DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 7.6 Vertical resolution in
480i systems can’t quite reach
the Nyquist limit of 240 cycles
(line pairs), owing to Kell and
interlace factors. Vertical resolution
is diminished, typically to
7 ⁄10 of 240 – that is, to 166 C/PH.
Equivalent horizontal resolution
to 166 C/PH is obtained by
multiplying by the 4:3 aspect
ratio, obtaining 221 C/PW.
Picture content consumes
about 85% of the total line
time. Dividing 221 C/PW by
0.85 yields 260 cycles per total
line. Line rate is 15.734 kHz;
260 cycles during one complete
line period corresponds to
a video frequency of about
4.2 MHz, the design point of
NTSC. There are 79 “TV lines”
per megahertz of bandwidth.
221
= 260
1-0. 15
480
221 C/PW
260 C/total line
= 4.2 MHz
332 TVL/PH
(“332 lines“)
As a consequence, early interlaced systems showed no
advantage in resolution over progressive systems of the
same bandwidth. However, scan lines were much less
visible in the interlaced systems.
Figure 7.6 above summarizes how vertical and horizontal
spatial frequency and bandwidth are related for
480i television. The image height is covered by
480 picture lines. Sampling theory limits vertical image
content to below 240 C/PH if aliasing is to be avoided.
Reduced by Kell and interlace factors combining to
a value of 0.7, about 166 C/PH of vertical resolution can
be conveyed. At 4:3 aspect ratio, equivalent horizontal
resolution corresponds to 4 ⁄3 times 166, or about
221 C/PW. For a horizontal blanking overhead of 15%,
that corresponds to about 260 cycles per total line
time. At a line rate of 15.734 kHz, the video circuits
should have a bandwidth of about 4.2 MHz. Repeating
this calculation for 576i yields 4.7 MHz.
CHAPTER 7 RESOLUTION 71
166 C/PH
× × 3
2
4
Expressed in TV lines, 166 C/PH
is multiplied by 2, to obtain 332.
× 2
TVL/PH
79
MHz