DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

See Notch filtering, on page 349.
NTSC Y’IQ system 30
In the NTSC studio, 5.5 MHz of composite bandwidth is
available; wideband chroma of about 1.3 MHz can be
easily maintained. However, only 4.2 MHz of composite
bandwidth is available for NTSC broadcast. If equiband
U and V components are modulated onto a 3.58 MHz
color subcarrier, only 600 kHz of chroma bandwidth is
achieved. The designers of NTSC considered 600 kHz of
chroma bandwidth to be insufficient, and they devised
a scheme to form modulated chroma from I and Q
components, where Q was bandlimited to about
600 kHz, but where I preserves a higher bandwidth of
about 1.3 MHz. NTSC’s wideband I scheme incurred
some increase in complexity over equiband U and V
encoding; however, the NTSC decided in 1953 that the
improved color detail would be worthwhile.
Television receiver manufacturers found the NTSC’s
I and Q scheme costly, however, and the scheme never
reached significant deployment in receivers. In the
1950s and 1960s, studio encoders generated wideband
I signals, without the benefits being realized by
consumers. During the 1970s and 1980s, equiband
U and V encoders became dominant. In 1990, SMPTE
adopted standard 170M, which endorsed U and V
encoding. Sadly, that act effectively banished wideband
chroma from terrestrial broadcasting: Chroma bandwidth
for NTSC broadcast is now effectively limited to
600 kHz. Virtually no equipment today uses NTSC’s
Y’IQ scheme. However, it is historically and theoretically
important, and it is very widely – and inaccurately
– documented. So, I feel obliged to cover it here.
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