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

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Old 10th Mar 2017, 6:27 pm   #1
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Default Line tilt?

I have been meaning to ask this for ages (and may have done so, apologies if I have).

Because the frame scan tilts TV lines is the correct angle for lines dead horizontal or tilted 1/302.5 (for 405 lines), it would make no real difference in the real world, just curious. Modern LCD/OLED TVs have perfectly horizontal lines almost by definition, as do modern TV cameras.
 
Old 10th Mar 2017, 8:51 pm   #2
TonyDuell
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Default Re: Line tilt?

I suspect (with no real proof) that the original TV cameras were scanned with 'tilted' lines. So the correct way is to have the lines tilted on the receiver too.
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Old 10th Mar 2017, 9:10 pm   #3
FERNSEH
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Default Re: Line tilt?

The flyback stroke is also tilted?

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Old 10th Mar 2017, 9:33 pm   #4
Panrock
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Default Re: Line tilt?

Considering the 202½ lines of one field on the 405-line system and a 4:3 aspect ratio, the 'tilt' is very small: 0.212 of a degree or 13 minutes of arc. The line flyback occurs faster, so that tilts even less.

You'll get this effect in any raster scanning system that uses one continuous trace. Interestingly, my 120-line Mirror Screw, whilst also being a raster scanning device, uses a fresh mirror edge for each line so the lines really are horizontal. Another advantage of using this mechanical method is that the line heights abut exactly, so providing a 'lineless' effect. Enough of this though since it's off-topic!

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Old 11th Mar 2017, 11:48 am   #5
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Default Re: Line tilt?

Just a small, but true anecdote. We were recording a programme one day and the set designer walked from the studio floor into the control rooms and looked at the bank of monitors. He said 'why is the set tilted? The studio floor is perfectly flat and level, as is the set, so how come?'. We were mystified and of course very sceptical. It turned out that due to a manufacturing fault by the broadcast camera manufacturer, all the Cmos sensors on all the cameras were glued 2.5 degrees off the horizontal when they were originally glued to the light splitting prisms!!!

Oops does not adequately cover the manufacturers response when they discovered the problem. They said it was down to a mechanical calibration problem during the alignment process. All the sensor blocks in all the cameras suffered in the same way and all were replaced by the manufacturer, must have cost a fortune. True story, at a national broadcaster in the UK.

That designer must have had some eyesight, better than we engineers.
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Old 11th Mar 2017, 2:48 pm   #6
PaulM
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Default Re: Line tilt?

In the late 1980s, whilst working on the design of linear line-scan 26" colour monitors for flight simulation, the problem of line tilt came home to roost. We were driving the line scan with huge, sweaty high-speed power amplifiers to achieve ultra-high linearity rather than the conventional resonant flyback system. At 875 line 50 field interlace the format was quite demanding for the late 1980s. Despite the problems, the amplifier drive worked, but the whole thing was a nightmare and it occurred to me to try zig-zag line scanning to save power (no flyback demanding high voltages). Our video source was computer generated and could easily output one line going forward and the next going backwards - so not a problem, at least in theory. The system was rigged up with a triangle line scan and oh my, what a mess! Despite our ultra high linearity amplifiers giving perfect results whether forward or backward scanning, we had cross-over in the middle, due, of course, to the line tilt. Changing the field scan to a staircase waveform then gave us parallel lines completely normal to horizontal. That made the field scan amplifier more complex, but still a reasonable design.

In the end the idea was going nowhere as despite the ultra high linearity, minute errors in the scan velocity caused a slight 'wobble' in verticals which couldn't be solved.

Great idea, probably not original thought, but it really brought home to me the issue of line tilt in raster CRT systems.

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