ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

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During playback of an interlaced video half of one frame is continually being
merged with the previous picture. If the encoder were to treat each frame as
progressive (a complete picture that is not interlaced) then inevitably artifacting
would occur when half of a new frame was merged with the remaining half of the
previous frame, particularly where the new frame differs significantly from the
last.
Interlacing artifacts therefore appear as
two slightly different pictures being
merged on alternate horizontal lines and
are most visible where there is fast
motion in a scene.
De-interlacing is the process of taking an
interlaced video and recombining the
fields so as to create a progressive, non-
interlaced output.
Consider that if the encoder were to assume interlaced content was in fact
progressive then when motion occurred in the image interlacing artifacts would
be present. Greater motion causes greater artifacting and inconsistency in the
texture between frames. The motion search must be able to match blocks in the
current frame back to similar areas in the reference frame and an inconsistent
picture will cause this process to fail. Consequently, a higher proportion of blocks
will be intra-coded leading to a higher bit spend per frame, forcing the rate control
to use higher quantizers and lowering the overall quality of the encoded video.
There are some circumstances where it is desirable to maintain the original
interlaced fields discreetly so as they can be recreated without artifacting for
display on an interlaced device. One example of such a situation is playback via
a DivX Certified device to an interlaced television.
To achieve this the encoder can be set to
Encode as interlaced . Fields will be
preserved, however encoding each field individually requires a higher bitrate than
is normally required when encoding a progressive source.
Why de-interlace?
Interlacing artefacts
Maintaining interlacing