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Old 02-23-2011, 10:35 PM
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Question Color TV Porthole sets?

Back in the day, round B&W CRTs were used, and Zenith came out with Porthole TV sets to make use of the entire CRT screen. I can't remember any color TV using a roundie CRT as a Porthole.
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Old 02-23-2011, 10:37 PM
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It wouldn't have worked because the top and bottom mask hides various registration marks in the screen used during manufacturing. Look closely at a bare round CRT and you'll see what I mean.
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Old 02-24-2011, 01:23 AM
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Anyhow by 1955, when the first 21" color sets came out, Porthole sets were long gone.
They never were a really good idea, just a way to claim more square inches without spending any money.
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Old 02-24-2011, 08:22 PM
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There were several issues:

Zenith saw that their idea was not winning out. Although porthole operation gives a larger image, there were sufficient cases of complaints about cutting off material that sets were then made with a switch to go to "full picture" mode instead of "full circle."

There were also problems in color tubes with scattered electrons from the bell of the tube fogging the picture when the corners of the picture were bright - porthole operation would be worse.

There would also be X-ray considerations, and the special design of the horizontal sweep and high voltage, especially converngence if the size switch was required.

Professional monitors had an underscan setting to show the edges of the scanned picture, and this resulted in double sets of convergence controls.
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Old 02-25-2011, 05:01 AM
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I recall asking about the porthole colour sets a few years back. Just wondering in general when viewing a picture on the bare round tube outside of its bezel does the picture only extend so far up the tube before being cut off or does it extend all the way to the top?
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Old 02-25-2011, 09:15 AM
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Well let's see, if the image extended to the top and bottom of the tube face, why would it be masked?

The correct aspect ratio of the transmitted image is always 4 wide by 3 high, no matter what it's displayed on.
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Old 02-25-2011, 10:18 AM
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Quote:
Originally Posted by Aussie Bloke View Post
I recall asking about the porthole colour sets a few years back. Just wondering in general when viewing a picture on the bare round tube outside of its bezel does the picture only extend so far up the tube before being cut off or does it extend all the way to the top?
Only as far as the phosphor coating on the face. Remember that the rest of the tube has to have an electrically conductive material on it to make it a capacitor.
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Old 02-25-2011, 03:27 PM
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Round could come back into vogue again
- Like it was in the Space Age mid Fifties
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File Type: jpg Round.jpg (70.2 KB, 41 views)
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Old 02-25-2011, 03:47 PM
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Picture height and width was not very well regulated on tube sets, so the wise assembly line or serviceman would set them at least 10% over to avoid complaints about black edges on the screen.
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Old 02-25-2011, 05:05 PM
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Quote:
Originally Posted by old_tv_nut View Post
Picture height and width was not very well regulated on tube sets, so the wise assembly line or serviceman would set them at least 10% over to avoid complaints about black edges on the screen.
Not just that but a lot of gear adds even more overscan these days... For example, I have a Mythtv-DVR wired to the satellite box here - If watching normally, the station "bug" is in the lower left corner of the tube... but if watching a recorded show there's a good 1-2" extra of picture around it that the tuner grabs but the TV never shows.
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