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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 15th Dec 2004, 3:17 pm   #1
shaft
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Default "Raster" lines

A basic question for the forum members if I may.

I currently have 2 working 405 line TVs. Both sets display a number of fine white diagonal lines when the brightness is turned up high enough. These lines never move or change position, and are generally not visible when viewing a normal picture, other than in some dark scenes. I seem to recall similar lines being visible on family TV sets when I was young , so I assume they are characteristic of 405 technology.

Can anyone explain (in simple terms) what these lines are and why they don't appear on modern sets?

Sorry if it seems a daft question, but I'm sure most of you here will know the answer.

Many thanks,

Cliff
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Old 15th Dec 2004, 3:24 pm   #2
ppppenguin
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Default Re: "Raster" lines

These are flyback lines. In an ideal world the beam would be turned off while the field timebase scans quickly back to the top of the screen. On many older sets the beam blanking is non-existent or inadequate. If there is any video information above black level during the flyback it will be all too visible. Since the flyback often encroaches into the top or bottom few lines of active video the problem is almost inevitable. The problem afflicted many 625 sets when teletext first arrived.

Setup (or pedestal) is the practice of putting black level a little above blanking level. This has been part of the 405 standard at times and is still part of the US 525 standard. This helped those sets with poor flyback blanking. It's also a total waste of transmitter power today. Just hangs around because it's the standard.
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Old 15th Dec 2004, 3:40 pm   #3
oldeurope
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Default Re: "Raster" lines

What is your 405 source?
To reduce this effect I reduced the V- sync level
and blanced the following 16 lines (Videotext)
with D33 Blatt3 in my converter.

Maybe you turned the brightness to high.
Black must be black not grey .

Kind regards

Darius
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Old 15th Dec 2004, 3:43 pm   #4
Station X
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Default Re: "Raster" lines

Quote:
These are flyback lines
Yes, but why do we see a series of parallel lines at a shallow angle and not just one running along the diagonal of the screen?

Also why is it that we can see the " colour " black on the screen, but when the set is turned off the screen appears grey?

Finally what happened to the white dot seen in the middle of TV screens when they were switched off? I assume it was caused by the EHT capacitors discharging with no beam deflection. This must have caused screen burns, so why was the circuit not designed to kill it.

Remember the ex shop window sets with Test Cards burned on to the screen?

Graham

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Old 15th Dec 2004, 4:08 pm   #5
oldeurope
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Default Re: "Raster" lines

You see a few lines because the V- flyback needs
a few lines time.

Your eys have an AGC control. The grey screen
looks dark when other parts are bright.

You must repair your set the dot must be suspressed
(Leuchtfleckunterdrückung)

Darius
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Old 15th Dec 2004, 4:23 pm   #6
AC/HL
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Default Re:

Graham,
You see several flyback lines because the time taken to return to the start is more than one line scan period.

I would assume that we " see " black more as a contrast to white rather than as an actual level. With the set off, ambient light shows the true reflected screen colour.

The switch-off spot, which is the undeflected (& unfocussed) remnants of the cathode emission & EHT voltage as you surmise, is not a problem despite it's bad press. The duration is too short. The actual cause of the damaged area in the middle of the screen is heavy ions produced alongside the useful electrons. They are not deflected much & cause the centre of the screen to burn & become dark. The problem of " Ion Burn " as it's called was cured firstly by the ion trap & later as a by-product of the aluminised screen.`
The lasting impression of a stationary picture is caused by similar burning, but by the electron beam. It can be alleviated by displaying a peak white raster for a long time, but all this does is to burn the rest of the screen, as a normal moving picture would do in time.

If you ever see a used old-style monochrome projection tube, you get a good before & after view of the burning of screen phosphor. Because the tube is round, the picture area is much darker than the unused area top & bottom.

Overlapped with Darius's reply, seems like we agree

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Old 15th Dec 2004, 4:23 pm   #7
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Default Re: "Raster" lines

During the frame flyback the line syncs are transmitted at double frequency to enable the line oscillator to re-sync in mid-line in order to effect the interlace.

What you are seeing are the two syncs per line, but with a heavy vertical displacement due to frame flyback.
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Old 15th Dec 2004, 4:34 pm   #8
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Default Re: "Raster" lines

The black on TV sets is an optical illusion. It only looks black by comparison with the lighter colours around it. For a superb description of optical illusions I recommend Richard Gregory's book " Eye and Brain, the Psychology of Seeing " .

Some sets suffered worse switch off spots than others. It will only do damage if it's very sharply focussed and lasts a relatively long time. Spot suppression could be done in 1 of 2 ways. Either make sure that the EHT discharges quickly at switch off. Or arrange that the beam is blanked at switch off.
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Old 15th Dec 2004, 6:15 pm   #9
shaft
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Default Re: "Raster" lines

Some ineresting replies - I knew you folk would have the answers

I must admit, I thought Graham had a good point when he asked how we see the " colour " black - I'd never given it a thought before!

For info, my 405 source is a Domino converter. It gives excellent results, and I can usually remove the flyback lines (as I now know they're called) by adjusting contrast and brightness settings.

More threads like this and I might start to sound as if I know something on the subject (some hope).

Many thanks,

Cliff
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