DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Back porch is described in
Analog horizontal blanking
interval, on page 405.
as 0 IRE; reference white level is 100 IRE. The range
between these values is the picture excursion.
In the analog domain, sync is coded at voltage level
more negative than black; sync is “blacker than black.”
The ratio of picture excursion to sync amplitude is the
picture:sync ratio. Two different ratios are standard:
10:4 is predominant in 480i and computing; 7:3 is
universal in 576i and HDTV, occasionally used in 480i,
and rarely used in computing.
Setup (pedestal)
In 480i composite NTSC video in North America, reference
black is offset above blanking by 7.5% ( 3 ⁄ 40 ) of the
picture excursion. Setup refers to this offset, expressed
as a fraction or percentage of the picture excursion. In
a 480i system with setup, there are nominally 92.5 IRE
units from black to white.
Blanking level at an analog interface is established by
a back porch clamp. However, in a system with setup,
no signal element is present that enables a receiver to
accurately recover black level. If an interface has poor
tolerance, calibration error, or drift, setup causes problems
in maintaining accurate black-level reproduction.
Consequently, setup has been abolished from modern
video systems: Zero setup is a feature of EBU N10
component video, all variants of 576i video, and HDTV.
In all of these systems, blanking level also serves as the
reference level for black.
480i video in Japan originally used setup. However, in
about 1985, zero setup was adopted; 10:4 picture-tosync
ratio was retained. Consequently, there are now
three level standards for analog video interface.
Figure 27.2 overleaf shows these variations.
The archaic EIA RS-343-A standard specifies monochrome
operation, 2:1 interlace with 60.00 Hz field
rate, 7 µs horizontal blanking, and other parameters
that have no place in modern video systems. Unfortunately,
Most PC graphics display standards have inherited
RS-343-A’s 10:4 picture-to-sync ratio and 7.5%
setup. (Some high-end workstations have zero setup.)
CHAPTER 27 VIDEO SIGNAL PROCESSING 327