From: Ben Harris <bjh21@bjh21.me.uk>
Date: Sun, 6 Nov 2016 16:16:39 +0000 (+0000)
Subject: Move pixel aspect ratio discussion from NOTES to bedstead.c.
X-Git-Tag: bedstead-001.002~19
X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~bjharris/git?a=commitdiff_plain;h=310244e70e0d7d5feffd94b8b24809f5ce7ceafc;p=bedstead.git

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

diff --git a/NOTES b/NOTES
index c14c991..84bc399 100644
--- a/NOTES
+++ b/NOTES
@@ -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:
diff --git a/bedstead.c b/bedstead.c
index 72922b4..de42f9b 100644
--- a/bedstead.c
+++ b/bedstead.c
@@ -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 {