts lead me to wonder if existing optical red obstruction lamps could be modulated, and still perform their intended warning function.. In my area alone, there are dozens of power station, hill top pylons, telephone relay masts, etc. etc. All with red lamps - large and bright ones.. Now if they could be modulated, we could be swimming in usable signals!

Since the light is not directly modulated with video and audio, but instead merely carries varying amounts of the (amplitude-modulated) 41.5+45.0 MHz 'subcarriers', it doesn't flicker but looks as steady as a DC light, regardless of picture content. Trust me, I've tried it! So yes, in theory the warning and signal functions could be combined. Or a hidden signal-bearing infrared component added to the visible warning light - which would be something to aim at during set up!

As I have stated before, there's no reason to restrict the 'subcarrier' to a 41.5/45.0 MHz analogue television service either. You could have a mass of digital signals, or the entire analogue radio spectrum, coming from each mast! However, for weather reasons, this would be a 'high days and holidays' service only, so would be unlikely to ever take off. This is one reason I see it more suited for a vintage television service, with highly motivated - and tolerant! - viewers.

Some big challenges to overcome before we get to that stage though.

Steve

 
Posted : 25/03/2016 1:43 pm
Cathovisor
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This is not going to make me popular here, but... why?

Even assuming you find you can broadcast with it, what would you actually broadcast, and more importantly, how will you pay for suitable material to broadcast?

 
Posted : 25/03/2016 4:07 pm
Panrock
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That's a fair question to ask Catho, bearing in mind the discussion here has been purely technical. I have an answer but there are 'sensitive' aspects that make it unsuitable for the public internet. The reasons should become clear in the private message I'll send you.

Steve

 
Posted : 25/03/2016 4:43 pm
Panrock
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Many thanks to the mods for tolerating this rather off-topic thread for so long!

This can now be closed if you like, since I have created a new web page where all the news on this will be brought together in future. It's HERE.

Cheers,

Steve

 
Posted : 13/04/2016 1:55 pm
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