2019 Convention

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

2019 Convention

Postby Panrock » Sun Mar 31, 2019 3:15 am

Three or four years ago, Jeremy allowed me to 'push the boat out' a bit (as regards NBTV) by showing 405-line television, encapsulated in a complete System A signal on VHF Channel B1, passed across the convention room on a modulated light beam.

I haven't had time this year to do any NBTV as such, since all my efforts have been devoted to building the new London 405-line transmitter (currently we are facing the challenge of how to make an efficient vision-sound combiner, working at power output level).

Anyway, this year I needed to rustle up something quick to bring to the convention along that could be made ready in a few days. I've pushed the boat out a bit further. So this time I shall be popping a System B signal in colour on Channel E4 onto a light beam and sending it across the hall. The resulting picture and sound will be viewable on my 1982 JVC 5-inch colour set.

These days, modulating light beams at prodigious rates with digital signals is routine for fibre internet and the like. What is unusual about my efforts is that this is high speed linear modulation of an LED at VHF, preserving all the gradations and characteristics of the analogue AM and FM carrier signals. Maybe someone better informed than me can comment on the stringency of the linearity requirement for digital...

Steve O
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Re: 2019 Convention

Postby Harry Dalek » Sun Mar 31, 2019 8:01 am

Panrock wrote:Three or four years ago, Jeremy allowed me to 'push the boat out' a bit (as regards NBTV) by showing 405-line television, encapsulated in a complete System A signal on VHF Channel B1, passed across the convention room on a modulated light beam.

I haven't had time this year to do any NBTV as such, since all my efforts have been devoted to building the new London 405-line transmitter (currently we are facing the challenge of how to make an efficient vision-sound combiner, working at power output level).

Anyway, this year I needed to rustle up something quick to bring to the convention along that could be made ready in a few days. I've pushed the boat out a bit further. So this time I shall be popping a System B signal in colour on Channel E4 onto a light beam and sending it across the hall. The resulting picture and sound will be viewable on my 1982 JVC 5-inch colour set.

These days, modulating light beams at prodigious rates with digital signals is routine for fibre internet and the like. What is unusual about my efforts is that this is high speed linear modulation of an LED at VHF, preserving all the gradations and characteristics of the analogue AM and FM carrier signals. Maybe someone better informed than me can comment on the stringency of the linearity requirement for digital...

Steve O

You can do the water transmitting if you like as well on the nbtv side might get a few oohs and ahhs as well :wink:
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Re: 2019 Convention

Postby Panrock » Sun Mar 31, 2019 9:30 am

Harry, what about a hose filled with water carrying my optical signal, running across the convention hall, as a "light pipe" ?? :lol:

Steve O
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Re: 2019 Convention

Postby Harry Dalek » Sun Mar 31, 2019 1:07 pm

Panrock wrote:Harry, what about a hose filled with water carrying my optical signal, running across the convention hall, as a "light pipe" ?? :lol:

Steve O


Never know it seems to like or work well on reflections Steve ! water hose DX :wink:
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: 2019 Convention

Postby Steve Anderson » Tue Apr 02, 2019 12:42 am

Steve, with so little time left you are really 'pushing the boat out' After all light, a few THz, is the carrier. Much in the same way I did the optical NBTV Tx/Rx in the newsletter...don't ask me when, perhaps a decade ago!

Most LEDs & photodiodes should cope with a few MHz of bandwidth (baseband), but at RF frequencies the chances of free-field success are limited. Internet hi-speed connections are all done via fibre, sure, but that is a guided system, much like a waveguide. What you shove in at one end you'll get a reasonable proportion arriving at the far end. Free-field, dubious.

You've already done something similar yourself with the line-of-sight system you devised. Could you not adapt that? (I don't recall if you did it at baseband or RF).

Steve A.
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Re: 2019 Convention

Postby Panrock » Tue Apr 02, 2019 6:06 am

Hi Steve A,

Steve Anderson wrote:Most LEDs & photodiodes should cope with a few MHz of bandwidth (baseband), but at RF frequencies the chances of free-field success are limited. Internet hi-speed connections are all done via fibre, sure, but that is a guided system, much like a waveguide. What you shove in at one end you'll get a reasonable proportion arriving at the far end. Free-field, dubious.

You've already done something similar yourself with the line-of-sight system you devised. Could you not adapt that? (I don't recall if you did it at baseband or RF).

That is exactly what I have done. And with RF. Ever lazy and looking for a 'free lunch' I wanted something that should be quick and easy to arrange, with most of it already built. So I simply upgraded my 2016 Ch B1 system (45Mhz etc.) to Ch E4 (63MHz etc.) and hey presto, I am now getting 625-line colour and sound, whereas before it was 405-line black-and-white.

This is free field (directional, with lenses). At VHF. With modulated light.

Now, with the 50dB RF amplifier and resistive combiner, fed from a WC-01, I am only modulating the LED between 20 and 40mA so I guess the optical modulation depth must be around 50% (the RF carriers' modulation depth is something else of course). But it works, and the picture looks linear, with colour and sound. The output is slightly down on the 405 version though, since I am now pushing the special LED beyond its stated frequency spec. As you say, standard LEDs are slower... though I've had results from them into the tens of MHz.

There's another thread discussing this at https://www.vintage-radio.net/forum/sho ... p?t=155334

The picture shows a freezing cold friend helping me with a 'field test' of the previous system. I think you've seen this before. The range is set by the beam-spread away from the parallel using 'club' bullseye lenses. I reckon they are pretty good for the pound or two they must have cost.

Steve O
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