DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

TEST SCENE
SCANNING
FIRST FIELD
Image row pitch
SECOND FIELD
Figure 6.6 Twitter would
result if this scene were
scanned at the indicated line
pitch by a camera without
vertical filtering, then
displayed using interlace.
Twitter
The flicker susceptibility of vision stems from a widearea
effect: As long as the complete height of the
picture is scanned sufficiently rapidly to overcome
flicker, small-scale picture detail, such as that in the
alternate lines, can be transmitted at a lower rate. With
progressive scanning, scan-line visibility limits the
reduction of spot size. With interlaced scanning, this
constraint is relaxed by a factor of two. However, interlace
introduces a new constraint, that of twitter.
If an image has vertical detail at a scale comparable to
the scanning line pitch – for example, if the fine pattern
of horizontal line pairs in Figure 6.6 is scanned – then
interlaced display causes the content of the first and the
second fields to differ markedly. At practical field rates –
50 or 60 Hz – this causes twitter, a small-scale phenomenon
that is perceived as a scintillation, or an extremely
rapid up-and-down motion. If such image information
occupies a large area, then flicker is perceived instead
of twitter. Twitter is sometimes called interline flicker,
but that is a bad term because flicker is by definition
a wide-area effect.
Twitter is produced not only from degenerate images
such as the fine black-and-white lines of Figure 6.6, but
also from high-contrast vertical detail in ordinary
images. High-quality video cameras include optical
spatial lowpass filters to attenuate vertical detail that
would otherwise be liable to produce twitter. When
computer-generated imagery (CGI) is interlaced, vertical
detail must be filtered in order to avoid flicker. A circuit
to accomplish this is called a twitter filter.
Interlace in analog systems
Interlace is achieved in analog devices by scanning
vertically at a constant rate between 50 and 60 Hz, and
scanning horizontally at an odd multiple of half that
rate. In SDTV in North America and Japan, the field rate
is 59.94 Hz; line rate (fH) is 525 ⁄2 (2621 ⁄2) times that
rate. In Asia, Australia, and Europe, the field rate is
50 Hz; the line rate is 625 ⁄2 (3121 ⁄2) times the field rate.
CHAPTER 6 RASTER SCANNING 57