DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Y’
I
Q
1.3 MHz
600 kHz
SUBCARRIER
OSCILLATOR
sin
cos
+33°
+33°
QUADRATURE
MODULATOR
-1
ADDER CHROMA
BANDPASS
FILTER
∑
BURST
INVERTER
Y’IQ encoding
BURST
INSERTER
CHROMA
MODULATOR
Y’/C
COMBINER
The block diagram of a Y’IQ encoder is shown in
Figure 30.2. The I and Q components are lowpass
filtered to 1.3 MHz and 0.6 MHz respectively, then
modulated in a manner similar to Equation 28.7 on
page 338:
C = Qsin( ωt+ 33° ) + Icos( ωt+ 33° ) ; ω=
2π
fSC
Eq 30.4
A decoder for Y’IQ is shown in Figure 30.3 at the top of
the facing page. Q is decoded using a 600 kHz lowpass
filter. Wideband I is decoded using a Nyquist filter: The
low-frequency component of I (from 0 to 0.6 MHz) is
recovered from both sidebands of modulated chroma,
and its high-frequency component (from 0.6 MHz to
1.3 MHz) is recovered from the lower sideband alone.
Filtering at the encoder and decoder are sketched in
Figure 30.4 at the bottom of the facing page.
A Y’IQ decoder recovers wideband I and narrowband Q
only if the signal was encoded on the [I, Q] axes from
wideband I and narrowband Q components. Until
1960s, it was reasonable to assume that an NTSC signal
was encoded in this way, as the NTSC intended.
368 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
∑
COMPOSITE
NTSC
VIDEO
Figure 30.2 NTSC encoder using Y’IQ is similar to the NTSC encoder using Y’UV components
presented in Figure 28.4, on page 340. Here, I and Q components are presented to the encoder.
Historically, the I and Q filters had unequal bandwidths as indicated in this diagram, but today’s
4f SC NTSC studio encoders use equiband 1.3 MHz filters. The continuous-wave sine and cosine
color subcarrier signals are phase-shifted 33° prior to being presented to the quadrature multipliers.