DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Oversampling to double the number
of lines displayed during a frame
time is called line doubling.
synthesizes data for a larger image array (as in the
middle left of Figure 6.8). The synthetic data can be
displayed with a spot size appropriate for the larger
array, and all of the scan lines can be illuminated in
each 1 ⁄ 60 s instead of just half of them. This technique is
spatial oversampling. For a given level of scan-line visibility,
this technique enables closer viewing distance
than would be possible for progressive display.
If such oversampling had been technologically feasible
in 1941, or in 1953, then the NTSC would have
undoubtedly chosen a progressive transmission standard.
However, oversampling is not economical even in
today’s SDTV studio systems, let alone HDTV or
consumer electronics. So interlace continues to have an
economic advantage. However, this advantage is
eroding. It is likely that all future video system standards
will have progressive scanning.
Oversampling provides a mechanism for a sensor PSF or
a display PSF to have negative weights, yielding a
spatially “sharpened” filter. For example, a sharpened
Gaussian PSF can be obtained, and can achieve performance
better than a Gaussian. With a sufficient degree
of oversampling, using sophisticated filters having sinclike
PSFs, the interchange signal can come arbitrarily
close to the Nyquist limit. However, mathematical
excellence does not necessarily translate to improved
visual performance. Sharp filters are likely to ring, and
thereby produce objectionable artifacts.
If negative weights are permitted in a PSF, then negative
signal values can potentially result. Standard studio
digital interfaces provide footroom so as to permit
moderate negative values to be conveyed. Using negative
weights typically improves filter performance even
if negative values are clipped after downsampling.
Similarly, if a display has many elements for each digital
sample, a sophisticated digital upsampler can use negative
weights. Negative values resulting from the filter’s
operation will be clipped for presentation to the display
itself, but again, improved performance could result.
194 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES