The Engineer's Guide to Standards Conversion - Snell

The Fourier transform of the frequency response yields the impulse response, and
this must then be sampled in two dimensions to obtain coefficients. The impulse
must be displaced by all of the necessary interpolation phases in two dimensions,
and sampled at each one into a coefficient set. As the impulse is symmetrical in two
axes, it is only necessary to store one quarter of it in ROM, the remaining three
quarters can be obtained by mirroring the vertical and/or horizontal ROM
addresses.
A vertical aperture of eight points (four per field) is sufficient for adequate
suppression of vertical artifacts, and a temporal aperture of four fields is wide
enough to remove temporal artifacts. Four field standards converters are too
expensive for some applications, and cost effective machines having two field
apertures are available. With such a short temporal aperture, it is not possible to
reach an acceptable compromise between roll-off and ripple.
Eliminating 5 Hz beating is very difficult because positioning a response null to
eliminate it results in passing the frequencies responsible for judder and vice-versa.
It is possible to increase the temporal aperture to six fields, and in theory this
produces a sharper cut-off and better suppression. However, on real input signals
the improvement will not be realised because of temporal aliasing actually in the
input signal. Another consequence of increasing the temporal aperture is that
motion portrayal is compromised.
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