Mechanical Television primer

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Offline albert

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Mechanical Television primer
Re: Mechanical Television primer
« Reply #26 on: Wed November 29, 2017, 04:33:17 AM »
Thank you, I'll try this!

Today I would like to look at early methods of synchronizing the Nipkow disc to the video signal. This was a difficult problem-perhaps THE problem-of early television.
Bairds first experiments simply had the transmitter and receiver discs running on the same shaft....until he had worked out the problems of capturing and amplifying the video signals.
But as soon as transmitter and receiver were in separate rooms, there had to be another way...
And when the first TV broadcasts started, the Nipkow receiver had to comprise a means of synchronization.
The easiest way would have been to use the 50 or 60 hz line frequency for this. But in the beginning of the "electric" era, the line frequency could differ during the day or even in different parts of the city or country. So that did not work in the early days.

The solution most often used was the "phonic wheel". Invented by LaCour in 1875, the phonic wheel was the first synchronous motor. Baird used a variation of this on his "televisor".
In its simplest form, the phonic wheel had a toothed gear on the motor shaft with the same number of teeth as there were lines in the video image. Two electromagnetic coils were set 180 degrees off this wheel with their pointed iron cores as close as possible to the teeth of the wheel.
The video signal was amplified, inverted (?) and run directly into the phonic wheel assembly on the motor shaft driving the disc. For this to work, it was necessary to regulate the "wild" motor speed as closely as possible to the desired speed of 750 rpm.
The video signal à la Baird contains a sync pulse indicating a new line and another one indicating a new frame every 30 lines.
This was simply a black "bar" around the image. 
When the motor ran too fast the impulses in the coil would "brake" the gear a little , pulling on the back side of the teeth.
When the motor was too slow the coils would give a small acceleration to the gear teeth, pulling on the advancing side of the wheel's teeth.
This worked in theory and the image would stop rolling vertically. ....but only if there was no black signal between different scenes and no scenes that were too dark. If this happened the sync would be lost.
The primitive system would take care of the general speed of the disc but not of the "line sync" or phase . If this was wrong you would see a picture split in half vertically.
To correct for this phase error, the entire assembly of the coils on the motor could be rotated  slightly with a knob on a gear.

Here are the original patent drawings and a page from an original Baird manual showing the motor and synchronizer. Note that the gear wheel on the motor shaft was made from many thin metal leaves like a modern transformer core.

"Watching TV" at the time was work and no job for "couch potatoes! "




« Last Edit: Wed November 29, 2017, 04:59:23 AM by albert »

Offline donaldk

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Mechanical Television primer
Re: Mechanical Television primer
« Reply #27 on: Wed November 29, 2017, 06:59:37 AM »
Issue with attaching and embedding images is that we get to see them twice.

Here are the original patent drawings and a page from an original Baird manual showing the motor and synchronizer. Note that the gear wheel on the motor shaft was made from many thin metal leaves like a modern transformer core.

Mechanical Television primer

"Watching TV" at the time was work and no job for "couch potatoes! "

Offline albert

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Re: Mechanical Television primer
« Reply #28 on: Sun December 03, 2017, 08:28:30 AM »
Since the Nipkow disc had many disadvantages, people looked for alternatives early on. To use the mechanical process for projection of the image onto a screen, the mirror drum invented by Lazare Weiller was used and improved for tv purposes by German professor Karolus in the 1930s.

This was a heavy piece of engineering and it required lengthy adjustment since the mirrors had to be perfectly aligned. Due to the weight mirror drums were difficult to synchronize.
Another idea was the mirror screw, the images could be viewed directly and they were a lot brighter than anything on the Nipkow disc. But again, a great deal of precision was necessary.


Offline albert

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Re: Mechanical Television primer
« Reply #29 on: Sun December 03, 2017, 08:35:50 AM »
A very nice pocket sized televisor was made in France in the 1930s, the Brami Visiola.  It used a 30 Line  mirror screw. To view either German/French TV or UK/Baird type tv, the whole device could be rotated to run either in a vertical or horizontal position.
As all mirror screw devices, this one had to be lit by a very long, small neon lamp that was longer than the screw itself.
This  was the "smartphone" size display of its day.


Offline donaldk

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Re: Mechanical Television primer
« Reply #30 on: Mon December 04, 2017, 07:00:18 AM »
Albert, a mirror wheel like that was also used in a French public Television projector, using a lightvalve as lightsource. Must search for the name, and source as i saw the reference years ago. Machine was from 1936 it said.

Edit: Scophony https://en.wikipedia.org/wiki/Scophony, UK/Germany http://www.earlytelevision.org/scophony_projection_tv.html http://www.earlytelevision.org/pdf/pop_sci_7-49.pdf

Small projector:
Mechanical Television primer

Mechanical Television primerMechanical Television primer

The Schlierenstop made a return in the GLV laser projectors, most notably the ESLP.

