Mechanical Television Project for Spark Museum in Bellingham

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Mechanical Television Project for Spark Museum in Bellingham

Postby maallyn » Wed Jul 26, 2017 7:54 am

Folks:

I have just completed the design and construction of a living exhibit for mechanical television at the Spark Museum of Electrical Invention in Bellingham, Washington.

We still have a bit of causmetic and interpretive work to do before we have the exhibit set up in the galleries. We hope to have it done by late fall.

However, you can see what I have done, as well as an explanation on how I created the signals on my youtube video that I have created. It is at:

https://www.youtube.com/watch?v=VqCBjtfpvaU&t=15s

The Spark Museum owns a Baird Televisor as well as an experimental kit mechanical television. Both are from period (1920's). They are not recent experiments.

My role is to create the signals appropriate for the neon lamp and to have the signal synchronize with the spinning of the disk so that the museum visitor does not have to fiddle with the motor speed to synchronize the picture.

If you have any further questions, you can email me at allyn (at) well.com

Enjoy!

Mark Allyn
Bellingham, Washington
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Andrew Davie » Wed Jul 26, 2017 10:36 pm

Hi Mark. I enjoyed watching the video and explanation of your design/project. Especially as I've been tackling many of the same issues myself with my Arduino televisor.
Things I found interesting/unusual/worthy of comment... in no particular order...
a) you mentioned frame rate for the Baird - 18fps . I never realised this was the case until now - I had always assumed the "club standard" 12.5fps was modelled on Baird's system but it appears not to be so. I have to find out why we're using 12.5fps. Odd.
b) It was interesting to me you are using a single hole for frame sync. It wasn't clear how you achieved that (that is, getting the first scanline in the right position). I assume your Pi software is (re)starting a frame when it sees the synch hole, rather than running independently and hoping that the disk is up to speed at some point. That is, do you detect a synch pulse, then start "sending" the image?
c) I thought the 512x512 GIF was both overkill and a significant limitation. Probably fit for purpose, but it would be nice to have period-clips (consider, for example, Don MacLean's restorations of actual 1930s TV recordings). I converted these to my own format - "NBTV8" - for display on my Arduino televisor. I did contact Don for permission, which he kindly gave, but also he seemed to be very positive to allowing these restorations/digitisations to be shown in museum situations. I think these would be very useful/relevant for your project.
d) The 4-bit resolution surprised me. I understand the human eye is sensitive to 6-bit. What was the reason/limitation/purpose of limiting the display to just 4-bits? I know it would be difficult to change now - you'd have to increase your resistor ladder and add more trimming pots and opamps etc. But yeah.... is 4-bit really representative of the quality that was achievable "back in the day"?
e) Rather than one system to provide the same image/data to two televisors, and the conversion issues of your 512x512 this brings... I had the advantage of designing for just one output system and consequently could pre-convert MPG data into WAV files stored on a SD card (and, as it happens, selectable on a touch-panel LCD for playback). I did have some ambition to get this into a museum context somehow.
f) A note on the sound in your videos - I am hard of hearing, and found the significant variation in volume quite hard to deal with. At some points you were hard to hear, and sometimes way too loud.
g) As to the "electrical noise" of the baird motor connected to the disc and thus interfering with the infra-red sensor - this was not clear to me what exactly the problem was. Wouldn't the sensor be isolated from the motor and thus not electrically affected? Are we talking about some sort of EM interference and induced signal? I'm a bit of a novice with electronics so just trying to understand exactly the issue requiring the shielded box.
h) I was surprised at the seeming flimsiness of the baird disk. Is this an original disk?
i) finally, since you synch with an IR led, I assume that you have added a synch hole to the disks on both televisors. Did this mean modification of the originals, or were there some sort of holes already present.

So that's it. Thanks for posting the video link!

PS: as an aside - I'm working on a project (CSIRO in Australia) which is/was using Edisons. I guess we have to find an alternative now :)
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Lowtone » Thu Jul 27, 2017 3:05 am

Andrew Davie wrote:a) you mentioned frame rate for the Baird - 18fps . I never realised this was the case until now - I had always assumed the "club standard" 12.5fps was modelled on Baird's system but it appears not to be so. I have to find out why we're using 12.5fps. Odd.

Interesting, i've always read this was 12.5 wich makes sense because it is derivated from the 50Hz mains.
What is the origin of the 18 ?
r a d i o P T T v i s i o n
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby McGee2021 » Thu Jul 27, 2017 11:11 am

Lowtone wrote:
Andrew Davie wrote:a) you mentioned frame rate for the Baird - 18fps . I never realised this was the case until now - I had always assumed the "club standard" 12.5fps was modelled on Baird's system but it appears not to be so. I have to find out why we're using 12.5fps. Odd.

Interesting, i've always read this was 12.5 wich makes sense because it is derivated from the 50Hz mains.
What is the origin of the 18 ?


While at the early television museum getting measurements off of a televisor, i managed to measure the speed of the disc and the speed was 750 rpm. If you convert it from rpm to rps, than you get 12.5 Rotations per second. Thus, the frame rate would be 12.5.
John Logie Baird was obviously the man who sowed the seeds but did not reap the harvest.
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Klaas Robers » Thu Jul 27, 2017 8:13 pm

Andrew, the speed was indeed 12.5 f/s. This was mainly to make interference from the 50Hz mains less visible. For the USA with 60 Hz mains, 15 Hz is more apropiate. However in the shown system the video is synchronised with the current, free running, speed of the disc. Then it is not that important what the disc speed is. Synchronisation done in electronics (computers) is much easier than synchronisation done mechanically. Electronics has no inertness and you can simply jump to the beginning of a frame when you see the first hole.

But what struck me is that the displayed pictures are very narrow in the horizontal direction. The dancing Mark looks far too slim to me. He could have compensated for this by stretching the .GIF picture by a factor of 2 in the horizontal direction. That gives a very obese Mark on the computer screen, but better pictures on the televisors. May be he can do that still.

And it was remarkable that the Baird televisor gave a white picture. I got the impression that this televisor is one of the replicae that the late Denis Assemann made, say 15 years ago. Denis died far too early. He had golden hands.
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Andrew Davie » Thu Jul 27, 2017 11:05 pm

Klaas Robers wrote:Andrew, the speed was indeed 12.5 f/s. This was mainly to make interference from the 50Hz mains less visible. For the USA with 60 Hz mains, 15 Hz is more apropiate. However in the shown system the video is synchronised with the current, free running, speed of the disc. Then it is not that important what the disc speed is. Synchronisation done in electronics (computers) is much easier than synchronisation done mechanically. Electronics has no inertness and you can simply jump to the beginning of a frame when you see the first hole.

But what struck me is that the displayed pictures are very narrow in the horizontal direction. The dancing Mark looks far too slim to me. He could have compensated for this by stretching the .GIF picture by a factor of 2 in the horizontal direction. That gives a very obese Mark on the computer screen, but better pictures on the televisors. May be he can do that still.

And it was remarkable that the Baird televisor gave a white picture. I got the impression that this televisor is one of the replicae that the late Denis Assemann made, say 15 years ago. Denis died far too early. He had golden hands.


Thanks Klaas. 12.5fps it is. I recall that in the video I heard Mark talking about the LED array on the Baird. At the time my impression was that the Baird display tube had been replaced. I had surmised/asked Mark in my post above if the synchronisation was by waiting for the first hole - this is kind of clear/apparent when the synchronization is enabled and it's pretty much instant.
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Klaas Robers » Fri Jul 28, 2017 1:06 am

His IR sensor uses simply the scanning holes and senses the first hole of the field. Then this "first" pulse is stretched by a mono stable multivibrator, becaus otherwise he gets more pulses following the first pulse, from line 2, 3, 4 and may be even 5. This mono stable jumps over these extra sense pulses. Of course he could have done that as well in software, but everyone does things in the way he is best familiar with.

The same is with the moving GIF. If you are able to record camera pictures in 512 x 512 pixels at 18 frames per second, then that is your standard. That is why he had to average the pixels to 30 x 30 at play back. This could have been done as well in advance, in the PC. This would have saved him a factor of 291 in storage capacity, or he could have stored a 291 times as long video clip.
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby Andrew Davie » Fri Jul 28, 2017 1:30 am

Klaas Robers wrote:His IR sensor uses simply the scanning holes and senses the first hole of the field. Then this "first" pulse is stretched by a mono stable multivibrator, becaus otherwise he gets more pulses following the first pulse, from line 2, 3, 4 and may be even 5. This mono stable jumps over these extra sense pulses. Of course he could have done that as well in software, but everyone does things in the way he is best familiar with.


Ah. Yes I remember him talking about that but I misunderstood. I thought the IR hole was too wide and/or his IR was inconsistent and returning pulse/no pulse near the hole margins, and it was therefore being "fixed" with the multivibrator. Now I understand. Ta for the explanation.

Klaas Robers wrote:The same is with the moving GIF. If you are able to record camera pictures in 512 x 512 pixels at 18 frames per second, then that is your standard. That is why he had to average the pixels to 30 x 30 at play back. This could have been done as well in advance, in the PC. This would have saved him a factor of 291 in storage capacity, or he could have stored a 291 times as long video clip.


Gotcha.
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Re: Mechanical Television Project for Spark Museum in Bellin

Postby smeezekitty » Fri Jul 28, 2017 6:53 pm

I live in Washington but have never been to the Spark Museum. The more I hear about it the more I want to go though
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