Interlaced Video Scans is an obsolete bandwidth-saving technique developed
during the establishment of early television standards to reduce the amount of
image data needed to render a perceived complete picture. The NTSC video format
definition is 30 (29.97) interlaced frames per second at 480 vertical lines of
resolution per video frame. Each frame is composed of two fields, consisting of
262.5 scan lines per field for a total of 525 scan lines. At any one moment, only
half the picture is displayed. Through persistence of vision, a human perceives a
series of interlaced frames to be a complete picture.
One problem of interlaced scans is object and camera movements. Highlight
details against a dark background can clearly be seen as a comb effect (see below
example). The lines of the previous interlaced field trail the lines of the current
field in combination with the relatively low refresh rate of the NTSC standard to
create the effect.
Progressive video scanning eliminates this issue by displaying all resolution lines
in a frame at one time. Many current HD systems (disc players and HD monitors)
are capable of converting an interlaced signal into a progressive scan signal
(commonly known as line doubling). We heartily recommend line doubling
DVDs of silent films to create a more filmlike picture.
Unfortunately, interlaced video is still used in DVD home video products today.
More details on interlaced video scans may be found on Wikipedia.
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