DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Because an analog demodulator
cannot determine whether an
encoder was operating at 0° or 33°
phase, in analog usage the terms
Y’UV and Y’IQ are often used somewhat
interchangeably. See also Y’UV,
Y’IQ confusion, on page 311.
Hazeltine Corporation, Principles
of Color Television, by the
Hazeltine Laboratories staff,
compiled and edited by Knox
McIlwain and Charles E. Dean
(New York: Wiley, 1956).
Y’IQ coding is unique to composite NTSC: It has no
place in PAL, component video, HDTV, or computing. If
color components are kept separate, it is incorrect to
apply I and Q (or U and V) scaling or notation. PAL and
SECAM are based upon equiband U and V components;
component analog systems use equiband Y’P B P R ;
and component digital systems use equiband Y’C B C R .
Composite digital 4f SC NTSC systems defined in SMPTE
244M sample on the [I, Q] axes. These systems are
properly described as Y’IQ, but there is no requirement
for narrowband Q filtering, and it is rarely (if ever) used.
Narrowband Q
With subcarrier at about 3.6 MHz, and baseband
chroma component bandwidth of 1.3 MHz, the upper
sideband of modulated chroma extends to about
4.9 MHz. The designers of NTSC were faced with
a channel bandwidth of 4.2 MHz, insufficient to convey
the upper sideband of modulated chroma. Bandwidth
limitation of the composite signal would cause quadrature
crosstalk – that is, cross-contamination of the
chroma components above 600 kHz.
Vision has less spatial acuity for purple-green transitions
than it does for orange-cyan. The U and V signals
of Y’UV must be carried with equal bandwidth, albeit
less than that of luma, because neither aligns with the
minimum color acuity axis. However, if signals I and Q
are formed from U and V, as I will describe, then the Q
signal can be more severely filtered than I – to about
600 kHz, compared to about 1.3 MHz – without any
loss in chroma resolution being perceived by a viewer at
typical viewing distance.
The NTSC’s desire for 1.3 MHz orange-cyan detail led to
the narrowband Q scheme. I and Q color difference
components are formed from U and V by a 33° rotation
and an exchange of axes. The Q axis is aligned with the
NTSC’s estimate of the minimum color acuity axis of
vision. Q can thereby be more severely bandlimited
than I. The NTSC decided to bandlimit I to 1.3 MHz and
Q to 0.6 MHz (600 kHz). Quadrature modulation is
then performed with subcarrier phase-shifted 33°.
366 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES