The Engineer's Guide to Standards Conversion - Snell

Vertical
frequency
Stop
band
Temporal
frequency
Triangular
passband
Fig 3.5.2
In order to return to the baseband in an interlaced system a twodimensional
filter with a triangular response is required.
The interpolation process must incorporate a two dimensional filter having a
triangular passband shown in Fig 3.5.2 which passes the baseband spectrum and
rejects the images. The interpolator works in two dimensions to express the input
data at a different line and field rate. In some cases it is possible to construct a two
dimensional interpolator using two one-dimensional filters in series.
Fig 3.5.3 shows how this can be done. Unfortunately the result must always be a
rectangular two-dimensional spectrum and it should be clear that this is of no use
whatsoever for filtering an interlaced signal. Fig 3.5.4a) shows the structure of a
four field by four line standards converter. Field and line delays are combined so
that simultaneous access to sixteen pixels is available.
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