Baird's stereo experiments

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Baird's stereo experiments

Postby Panrock » Fri Apr 20, 2007 7:41 am

I'm to bring my 'new improved' colour stereoscopic Nipkow camera and monitor to the convention on Saturday and I can confirm it's all working properly. Though it exhibits some left-right flicker, so viewing is not recommended for epileptics.

This has led me to wonder. Baird demonstrated stereo 3D in the late 'twenties didn't he? But I don't believe this was also colour. However, didn't he demonstrate stereo colour in the early 'forties? Anyone know the details?

Ta

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Baird Colour Stereo Television.

Postby Stephen » Fri Apr 20, 2007 9:07 am

John Logie Baird demonstrated the stereo colour system shown in Figure 1 of his British Patent 552,582 in December 1941. It did not require any special glasses for viewing full colour images, but it did require the viewer to sit at a specific location in front of the display screen for the three dimensional effect. I have attached a copy of the patent for reference.
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British Patent 552,582
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Postby Panrock » Fri Apr 20, 2007 7:34 pm

Many thanks Stephen, This is most interesting.

I always find patents hard to read (maybe sometimes they are intended to be!) but as far as I can tell the following is the case...

Both Baird and I used beam splitters. But whereas Baird seems to have used a mirror array working from each half of the light entering the camera lens, I have used the full lens aperture with semi-silvered mirrors.

Both systems use similar left-right shutter discs. Baird appears to have used a two primary sequential colour system whereas mine uses three primary colours in parallel.

Baird seems to have used a cathode ray tube as a picture display device whereas mine is Nipkow. Of course our priorities are a little different. He was trying to push the technology as far forward as possible at the time, whereas I am trying to push it back!

Baird used a fixed viewing position. My 'visor box' basically imposes the same restriction. However I found it essential to use (angled) optical blocks in the light path between the beam splitter outputs and the eyes, in order to provide the convergence necessary for fusing of the left and right images.

Baird also seems to show his preference for the flying spot method of pick-up here too, which I have never used since normally it requires a special dark studio environment.

I shall await correction from others on my understanding of this patent. Something like this would take me all day to go through and properly digest!

Again, many thanks for this interesting bit of history.

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Baird Colour Stereo Television.

Postby Stephen » Sat Apr 21, 2007 12:14 am

Playful patent agents and attorneys often write patents to be as obscure as possible. Barristers may then later argue in court that the scopes of the patents are much broader that the inventors could have ever conceived.

Actually the system that Baird demonstrated as shown in Figure 1 is a three-colour system. On page 4, column 1, lines 8 through 15, Mr. Baird says: "Arranged to rotate in front of the cathode ray tube 1 is a colour filter 4 in the form of a disc divided into three transparent sections as for example a red section 5, a blue section 6 and a green section 7, arranged to be brought in succession between the tube and the object 3." He does describe a two-colour system in connection with the alternative embodiment shown in Figure 4.

This of course is a field-sequential system. His all-electronic Telechrome system that he was working on at the same time was a line-sequential colour system that eliminated the large field flicker effects of the field sequential colour system. This system had a triple interlace arrangement with 600 to 1800 lines. Mr. Baird utilised an electronic interlace system that he patented in the early 1930s that was not limited to an odd number of lines with a two-field odd/even arrangement like the Marconi-EMI/RCA scheme. His interlace system worked fine for any number of lines with any number of interlaced fields in a frame.

Mr. Baird had already considered the dot-sequential colour system as early as 1930 (British Patent 359,981-see another thread on the forum) but even in the patent for this scheme he points out the need for careful phase adjustment. The line sequential colour system was probably the best choice for the time, reducing large area colour flicker effects whilst being resistant to slight phase shifts. The problem that the line sequential system has, like the field sequential system, is lack of compatibility with monochrome receivers.

I have electronic copies of patents for most of Mr. Baird's fundamental inventions so if there is anything of special interest to you I can e-mail copies of any relevant patents.
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Baird Colour Television.

Postby Stephen » Mon Apr 23, 2007 10:28 am

I have just posted some additional patents by Mr. Baird in the "Patents and Articles" section of this forum. British Patent 562,334 shows the principles of Mr. Bairds line sequential colour system. It is usable for both progressive and interlaced scanning. He suggests an 1800 line triple interaced system, amongst others. British Patent 562,168 shows, in Figure 2, Mr. Baird's tri-colour CRT, the first of its kind by many years.

British Patent 423,101, filed 21 July 1933, shows Mr. Baird's electronic interlacing system that allows any number of interlaced fields with any number of frame lines. The interlacing system is simple and elegant. As shown in Figure 1, it is only necessary to superimpose a sawtooth waveform with a frequency that is a multiple of the picture frequency waveform onto to the picture sawtooth waveform to achieve interlacing. Figure 1 shows an example of triple interlacing with this system. Adjusting the amplitude of the superimposed sawtooth waveform adjusts line interlace interleaving without affecting vertical or horizontal frequency or size.
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Postby Klaas Robers » Mon Apr 23, 2007 11:56 pm

I doubt that a field sequential system could ever give reasonable colour rendering, when you use a normal black and white picture tube and a colour filter wheel. What only a few people know is that black and white picture tubes use two phosphors to simulate white, it is blue and yellow. If you have a black and white TV still operative and a magnifying glass, please look very carefully to the screen of the TV in a dark environment and you will see blue and yellow grains of phosphor on the "white" screen. A "snowing" screen is good enough, analogue transmissions are not needed for the observation.

With only yellow and blue you will never get red and green from the screen. This is the problem with our eyes that don't SEE the difference between a mixture of blue and yellow, a mixture of green and magenta, a mixture of red and cyan or a mixture of red, green and blue . All four can show us the same impression of white.

Of course white picture tubes could be made that synthesise white from red, green and blue, just like the better TL-tubes nowaday do, but as far as I know this has never been done. The need for better colour rendering initiated around 1960 an enormeous research effort to get an efficient and good red giving phosphor. This resulted in the Europium (strange earth metal from the range La-Lu, the so called Lantaniden) doped red phosphor.
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Colour television.

Postby Stephen » Tue Apr 24, 2007 12:29 am

I do not know what phosphors that Mr. Baird used for the CRTs in his field sequential systems, or for that matter what phosphors that Peter Goldmark used with the CBS variant in the US, but as far as I know they used both used commonly available phosphors at the time. In transmissive optics, red plus green produces yellow. It may be that the "yellow" phosphor has a broad spectrum comprising red and green wavelengths. Red plus green would then appear as yellow to the eye. It would only then only be necessary to add some blue phosphor to achieve a white raster. If this is the case, the yellow phosphor would provide both the the red and green for the colour wheels.
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Postby Klaas Robers » Tue Apr 24, 2007 9:44 pm

If you are lucky, yes. But it is quite simple to look through a red glass to a black and white TV screen and do the same with a green one. You should see a bright red and/or green picture.

But I fear it is not working like that. Phosphors give in most cases a rather narrow spectrum. It is just the physical way phosphorisation works that this will be the case. That is also the reason that colour rendering from TL colour 44 tubes (the same phosphor as in B+W TV's is that bad. Look at (reddish) skin tones. It is almost grey.
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