DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

A decoder cannot determine
whether a source was encoded
with narrowband or wideband
components. An analog decoder
cannot determine which axes were
used by the encoder.
With wideband U and V
components in the studio, full
I bandwidth could have been
retained in NTSC broadcast if
wideband chroma had been
decoded, then reencoded using
wideband I and narrowband Q
prior to lowpass filtering of the
composite signal to 4.2 MHz.
Such decoding and reencoding
has virtually never been done.
However, recovery of the wideband I component was
rarely implemented even in early consumer receivers, so
the promise of NTSC’s design was never fulfilled. Later,
the low chroma bandwidth realizable in consumer VCRs
had the effect of limiting the maximum usable color
difference bandwidth to less than 600 kHz in both
components. To recover color differences up to
600 kHz in an analog decoder, it is pointless to use the
Y’IQ scheme: a Y’UV decoder fitted with 600 kHz filters
is less complex, and might as well be used instead.
Owing to the technical difficulty of encoding Y’IQ properly
in the studio, and because few consumer receivers
recovered wideband chroma in any event, Y’IQ
encoding was gradually abandoned for studio encoders
during the 1970s and 1980s. In the studio, it became
common to encode U and V components both having
1.3 MHz bandwidth. In NTSC transmission, the
composite signal bandwidth cannot exceed 4.2 MHz.
This restriction is ordinarily imposed by naive lowpass
filtering. For a composite signal encoded with wideband
U and V, 4.2 MHz lowpass filtering caused crosscontamination
of all chroma components above
600 kHz. As these poor studio practices proliferated,
recovery of wideband I by a high-quality receiver
became more and more problematic: Cross-contamination
above 600 kHz at the transmitter became more
and more likely, and by about 1980, any prospect of
wideband chroma at the consumers’ premises was lost.
When SMPTE adopted the 170M standard in 1990,
wideband I and narrowband Q encoding was ostensibly
permitted. However, equiband 1.3 MHz encoding of U
and V components was permitted as well. A decoder
cannot possibly determine whether narrowband or
wideband components were encoded. In any event,
proper Y’IQ encoding as envisaged by the NTSC in 1953
was extremely rare by 1990 and is virtually unseen
today. Although wideband chroma is ubiquitous in the
studio, the establishment of SMPTE 170M had the
effect of permanently enshrining narrowband chroma
for broadcast: Upon lowpass filtering at the transmitter,
only narrowband chroma can be reliably recovered.
370 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES