displayed and then the second half frame is displayed shifted so that its lines fit between the lines of
the first frame. This is called
interlace and it succeeds in lowering
the frame rate without increasing the
flicker – it is used quit successfully,
for example in broadcast TV pictures.
Unfortunately, as far
as computer video goes its effects are very poor. Because of the way that
computer graphics often use alternate lines, or patterns of lines to make
intermediate shades or colourss,
this use of interlace often produces a strong and very noticeable flashing. It is possible to arrange a demonstration of interlaced video
so that it looks acceptable by avoiding tones or colourss that use regular odd/even line patterns.
Equally, it is possible to find displays that
make and interlaced Super VGA is
tolerable as long as you avoid
even grey tones. Some applications
packages are even altering there colours scheme to avoid flicker on interlaced systems – Ventura 4 for
instance gives the user the option of
changing the grey surround
to an even colours to reduce
flicker.
Some Super VGA boards support non-interlaced monitors and
even increased vertical scan rates
up to 70Hz. Changing to a non-interlaced monitor makes the biggest
difference but a scan rate of 70Hz is
also worthwhile. Notice that to make use of non-interlaced high scan rate modes you need a video card and monitor that
supports them.
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