Interlacing allows you to double the number of lines appearing on the video screen.
Generally,
in
non-interlaced mode,
200
lines
fill
the screen and a playfield of normal size
appears full-sized. In interlaced mode, normally, a maximum of 400 lines
fill
the screen.
Twice as much
data
is
displayed
in
the same vertical area as in non-interlaced mode.
In
interlaced mode, the scanning circuitry vertically offsets the
start
of every other
field
by half a scan line.
Line 1
Field 1
line
1
Field 2
\
Line 1
/
Line 2
Video Display
(400 lines)
(Same
physical
space
as
used
by
a 200 line non interlaced display.)
Figure
3-5:
Interlacing
Even though interlaced mode requires a modest amount of extra work
in
setting registers
(as you will see later on
in
this section), it provides
fine
tuning
that
is needed for certain
graphics effects. Consider the diagonal line in figure
3-6
as
it
appears
in
non-interlaced
and interlaced modes. Interlacing eliminates much of the jaggedness or "staircasing"
in
the edges of the line.
Playfield Hardware
39
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