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
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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