DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

525
910
768
483
NTSC
Figure 12.3 480i, 4f SC NTSC
sampling is line-locked. If the
analog sync edge were to be
digitized, it would take the
same set of values every line.
625
576
1135 4 ⁄ 625
PAL
948
Figure 12.4 576i, 4f SC PAL
sampling is not line-locked.
If you had to give 4f SC a designation
akin to 4:2:2, you might call it 4:0:0.
In industrial and consumer video, subcarrier often freeruns
with respect to line rate, and the advantages of
frequency interleaving are lost. Most forms of analog
videotape recording introduce timebase error; left
uncorrected, this also defeats frequency interleaving.
Composite digital SDTV (4fSC )
Processing of digital composite signals is simplified if
the sampling frequency is a small integer multiple of the
color subcarrier frequency. Nowadays, a multiple of four
is used: It is standard to sample a composite NTSC or
PAL signal at four-times-subcarrier, or 4fSC (pronounced
four eff ess see.)
In 4f SC NTSC systems sampling rate is about 14.3 MHz.
Because NTSC’s subcarrier is a simple rational multiple
( 455 ⁄2) of line rate, sampling is line-locked. In linelocked
sampling, every line has the same integer
number of sample periods. In 4f SC NTSC, each line has
910 sample periods (S TL), as indicated in Figure 12.3.
In conventional 576i PAL-B/G/H/I systems, the 4f SC
sampling rate is about 17.7 MHz. Owing to the complex
relationship in “mathematical PAL” between subcarrier
frequency and line rate, sampling in PAL is not linelocked:
There is a noninteger number (1135 4 ⁄625) of
sample periods per total line, as indicated in
Figure 12.4 in the margin. (In Europe, they say that
“Sampling is not precisely orthogonal.”)
During the development of early studio digital standards,
the disadvantages of composite video processing
and recording were widely recognized. The earliest
component digital video standard was Rec. 601,
adopted in 1984; it specified a component video interface
with 4:2:2 chroma subsampling and a sampling
rate of 13.5 MHz, as I described in the previous
chapter. Eight-bit sampling of Rec. 601 has a raw data
rate of 27 MB/s. The first commercial DVTRs were standardized
by SMPTE under the designation D-1. (In
studio video terminology, chroma subsampling is not
considered to be compression.)
108 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES