DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Twitter and scan-line visibility are
inversely proportional to the count
of image rows, a one-dimensional
quantity. However, sharpness is
proportional to pixel count, a twodimensional
(areal) quantity. To
overcome twitter at the same
picture angle, 1.4 times as many
image rows are required; however,
1.2 times as many rows and 1.2
times as many columns are still
available to improve picture angle.
However, to maintain the same sharpness as a progressive
system at a given data capacity, all else being
equal, in interlaced scanning only half the picture data
needs to be transmitted in each vertical scan period
(field). For a given frame rate, this reduction in data per
scan enables pixel count per frame to be doubled.
The pixels gained could be exploited in one of three
ways: By doubling the row count, by doubling the
column count, or by distributing the additional pixels
proportionally to image columns and rows. Taking the
third approach, doubling the pixel count would increase
column count by 1.4 and row count by 1.4, enabling
a reduction of viewing distance to 0.7 of progressive
scan. This would win back the lost viewing distance
associated with twitter, and would yield equivalent
performance to progressive scan.
Ideally, though, the additional pixels owing to interlaced
scan should not be distributed proportionally to
picture width and height. Instead, the count of image
columns should be increased by about 1.7 (1.4×1.2),
and the count of image rows by about 1.2. The 1.4
increase in the row count alleviates twitter; the factor
of 1.2 increase in both row and column count yields
a small improvement in viewing distance – and therefore
picture angle – over a progressive system.
Interlaced scanning was chosen over progressive in the
early days of television, half a century ago. All other
things being equal – such as data rate, frame rate, spot
size, and viewing distance – various advantages have
been claimed for interlace scanning.
• If you neglect the introduction of twitter, and consider
just the static pixel array, interlace offers twice the static
resolution for a given bandwidth and frame rate.
• If you consider an interlaced image of the same size as
a progressive image and viewed at the same distance –
that is, preserving the picture angle – then there is
a decrease in scan-line visibility.
74 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES