Baird 240-line system

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Baird 240-line system

Postby OmegaProductions » Sun Oct 29, 2017 4:21 am

Hello guys in NBTV! :D

Today, here is a question. You know John Logie Baird invented mechanical television which has 30 lines. In the 1930s, he did an improved version of his system, but upgraded to 240 lines and was used in BBC's experimental broadcasts, before Marconi-EMI had won the competition.

Well...I know you guys make a 30 and 32-line TV system, but what about 240-line TV system? It might be interesting to build a 240-line TV and watch back the pictures. It might be interesting and cool. :)

Anyways...that's all I have to say. Goodbye guys!
:D
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Scophony with 405 lines

Postby Robonz » Sun Oct 29, 2017 4:51 pm

Possibly the most advanced mechanical television would be the 1938 Scophony with 405 lines. You should build one. I am not sure anyone has built a modern day replica of a Scophony. Here is a picture to get you started. 60 lines for my next TV is going to be hard enough I guess. These engineers back in 1938 were pretty clever and they didn't have CNC machines to do the precision work. Amazing when you consider it.

1938-Scophony-10.JPG


1938-Scophony-9.JPG
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Re: Baird 240-line system

Postby OmegaProductions » Sun Oct 29, 2017 9:38 pm

Hummmmm...interesting! :?

This reminds me of mirror-drum television. This is cool that there is an mechanical version of a 405-line TV. It seems to be an exciting project. It's like Marconi-EMI's system, but a mechanical one. It does look hard for me to build, but I will build one of those in the future. Thanks for the post! :D
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Re: Baird 240-line system

Postby OmegaProductions » Mon Oct 30, 2017 12:16 am

Well...here could be my plan to build one of those TV sets. Let's just use a mirror drum for example.

The mirror drum camera in this diagram, in this picture...the image goes through the hole to the mirror drum. Then reflects to the other mirror and straight to the Nipkow disk, let's say the disk has 240 or 405 lines! Then to the photoelectric cell and then watch it back to the mirror drum receiver.
Schematic drawing of mirror drum TV camera.gif
Schematic drawing of mirror drum TV camera.gif (25.47 KiB) Viewed 5558 times

Neon_Crater_Lamp-Mirror_Drum_Sys.JPG
Neon_Crater_Lamp-Mirror_Drum_Sys.JPG (87.51 KiB) Viewed 5558 times

bairdmd8.gif
bairdmd8.gif (8.11 KiB) Viewed 5558 times


Anyways...that's all! Thanks for reading and goodbye! :D :)
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Re: Baird 240-line system

Postby Panrock » Mon Oct 30, 2017 3:39 am

Welcome OmegaProductions! I also started in this hobby when I was about your age... That was 53 years ago! Making your own mechanical television system is a fascinating thing to do. There's no harm in being ambitious, but the simpler you make things the more likely you'll succeed (really!) and you'll find plenty of know-how on this discussion board. There will be setbacks and disappointments along the way but that is the way you will learn.

As far as I am aware, the only surviving relic of a Scophony receiver is a High Speed Scanner, hidden away at the National Media Museum in Bradford.

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Re: Baird 240-line system

Postby OmegaProductions » Mon Oct 30, 2017 4:52 am

I did do a prototype of my mechanical television, but with Hot Wires at school. I know it's not about 240-line system, but I just want to tell you. Here's a video of it in action. :)

Here's the video at this link: https://www.youtube.com/watch?v=2vBgTcaBPX4
IMG_1506.PNG
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Re: Baird 240-line system

Postby ppppenguin » Mon Oct 30, 2017 8:18 am

Last November there was a programme on BBC4 to celebrate the 80th anniversary of BBC TV. One of the teams involved tried to re-create a Baird 240 line spotlight studio. They reckoned it wasn't feasible to do more than 120 lines with the time and resources available. The pictures were really quite good, though they were using some very modern photomultipliers and electronics. This lot fed a data capture card, then to a PC. I was consulted about the best way to get pictures into a modern TV system. I was unsure at first, as I didn't see the whole rig until the shooting at Hornsey Town Hall. It was soon obvious that the answer was a VGA to PAL converter.

My understanding is that to do 240 lines with a Nipkow disc it's almost essential to run the disc in a vacuum. Otherwise friction and windage are just excessive.

There is tantalisingly little Scophony kit that still exists. John Trenouth once showed me the Scophony motor they have at Bradford. I think it was just a motor, not a complete scanner.
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Re: Baird 240-line system

Postby Panrock » Mon Oct 30, 2017 7:50 pm

ppppenguin wrote:My understanding is that to do 240 lines with a Nipkow disc it's almost essential to run the disc in a vacuum. Otherwise friction and windage are just excessive.

Below I append the report I wrote discussing the problems (less the illustrations). BTW Paul Marshall raised a query about my mention of photomultipliers, but information later received seemed to confirm they were actually used.

The NBTVA's very own Vic Brown has since succeeded at 240-lines using four independent video feeds and a shuttering arrangement with a Nipkow. It's all in the latest NBTVA Newsletter. :)

ppppenguin wrote:There is tantalisingly little Scophony kit that still exists. John Trenouth once showed me the Scophony motor they have at Bradford. I think it was just a motor, not a complete scanner.

He showed this to me too, and on reflection I think you're absolutely right.

Steve O

-----------------------------------------------------------

Hi Terry,

A note as promised, with some information (as far as I can glean it) about the 240-line system and the Baird Spotlight Studio. I hope it may help a little when your team goes to Cambridge.


Overall physical structure and size

• The floor space required is shown on Fig 1 attached. This is based on Simon Vaughan's original diagram, with scaling provided by Martin Allen.

• The barrier between the scanner and booth looks like it may be of steel. Probably advisable!

• The ceiling panel (glimpsed on another photo) looks to be of board

See also Fig 2 and Fig 3, based on Simon's archive photos, with further annotations by me. These are my best guesses only. They could well be wrong.


Motor

• High speed, high power motor, governed at 6000 rpm.


Lamp

• Carbon arc now obsolete, produces high level of UV. A Xenon Arc may have to replace it. See Fig 4 (ex Wikipedia).

• The high pressure filling makes Xenon Arcs safety-critical to handle. Such lamps require specialised startup and high power current-source power supplies. Some form of forced cooling might be required.


Photocells

• Huge surface area photomultiplier tubes appear to have been used. Photomultipliers were very new at the time. These would be blue sensitive. They will require high voltage, low current power supplies, potential divider chains and effective ambient light cut-out protection. If this fails, they can be ruined.


Electronics

Valve-based signal amplification and sync processing would have been used.


The 240-line standard

• Please see Fig 5. The pertinent facts appear to be as follows:


240-lines.
No. of active lines: 220.
Aspect ratio: 4:3.
Frame frequency: 25 Hz.
Line frequency: 6000 Hz.

Frame period: 40.00 mS
Line period: 166.67 uS
Frame sync pulse length: 2.00 mS (12 lines)
Frame back porch length: 1.33 mS (8 lines)
Line sync pulse length: 13.33 uS (8% of line period)
Line back porch length: 3.33 uS (2% of line period)


The Nipkow disc and spiral shutter

• I've worked out some provisional specs for a disc. There's no need to worry about these right now, but they could come in handy later. They will certainly need double checking before the design is worked up in a CAD package. Basically, this is for a 4 x 60-hole spirals disc with an inner ring of holes for line syncs and a separate hole for frame syncs. Light through the holes would presumably trigger monostables with suitable time constants.

• The picture would soon become blurry and jagged if the positional tolerances of the Nipkow holes weren't perfect. Karen Orton has developed a software correction method.

• Because of the high rim speed, operation in a vacuum would be mandatory. Balance would be critical. Once drilled with Nipkow spirals, the intrinsic balance would be compromised. A separate balancing arrangement would likely to be required.

• The disc would need to be as thin as possible to avoid vignetting on the picture. Such a thin disc would be fragile and could shatter in use.

• In this conception, a spiral-shaped broad slot on another disc in front selects the appropriate 60-line Nipkow spiral. This front disc would rotate at 1500 rpm and be 1:4 geared to the 'main' Nipkow disc. The gear tooth profile would need to ensure silent operation.


Overall disc diameter 915.00 mm (3 ft).
Outer guard band: 7.50 mm.

Number of Nipkow hole spirals: 4 (concentric).
Number of holes per Nipkow spiral: 60.
Sync hole rings: 2 (60 holes for line and single hole for frame - placed to inside of disc).
Max radius of outer Nipkow hole spiral: 450.00 mm.
Min radius of inner Nipkow hole spiral: 434.29 mm.
Mean radius of all four Nipkow hole spirals: 442.15 mm.

Radius of line sync hole ring: 425.00 mm.
Radius of frame sync hole: 410.00 mm

Line segment outermost width (including sync & back porch): 47.12 mm.
Line segment innermost width (including sync & back porch): 45.95 mm.
Line segment outermost width (less sync & back porch): 42.41 mm.
Line segment innermost width (less sync & back porch): 41.35 mm.
Line segment mean width (including sync & back porch): 46.54 mm.
Line segment mean width (less sync & back porch): 41.88 mm.
Total (240-line) raster height: 31.41 mm

Thus the picture, off-disc, would measure approximately 42 x 31 mm.

Keystoning: 7%.

