A televisual curiosity

[ppppenguin]

I think I paid £250 for my copy a few years ago. Calling it the Marconi system is wrong. Marconi and EMI formed a joint venture to equip the BBC TV service. EMI devised the 405 line standard, provided the cameras and other studio equipment. Marconi provided the transmitters.

[Panrock]

Back to the mirror screw: here are the drive arrangements - a governed 1/8 hp washing machine (?) motor fired up through a variac - and the lines of LEDs that supply the coloured modulated light. I don't propose to count them now, but I think there are supposed to be 198 in three rows. To the front, is a run of flexible lenticular film. This has the effect of stringing out the LED 'bobbles' into continuous lines. There's no optical and lossy colour mixing required. Karen's Timing Corrector applies adjustable different delays to the colour channels; in effect these are 'convergence' controls.

The vision bandwidth required for 120-lines at 25 Hz 4:3 is 240 KHz. ISTR the WC-01 World Converter source actually does better than this.  The actual limit will be set by the LED drivers.

[ppppenguin]

There's a lot of energy stored in that mirror screw. As Steve said earlier, even a small balance weight getting loose will do real damage. All that energy has to get into the screw. How long does the motor take to spin it up to speed? Conversely how long to slow to a stop?

[Panrock]

Hi Jeffrey, 

I run it up gently over several minutes, starting it off with a manual tug on the toothed belt. I haven't timed it running down to rest, but it probably takes a minute or so.  At operating speed, the (governed) motor seems happiest and calmest at 160 volts on the variac. It's an American motor, so presumably was intended for 60 Hz mains. Running it at 240 volts off the UK 50 Hz mains for a period does make it become a bit too warm.

[valvekits]

It’s an amazing curiosity that is best appreciated by viewing it in the flesh, as I am sure those who have viewed it, would testify. When I went to Panrock’s demonstration at Loughborough a few years ago, I scrutinised this mechanical beast in detail as it was prepared for demonstration.

My first impression was a rugged construction with a set of mirror slats that look like they would have needed to be expertly polished and aligned. However, It clearly weighed a ton and was going to be spun up by a washing machine motor? I think the doubt was setting in even before Steve had fired it up and I suspect I probably didn't have high expectations.
Meanwhile the engineer in me was thoughtfully working out the safest place to stand, but in any event it turned out that “well back” also resulted in the best viewing position. Those of us who witnessed it for the first time were left staring in disbelief as the proof of the pudding delighted us all.

Photographs don’t seem to do it justice somehow.

Eddie

[Panrock]

Nice to see you here Eddie. My memory can play tricks but I think it was only demonstrated in 60-line form at Loughborough. It was later shown at the Plumptre Hall at Eastwood at 120-lines, in a higher state of slat edge polish, but then also needing the balance weight.

Steve

[valvekits]

I remember it was difficult to photograph. I was at Loughborough a couple of times, the second time the NBTV group lost one of it's members if I recall correctly. I only managed Eastwood when you did the TV by light transmission, which I also thought was quite impressive.

Eddie

[Panrock]

Yes, maybe it's hard to photograph because, not only is there no image storage whatsoever, but the travelling 'spot' of light building up the raster is a reflected virtual image and positioned as far behind the screw as the lines-of-light are in front.

There were two demos of an analogue TV channel over a light beam at Eastwood. I lost the beam during the second (Ch. E3/colour) and was on the point of giving up. It was only thanks to help from another member (David Gentle) that the situation was saved. The beam looks the same (steady) whether or not it carries RF modulation and I actually thought the modulator had failed.

Steve

[Amie]

....analogue TV channel over a light beam....

Steve

If its not too much trouble.... How do you modulate the beam?

Very interesting thread Steve,

Amie.

[Panrock]

Hi Amie. Thank you for the kind words.  There's further background on the project here: http://www.radiocraft.co.uk/opto.htm  The original reason for this project was because it looked like it could be a way to get 405-line television back on the air - legally - since unlike radio the light band (apparently) needed no licensing. Things have since moved on in this regard since and licensed analogue television in VHF Band I now looks definitely possible, but that's another story.

The beauty of this optical method was its simplicity. In essence, all you needed to do was place a photodiode in the modulated light beam and out would pop a ready made television channel  (vision and sound) that you could connect straight to the aerial socket of a set! Well, actually a few more components were needed but not many.

Two methods were used to modulate the beam and development notes are attached. The first was to use an RF amplifier module and connect it straight to a visible red LED, in parallel with some DC bias. The second used more LEDs, infra-red this time, and a MOSFET transistor driver circuit.  I started by using a special 'tuned cavity' LED to reach frequencies of 40 MHz and above. Later, I found that more standard LEDs would sometimes work, but at reduced modulation depth.

Steve