More Questions Re Aspect Ratio, Frame Size/Shape, and Holes

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More Questions Re Aspect Ratio, Frame Size/Shape, and Holes

Postby ac7zl » Wed May 16, 2007 4:19 am

I'm wondering what the consensus is with regard to the NBTV club standard and the issues of aspect ratio, frame size/shape, and the positioning of holes.

a) Given that a scan segment on the Nipkow disk is pie-shaped, the region over which a frame is scanned is actually trapezoidal, or keystone shaped, not rectangular.

The meaning of "aspect ratio" with respect to a rectangular frame is apparent enough. What is the meaning of "aspect ratio" as it applies to a trapezoid? Are we talking about the ratio of the outside vertical dimension to width, the inside dimension to width, or the the height of the trapezoid at its center with respect to width?

b) It seems to me that resolving question (a) is necessary because, without a definition of frame height, one cannot know the frame width. Without frame width, one cannot compute appropriate hole sizes (as 32 holes must fit within the width dimension.)

c) Speaking of holes, and assuming round holes...the easiest kind to make....I gather that there is benefit to planning for overlap between scanning holes in successive disk segments. Is there a rule of thumb for the amount of overlap? Given a fixed width, the amount of overlap has direct bearing on the horizontal positioning of hole-centers, and the maximum allowable hole size.

Your thoughts?

Pete
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Postby Klaas Robers » Wed May 16, 2007 7:26 am

Pete, read the NBTV handbook.
The aspect ratio is measured in the centre of the picture.
There is little difference if you measure over the circle or in a straight line.
The aspect ratio is including the sync. Because there is sync the aspect ratio of the active picture becomes smaller. The Baird 3:7 becomes almost 1:2 because of the sync, but also because of the wider lines at the edges of the picture.
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Postby ac7zl » Wed May 16, 2007 10:33 am

Klaas,

I found some clarification in the handbook. Thank you for pointing me to this resource. However...

A 32-line Nipkow disk has 11.25 degrees worth of "space" for a line of video...assuming there is no sync.

If we include sync, if the sync pulse is 0.25 mS long, and the rotational speed of the disk is 750 rpm (12.5 frames per second), I calculate that the 11.25 degree segment is essentially reduced by 0.8125 to 10.44 degrees. Obviously, this results in diminished vertical space for the frame to reside.

When you say that the aspect ratio "includes" the sync, are you saying that sync time ignored and that hole positions and sizes are computed as though the entire 11.25 degrees were available for picture information?

Or are you really saying that the aspect ratio "accounts for" the sync, in which case the hole positions and sizes would be computed on the basis of the corrected 10.44 degrees?

Thanks,

Pete
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Klaas Robers wrote:Pete, read the NBTV handbook.
The aspect ratio is measured in the centre of the picture.
There is little difference if you measure over the circle or in a straight line.
The aspect ratio is including the sync. Because there is sync the aspect ratio of the active picture becomes smaller. The Baird 3:7 becomes almost 1:2 because of the sync, but also because of the wider lines at the edges of the picture.
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Postby Klaas Robers » Wed May 16, 2007 5:58 pm

The calculated aspect ratio is when there is no sync and the total line would be available for video. In that case the aspect ratio of the clubstandard is 2:3 and the (original) Baird standard 3:7.

With sync the aspect ratio of the clubstandard wil go into the direction of 2:2,8 and the Baird standard 3:6,7. Additional mr. Baird widened his picture with wider lines left and right, which moves his aspect ratio even more towards 3:6, i.e. almost 1:2. It looks as if he found out that 3:7 is indeed not a handy aspect ratio.
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