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Move pixel aspect ratio discussion from NOTES to bedstead.c.

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Ben Harris 2016-11-06 16:16:39 +00:00
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commit 310244e70e
2 changed files with 18 additions and 9 deletions
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NOTES
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@ -1,12 +1,3 @@
Pixel aspect ratios: http://www.lurkertech.com/lg/video-systems/#sqnonsq
In 480i, industry convention is that you get square pixels with a pixel
clock of 12+3/11 MHz. In 576i, you need 14.75 MHz. The SAA5050 takes a
nominal 6MHz clock and uses both edges to output subpixels (duty cycle
between 0.4 and 0.6). This means that the nominal pixel aspect ratios
are 12+3/11:12 for 480i and 14.75:12 for 576i. These correspond to
pixel widths of 102.3 and 122.9 respectively.
Precise pixel layouts for mosaic graphics:
Based on a 6x10 grid of pixels:

18
bedstead.c
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@ -104,6 +104,24 @@
/*
* Design parameters. These can vary between fonts in the Bedstead family.
*
* To faithfully represent the output of an SAA5050, we need to know a
* little about TV pixel aspect ratios:
*
* http://www.lurkertech.com/lg/video-systems/#sqnonsq
*
* The SAA5050 can be used to drive both 480i ("NTSC") and 576i
* ("PAL") displays. In 480i, industry convention is that you get
* square pixels with a pixel clock of 12+3/11 MHz. In 576i, you need
* 14.75 MHz. The SAA5050 takes a nominal 6MHz clock and uses both
* edges to output subpixels (duty cycle between 0.4 and 0.6). This
* means that the nominal pixel aspect ratios are 12+3/11:12 for 480i
* and 14.75:12 for 576i. These correspond to pixel widths of 102.3
* and 122.9 respectively.
*
* 102.3 is close enough to 100 that we may as well just use square
* pixels, which will work better on modern displays. We round 122.9
* to 124 so that XQTR will be precisely an integer.
*/
struct param {