Baird's Bulb Screen

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Baird's Bulb Screen

Postby Lawnboy » Sat Nov 18, 2017 10:51 am

Lately I have taken an interest in Baird's large screen 30 line bulb display, specifically the commutator. How did he manage to switch 2100 contacts sequentially? There doesn't seem to be much info around on the design, but I have found a few pictures, which are interesting. The contacts seem to be divided into 60 bars, each with two sets of 14 terminals on them, that totals 1680. Where are the other 420?
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Re: Baird's Bulb Screen

Postby ppppenguin » Sat Nov 18, 2017 7:39 pm

Fascinating pictures which I've not seen before.

Baird wasn't the only company doing this. Bell Labs did something similar with a 50x50 array of what, in a photo, look like cold cathode assemblies. This is written up in a paper in a late 1920s edition of the BSTJ. I know I've got the relevant volume but can't find it at the moment. There are photos showing a massive bunch of cables attached to a rotating device.
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Re: Baird's Bulb Screen

Postby OmegaProductions » Sun Nov 19, 2017 1:57 am

OMG! Now that's what I call a large TV screen! :shock:
That does look complicated when you tried to put all wires at once to even turn this thing on if your asking me! :oops:
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Re: Baird's Bulb Screen

Postby Lawnboy » Sun Nov 19, 2017 5:01 am

Baird wasn't the only company doing this. Bell Labs did something similar with a 50x50 array of what, in a photo, look like cold cathode assemblies. This is written up in a paper in a late 1920s edition of the BSTJ. I know I've got the relevant volume but can't find it at the moment. There are photos showing a massive bunch of cables attached to a rotating device.

Yes they did, and some photos of it are here http://www.earlytelevision.org/bell_labs_matrix.html but the Bell Labs device was switching the modulated output of a tesla coil to a single, long neon tube with 2500 external anodes instead of filament lamps. Still, looking at the brush of that system they may have been matrixing (for lack of a better term) the contacts somehow. Could Baird have been doing the same thing?
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Re: Baird's Bulb Screen

Postby ppppenguin » Sun Nov 19, 2017 6:37 pm

Thanks for the link. That's the Bell system I've seen in the BSTJ. More and better photos than in the BSTJ too.
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Re: Baird's Bulb Screen

Postby Klaas Robers » Tue Nov 21, 2017 1:33 am

The problem is that the incandescent lamps were glowing on very short pulses. Even if you use lamps 6V 300 mA (bike rear light) then still the current should mbe rather large, as it encounters always cold filaments. The current should be so large that a lamp will evaporate when the commutator stops. Difficult to tune right. And it will cost many lamps, also during operation.
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Re: Baird's Bulb Screen

Postby ppppenguin » Tue Nov 21, 2017 2:10 am

The multilamp method was around until quite recently, albeit using very different technology. The Sony Jumbtron used a large array of CRTs. I though it was 1 CRT per pixel but the wiki article says is was a few pixels per CRT: https://en.wikipedia.org/wiki/Jumbotron

The technique is still in use with LEDs.

I bet the switching is all electronic, both for the jumbotron and LED displays. No mechanical commutators.
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Re: Baird's Bulb Screen

Postby Panrock » Tue Nov 21, 2017 5:32 am

Our very own David Gentle of the NBTVA successfully made and demonstrated a commutator-based system a few years back.

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Re: Baird's Bulb Screen

Postby Lawnboy » Sun Nov 26, 2017 12:26 am

Thanks Steve, I will try to get in touch with him since he has obviously gotten much farther on this than I have!
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Re: Baird's Bulb Screen

Postby Klaas Robers » Sun Nov 26, 2017 10:34 pm

I have seen David's contraption. His main problem was the commutator that wears out very fast. It worked in the beginning but after several minutes (!) the contacts became unreliable. And then, the number of wires and solder joints.....

You should absolutely contact him to hear his stories.
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