DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Certain elements of the vertical
interval in NTSC – particularly
VITC and closed caption data –
are mandated to have zero setup.
These elements can’t be carried
through component equipment,
because Rec. 601’s footroom
does not extend to 0 IRE.
In the past, to be compliant with FCC regulations for
broadcast, an NTSC modulator was supposed to
operate on I and Q components, where the Q component
was bandwidth limited more severely than the I
component:
C = Qsin( ωt+ 33° ) + Icos( ωt+ 33° ) ; ω=
2π
fSC
Eq 42.2
The bandwidth limitation of Q, to about 600 kHz, was
specified in the original design of NTSC to permit accurate
recovery of the 1.3 MHz wideband I component
from a signal transmitted through a 4.2 MHz channel.
Y’IQ was important in the early days of NTSC, but
contemporary NTSC equipment modulates equiband U
and V, where only 600 kHz of bandwidth can be recovered
from each color difference signal. This practice has
been enshrined in SMPTE 170M. See Y’IQ encoding, on
page 368.
The Y’ and C components should be time-coincident
within ±25 ns. Error in chroma timing is known as
chroma-luma delay.
Setup
480i composite video usually has 7.5% setup, which
I introduced in Setup (pedestal), on page 327: Luma is
scaled and offset so that reference white remains at
unity, but reference black is raised by the fraction 7.5%
of full scale. The luma signal remains at zero – reference
blanking level – during intervals when no picture is
being conveyed.
When setup is used, the same scale factor that is
applied to luma must also be applied to chroma, to
ensure that the peak excursion (luma plus modulated
chroma) remains at + 4 ⁄3 of reference white, or, in other
words, to ensure that the peaks of 75% colorbars
exactly match reference white at 100% (100 IRE):
601 3 37 601
setupY'
= + Y'
40 40
37
C = C
setup 40
Setup originated in the vacuum tube era to prevent
retrace lines from becoming visible due to unstable
514 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
Eq 42.3