[Tig] Theorem sought for aspect ratio

Russell Branch russell at innomedia.co.uk
Fri Dec 19 15:37:27 GMT 2008


Hi Rob,

 

You are quite correct, the pythagorean principle holds true, (hypotenuse
given by square root of the sum of the squares) so for a 4: 3 screen, the
hypotenuse is a 5, such that a 20 inch screen has a base dimension of 4/5
(16 inch) and a height of 12 inch (3/5)

 

However, it is slightly more complicated than that, since in the CRT picture
tube world, it was traditional to specify the CRT diagonal in inches from
outer rimband to outer rimband, meaning that for a 24 inch grade 1 CRT, the
actual visible picture diagonal is about 1 1/2 inch less than the nominal
dimension.  (Oddly enough, in France the authorities insisted that screen
dimensions be given in cms, and these were the actual viewable dimensions
rather than the gross outside so we used to have a mathematical (or
marketing!) dilemma that the TV's outer box would list both inches and cms,
and the two were not the same actual length.)

 

For LCD, it is more complicated still - for instance a 1920 x 1200 display
such as the Cine-tal has a ratio of 16:10 giving us a ratio of 16x along the
bottom, 10x up the side and approx 18.9x diagonal, so for a 24 inch display
the base will be (16/18.9) x 24 = 20.3 inch.  However, a) you have to
remember that the dimensions on a LCD are for the actual viewable area,
giving slightly more bang for your buck than for a CRT; and b) that although
the vertical dimension is correct for the screen at full 1200 line DVI
resolution, you should make the calculation at 16:9 if you are working HD
since, at least in the case of the Cine-tal, the extra 120 superfluous lines
over and above the 1080 of active video are used for information and menu's
(so diagonal ratio is 18.35:16:9 giving the height of the active image area
as (9/18.35) x 24 = 11.77 inch.

 

Russell Branch

 

Formerly a Toshiba Senior TV design engineer and Toshiba's European CRT
specialist (happy days)

 

Managing Director

InnoMedia Systems Ltd

Tel/Fax: +44 1473 231963

Mobile: +44 7900 692000

e-mail: russell at innomedia.co.uk

 

Disclaimer - InnoMedia is the importer and European master distributor for
Cine-tal monitors

 

 




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