Mechanical Television primer
GE mechanical projection demo 1930, above, others doing the same that year: http://www.sportsvideo.org/blogs/?blog=schubin-cafe&news=getting-the-big-picture, others: http://www.earlytelevision.org/mechanical_theater_tv.html
« Last Edit: Mon December 04, 2017, 10:13:46 AM by donaldk »

Offline albert

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Re: Mechanical Television primer
« Reply #31 on: Thu December 07, 2017, 08:55:30 AM »
Donaldk, you're right- the first public demonstration of TV  in France made use of a mirror drum device and projected images. René Barthélémy was the inventor, not  a lone wolf like John Baird, but an accomplished mathematician/physicist - he was employed by "La Compagnie des Compteurs" in Montrouge, that was the company that did all the gas and electricity counters in France at the time. They had an incredibly skilled staff of mechanical engineers and so their equipment was very well made- a far cry from Baird's plywood and chewing gum approach. Everything was solidly engineered. Soon after these  humble beginnings, this staff switched to electronic cathode ray systems for recording and displaying tv images.
The scophony system you have mentioned was also using a mechanical scanner but with an interesting light valve that was years ahead of anything else. It used a liquid filled cell into which an ultrasonic crystal introduced vibrations at very high frequency, the Jeffree cell. I could not find much more detailed information on that.
The first French TV studio was not far from the Eiffel Tower, and it used a 60 line Nipkow disc camera system. Due to the tremendous light loss with the discs, the intensity of the light in the studio had to be above 20.000 lux. The air would be heated to 50  Celsius in no time. So they had to put in a tremendous amount of air conditioning. The outlets of this system made the studio floor look like the deck of an ocean liner.
The Eiffel Tower, by the way, owes its continued existence to the fact that it was an ideal place to put the first powerful radio and then TV transmitters. It was originally scheduled to be taken down after the 1900 world expo for which it was erected. Many people at the time were less than enamored with the Eiffel Tower and thought it was a disgrace to the beauty of Paris! That has changed a bit!

The images below come from these pages- a history of early television in France- in French.

http://819lignes.free.fr/Histoire_de_la_television_francaise.html

There is a great conference held at the cinemathèque in Paris in the summer of 2017- it features Don Mc Lean, the engineer who decoded the few remaining discs of Baird's phono vision- its slow because Don speaks English and is then translated to French. The second half of the conference is on early French TV by a French expert - Bernard Tichit- in French. For patient people only- 2 hours 33 minutes....





« Last Edit: Thu December 07, 2017, 09:06:49 AM by albert »

Offline albert

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Re: Mechanical Television primer
« Reply #32 on: Fri December 08, 2017, 11:34:40 AM »
Donaldk, could you please explain what you mean by Schlieren stop. I know about Schlieren optics and I believe this was used in the Scophony cell- as well as in the later Eidophor big screen projector.

Offline donaldk

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Re: Mechanical Television primer
« Reply #33 on: Fri December 08, 2017, 12:27:18 PM »
It is indeed used to block part of the light. I presume it is the difference between English and German. In the small picture of the Scophony system it is labelled, the larger picture on the reight does identify other parts, so I posted both. I did not know it was in the Eidophor before this discussion, but I read about it for te first time in articles about the ESLP and the Sony/GLV Grating Light Valve projectors.

Offline albert

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Re: Mechanical Television primer
« Reply #34 on: Sat December 09, 2017, 07:06:28 AM »
Here is a link to an old movie describing a tv setup with a Nipkow disc "flying spot scanner" @ 48 lines from GE Company in Schenectady 1931.

https://www.youtube.com/watch?v=2WiV0FIFpHE&feature=youtu.be

The Nipkow disc on the transmitter side projects the bright spot from a high power incandescent lamp house onto the subject. The reflected light is taken up by massive photo cells and this is what drives the receiver side. The machine is shown in all its details which makes for interesting viewing. -
You can also see the results of the machine filmed directly off the receiver. This gives a good idea of what these images looked like at the time.

https://www.youtube.com/watch?v=2WiV0FIFpHE&feature=youtu.be

The receiver uses a lens disc to project the image onto a screen. The light from an arc lamp passes thru a "light valve"- again a Kerr cell or something similar to it.

This obviously was a well engineered system.


« Last Edit: Sat December 09, 2017, 07:15:49 AM by albert »

Offline donaldk

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Re: Mechanical Television primer
« Reply #35 on: Sat December 09, 2017, 10:05:00 AM »
According to wikipedia the Jeffree cell used by Scophony let 200 times more light through compared to the Kerr cell.

Offline albert

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Re: Mechanical Television primer
« Reply #36 on: Tue December 12, 2017, 05:24:26 AM »
I have found this website - it has the most stringent info on the Jeffree Cell so far...

http://www.bluehaze.com.au/modlight/UltrasoundMod.htm

I think this type of cell could be used in holographic imaging today.

Offline albert

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Re: Mechanical Television primer
« Reply #37 on: Tue December 12, 2017, 07:57:24 AM »
More on Scophony. This seems to be the most overlooked TV system ever.
Think about it- they had in 1938 a motor driving a small mirror scanner at more than 30 000 rpm in sync with the video signal. And they had the ultrasonic modulator light valve.
Hard to believe. When I think how the Ampex people struggled later on with their head wheel for their first 2 inch quad video recorder....this had to run @ 10. 000 rpm - in Europe @ 15. 000- and the entire head wheel/motor assembly had a lifetime of only 300 hours at first.


Offline w.mayer

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Re: Mechanical Television primer
« Reply #38 on: Fri December 15, 2017, 01:45:24 PM »
Still very nice to read here!

Is it not strange what kind of big development we only got in less than 80 years!
So think what we will have in 80 years from now.....
Hope I can at least see the next 20 to 25 years........