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Rastergraf
5-16 Programming On-board Devices and Memories
To change the width of the image:
The best way to change the width of the image is to change the pixel clock
frequency. If you want to change the pixel clock but not any other timing
parameters, then increasing the frequency will result in a narrower image
and decreasing it will result in a wider image While there are ways to
change the width (horizontal size) of the image without changing the pixel
clock, they affect other timing parameters and can lead to complications.
Note:
To keep the timing intervals the same when changing the pixel clock you
have to enter new horizontal timing parameters.
To change the vertical frequency:
The vertical frequency is also known as vertical refresh rate or vertical
scan rate. Indications that the vertical frequency needs to be changed are a
picture which rolls up or down. Sometimes the appearance is of multiple
pictures, one on top of another, with multiple horizontal lines. A very slow
vertical frequency will cause the image to flicker. Some monitors display
no picture when the vertical frequency is out of its bandwidth. Since the
same symptoms can be caused by no sync at all, make sure that the cable
is connected correctly and that the monitor is configured correctly.
To change the vertical position:
To shift the image
up
increase the vertical front porch by the same
amount. Perform the converse procedure to move the image
downward
.
To change the height of the image:
There are two ways to change the height (vertical size) of the image.
1) Change the number of lines. The image aspect ratio remains the same.
2) Change the vertical frequency. Increasing the vertical frequency will
result in a shorter image, decreasing it will result in a taller image.