High speed video

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psycho
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High speed video

Post by psycho » Mon Apr 05, 2010 2:54 pm

I was explaining how the picture tube of a TV works to my 9 year old because our digital camera catches the glitches in the frames and he asked me why.

I am curious to know if anyone has seen a video (preferably downloadable) taken with a high speed camera of a regular TV drawing it's frames? I wouldn't even know how to look it up since I vary rarely watch videos on the net --- crappy ISP (Satellite. Need I say more?). I am not sure of what the camera would actually capture but I think it would be cool to check out.

Kevin

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haklesup
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Re: High speed video

Post by haklesup » Mon Apr 05, 2010 5:24 pm

What you see would depend on the settings of the high speed camera and how or ifit was synched with the TV. In general you will see just a narrow horizontal band of the picture. The width depends on the exposure time, where the band appears depends on the moment the exposure took place and how many bands you see depends on the frame rate of the camera.

With a still camera, if the exposeure were less than 1/24th of a second, you would get a aprtial frame. If a little longer you get a full frame with part of it birghter than the rest (as part got scanned twice while the lens was open). With a long exposure, the photo looked good.

Most of any examples I would have are old fashond film pictures of an analog TV set. So these are not scanned in yet as JPGs or anything so I can't post. I'll take a look and see if I can find one I can scan in. (usually these went to the trash, so I dunno)

To demonstrate to your kid, try this. Hunt down an old 35mm SLR camera. Open the back and look through the lens when you snap a picture, try this with several different shutter speeds with the aperature wide open and the focus set to infinity. Without even exposing film, you should be able to see partial frames with the naked eye (especially a well focused childs eye). I wouldn't be suprised to see the same thing on a modern digital set except the refresh rates can be as high as 240 Hz but 60Hz is still typical.

SETEC_Astronomy
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Re: High speed video

Post by SETEC_Astronomy » Mon Apr 05, 2010 5:45 pm

It's only a small portion of the screen but you can still see it in this video -> High Speed Refresh Capture -- Found Here -> High Speed Video three other related videos accompany the one I linked.

A decent high speed pic can be found on this wiki page -> Refresh Rate - Wiki

psycho
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Re: High speed video

Post by psycho » Mon Apr 05, 2010 5:53 pm

-- edit
SETEC: You and I posted at the same time...
--

Wouldn't a high speed camera (like the ones used on mythbusters) catch the frame as it was being drawn so you could see the progression of the scanning of the beam? They can trace a bullet with those things...

I have seen one film of a water balloon popping and it was really neat.

On the subject of TVs, I was under the impression that there were NO analog TV signals transmitted anymore due to the new FCC laws. But, I can get 5 weak stations (good old snowy pictures). I don't understand how they can still transmit. One of them is some stupid shopping channel.

Kevin

SETEC_Astronomy
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Re: High speed video

Post by SETEC_Astronomy » Mon Apr 05, 2010 6:31 pm

psycho wrote:On the subject of TVs, I was under the impression that there were NO analog TV signals transmitted anymore due to the new FCC laws. But, I can get 5 weak stations (good old snowy pictures). I don't understand how they can still transmit. One of them is some stupid shopping channel.
As I remember it broadcasters could continue analog transmissions for a certain time after the digital transition. The date was more or less a starting date for digital, not the death of analog. There are a lot of plans for the newly freed space though so I don't imagine they'll keep transmitting long.

High speed camera, bullets.... Have you seen this? 1 Million fps Bullet Impacts

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Bob Scott
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Re: High speed video

Post by Bob Scott » Mon Apr 05, 2010 9:35 pm

psycho wrote:On the subject of TVs, I was under the impression that there were NO analog TV signals transmitted anymore due to the new FCC laws. But, I can get 5 weak stations (good old snowy pictures). I don't understand how they can still transmit. One of them is some stupid shopping channel.
If all US analog broadcasting has ended, there are still a couple of possibilities. Remember that analog is still available on Cable. There may be a bad ground in the cable --a broken shield-- that will cause, as it is called in the cable industry, egression. The analog signal leaks out. This is quite a common fault especially with older connectors on trunk lines.

The other possibility is long distance reception of Canadian analog broadcast signals. That will not end until ~2012.
-=VA7KOR=- My solar system includes Pluto.

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haklesup
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Re: High speed video

Post by haklesup » Tue Apr 06, 2010 9:53 am

Wouldn't a high speed camera (like the ones used on mythbusters) catch the frame as it was being drawn so you could see the progression of the scanning of the beam? They can trace a bullet with those things...
Factoring in the sustained luminescence of the phosphor after the beam hits it, yes. I wouldn't expect to see a dot tracing though because of that. You could hypothetically characterize that luminescence of the phosphor.

Residual analog signals could also be retransmitted signals by a nearby neighbor or business for closed circuit use but I can't imagine why one would want to rebroadcast a shopping channel

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