the heart of the modeline is the pixel clock (or pclock
for short), a conversion factor that translates pixels (digital)
into seconds (analog). Pixel clock = pixels / second.
We can calculate the pixel clock using
the following formula
Pixel Clock = Number of Pixels per Line
x Number of Lines x Refresh Rate
Be aware that the Number of Pixels
per Line includes both the number of active pixels
(ones that display data) and the number of blanking pixels
(buffering time for the CRT). Similarly, the Number
of Lines includes both active lines (ones that display
data) and blanking lines (buffering time for the CRT).
So, for example, we get the following pixel
clock for our modeline above
Pixel Clock = 20 Pixels per Line x 15 Lines
x 1 refresh per 60 seconds = 5 pixels per second
In order to understand the usefulness of
pixel clocks, we need to explore how they are used to manipulate
signal timing. Pictured below is a snap shot in time of
the VGA signals above. From it we can see how horizontal
pixels are divided into active ones (ones that display data)
and blanking ones (buffering time for the CRT).