Aspect ratio of each quarter section (not including sync & back porch): 16:3

Nipkow/sync hole diameter 0.13 mm.

Rim speed at 6000 rpm: 287.46 m/s = 1035 Km/h = 643 mph.

A g-force check will be required to establish whether the stresses in the disc are manageable.


Shrouding/structure/pump

• There will be some heavy engineering involved with this project. Efficient vacuum generation and sealing will be needed, and the manufacture of a massively strong bell housing around the disc.


Optics

• A projection lens would be needed, focussed on the disc surface

• Projection magnification would need to be about x14 to give a 600mm wide scanned area.. The same factor would determine the relationship of distances in front of and behind the lens. The lens would need to be of sufficient aperture to gather the entire light cone diverging out of the Nipkow holes at its operating distance. Generally, the operating distances should be the maximum possible, to keep the diverging light cone narrow. This would also allow a thicker disc.


Ventilation

Safety

Insurance

Time needed!

Cost


• The above are factors I cannot comment on.

• Finally, fig 6. shows a lady, believed to be Madeleine Carroll, on a Baird Test Card of the time.


Good luck! :-)

Steve



Last edited by Panrock on Tue Oct 31, 2017 8:22 am, edited 1 time in total.
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Re: Baird 240-line system

Postby OmegaProductions » Tue Oct 31, 2017 6:33 am

Hhhummm...interesting! :?

I would indeed use the Baird testcard (but I'll do that in the future.) :)

In the future...I'm just doing the nipkow disk/mirror drum camera with 240-lines and can be shown back in the average mirror drum television! That's all folks! :D :)
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Re: Baird 240-line system

Postby OmegaProductions » Tue Oct 31, 2017 6:47 am

Hey guys in NBTV! :D

I have drawn this prototype on Baird's 240-line mirror drum television system, and here it is! Hope you enjoy!!!! :D :) :wink:
Baird's 240-line mirror drum TV drawing! (29th-10-2017).png
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Re: Baird 240-line system

Postby ppppenguin » Thu Nov 02, 2017 5:19 am

Steve, I can confirm that photo multipliers were used in the BBC80 production. I saw them myself and chatted with the engineer who was in charge of them.

A 240 line mirror drum would be rather scary. The G forces are very much against you. I can't cite a reference for this but I have a feeling that the optical efficiency of mirror drums drops even more alarmingly than Nipkow discs as you push up the number of lines.
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Re: Baird 240-line system

Postby OmegaProductions » Thu Nov 02, 2017 7:52 am

Hi there! Here is the diagram and construction plans for Baird's 240-line system TV set and TV camera. :)
https://www.youtube.com/watch?v=8hHNB8x ... e=youtu.be
Baird's 240-line camera on PowerPoint!.png
Baird's 240-line camera on PowerPoint!.png (315.42 KiB) Viewed 5465 times


However ppppenguin, I do get your point about why it is scary trying out a mirror drum with 240-lines, using a G force on it. :?
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Re: Baird 240-line system

Postby Robonz » Fri Nov 03, 2017 9:55 am

Hi Omega, great job on the videos and artwork. really good stuff. The Baird 240 is impractical at best in my opinion. Do you think it might be best to concentrate your efforts on a "good picture" using 32 lines?

I did some rough calculations on what it would cost to make a couple of 240 line prototypes in a "business model" and I would guess around the 2 million dollar mark (in today's money). I didn't realize how much funding Baird must have had in the day.

Cheers
Keith
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Re: Baird 240-line system

Postby OmegaProductions » Sat Nov 04, 2017 4:39 am

Yes. I was just saying what the construction of the 240-line TV set/cameras would be in the future.

But however...I'm still experimenting with my 20-line mechanical TV made with Hot Wires. Andrew Davie told me that MP3 files make up terrible quality picture and said to use uncompressed WAV format for playback. I could use that in the future, but it's disassembled for now. If you want to see my 2nd test video, then here's the link: https://www.youtube.com/watch?v=BInAMWblzhA


youtu.be/BInAMWblzhA


Thanks for reading and goodbye! :D :) :wink:
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Re: Baird 240-line system

Postby Klaas Robers » Mon Nov 06, 2017 9:14 pm

I think that you should have used a piece of frosted paper or frosted glass in between the LED and the disc. Now we see only the iluminated LED and no light around it, where the picture should appear.

And I agree with the remark that it is worthfull to concentrate on a 32 line monitor and get a viewable picture. The big advantage is that there is quite some video signals for 32 lines available, and then at least the video signal is Ok, so you don't need to make that yourself for the first experiments.

One thing at the time. But GO ON! Make small steps and look what you have done. Proceed when it is good, step back when it is wrong or not good enough. Things never get better automatically, most of the time they get worse.

Success!!
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