Baird scanning disc with sixteen holes

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Baird scanning disc with sixteen holes

Postby Stevie V » Tue Nov 15, 2016 8:33 am

Hi. I'm new to the forum. I am very curious as to how Baird's 30-line system worked. I've read several articles about it which are often accompanied by photos of him with a scanner that has what looks like a cardboard disc with sixteen holes with lenses. The holes are arranged in two identical spirals so that each spiral has eight holes and takes up half the disc. Also, any two holes that are opposite each other appear to be the same distance in from the edge of the disc so they would be scanning the same area. If this was indeed the scanner for the 30-line system, I can't see how he was able to get a thirty-line picture when there were only eight hole positions.
Can anybody explain to me how it worked or suggest any books or websites that explain it properly? There are also two other discs as part of the system but I've never been able to find out what they are for. I've seen a lot of discussion about 32-line pictures so does this mean that Baird's system somehow got four lines from each hole and that his system really was a 32-line, not a 30-line system?
Many thanks.
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Re: Baird scanning disc with sixteen holes

Postby Panrock » Wed Nov 16, 2016 12:45 am

I've been looking at my copy of "John Logie Baird: A Life" by Antony Kamm and Malcolm Baird. My understanding of how this early apparatus actually worked (and how well) is only rough but it seems to have been something like this:

The usual illustration shows the "camera" for the system. Rotating lenses rather than holes in the front disc were used to gather more light. A slotted disc, running at an indeterminate speed behind, then chopped up the signal so it would emphasise the high frequencies more - in a desperate attempt to compensate for the sluggish response of the selenium pick-up cell at the far end. Finally, a disc containing spiral slots further back and rotating at a suitable sub-multiple of the speed of the front lens disc, acted as a line multiplier, selecting out different 'bands' of the picture, and so multiplying each spiral of 8 lenses by 4: creating a 32-line raster.

Others here may have a better understanding.

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Re: Baird scanning disc with sixteen holes

Postby Dave Moll » Wed Nov 16, 2016 6:33 pm

I find it interesting, however, that this would have given 32 lines rather the Baird's more usual 30 lines.
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Re: Baird scanning disc with sixteen holes

Postby Klaas Robers » Sat Nov 19, 2016 7:51 am

I always had the idea that the slotted disc ran at a high speed (many slots per second) because in that era DC amplifiers were a "no go". So this disc created an amplitude modulated light on an optical carrier of say 30 kHz, which could be amplified using R-C coupled radio tubes, "detected" and aplied to the neon lamp.

I never understood the function of the disc with the spiraled slots. JLB should meet me and explain them to me. But may be that if we were going to copy his set-up we would find that out automatically......
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Re: Baird scanning disc with sixteen holes

Postby gary » Sat Nov 19, 2016 10:22 am

Klaas Robers wrote:
I never understood the function of the disc with the spiraled slots. JLB should meet me and explain them to me. But may be that if we were going to copy his set-up we would find that out automatically......


According to Doug Pitt (see article "Variable Scanning System by Nick Hammond" NBTVA Newsletter) the spirals were part of a variable line standard system using the intersection method.

In regards to the "double-8" lenses Don McLean has this to say in his book "Restoring Baird's Image":

"This had a distinctive 'double-8' Nipkow disc - two 8-aperture spirals on one turn of the disc. The double spiral, though unusual, served three purposes. First, the double spiral halved the rotational speed needed to create a picture. This meant that the centrifugal force on the lenses in the disc dropped to a quarter of that of a single spiral. Secondly, a double spiral disc is naturally balanced - unlike a single spiral. Thirdly, the Nipkow disc could have operated as a 'closed-loop' TV system on one Nipkow disc - imaging on the downward scanning side, viewing on the upward-scanning side. Another point, though cosmetic, is that a single spiral 8-aperture Nipkow disc would give a massively arc-scanned picture with a sweep of 45 degrees. The double-spiral approach would halve that to a more reasonable 22.5 degrees."

Klaas, if you do meet up with JLB could you give him my regards please? Also remind him that I have a spare room reserved for him here in the lovely Southern Highlands of NSW, where the weather would suit his constitution, and where, one day in every year, my village turns into the magical "Brigadoon".
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Re: Baird scanning disc with sixteen holes

Postby Lawnboy » Sat Nov 19, 2016 10:49 am

I've actually been researching this system recently. The "double-8" transmitter is not likely the one where Baird achieved "true" television, but instead could only transmit two tone images in reflected light, just a step beyond shadowgraphs. The very first 30 line transmitter is still a bit of a mystery, however Don McLean analyzed the first off-screen photo of television as well as other photos taken from the same period, and has shown that the camera disc had a single-spiral of 30 lines.
Interestingly the spiral slotted disc on the double-8 (not the serrated disc) doesn't seem to be present in the earliest photos of the machine, leading me to believe that it was added on later. This meant that the device could only produce an 8 line image. Baird published an article about it in The Wireless World and Radio Review issue of January 21, 1925. In it he states that "The letter 'H,' for example, can be clearly transmitted, but the hand, moved in front of the transmitter, is reproduced only as a blurred outline." That seems to indicate a low line count.
The large white disc with the tiny square holes is the display disc, mechanically linked to the lens disc, acting as a viewfinder. It is sandwiched between a pair of smaller slotted discs, one on each side, apparently only to support the display disc.
Check out Don McLean's paper "The Achievement of Television: The Quality and Features of John Logie Baird's System in 1926"
http://www.tandfonline.com/doi/pdf/10.1 ... 0000000048
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Re: Baird scanning disc with sixteen holes

Postby Klaas Robers » Sat Nov 19, 2016 11:01 pm

Lawnboy, this is an interesting document. As Don McLean writes: "Unhappily John LB was not an archiver", although he gave his "Double 8" system to the Science Museum, after he discovered that this was not the way to go. But it is remarkable that he experimented with 32 lines, in an 1:4 interlaced way. I never believed in interlace, this only works if you are staring at a certain point of the screen, or if you are taking a photograph of the screen.

This document also learned me that, if you believe that the radio infrastructure will not change to much higer bandwidths, just to facilitate television, and you have to fit into the narrow bandwidth of the existing MW radiotransmitters, that 30 lines, or there about, is the only choise for an emerging television system. We see this too with clubmembers that are experimenting with more lines: there is no simple common way to store your video signal.
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