DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Studio SDTV has 720 S AL ;
resolution higher than
540 TVL/PH is pointless.
The NTSC, in 1941, was well aware of the Kell factor,
and took it into account when setting the monochrome
television standard with 525 total lines and
about 480 picture lines. The numbers that I have
quoted work out perfectly to achieve matched vertical
and horizontal resolution, but there is no evidence that
the NTSC performed quite this calculation.
The relationship between bandwidth (measured in engineering
units, MHz) and horizontal resolution
(measured in consumer units, TVL/PH) depends upon
blanking overhead and aspect ratio. For 480i systems:
1MHz 1 SPW · 2·
·
fH
AR STL 1MHz
3 711
= · 2·
·
15. 734 kHz 4 858
TVL/PH
= 79
MHz
In 480i video, there are 79 TVL/PH per megahertz of
bandwidth. NTSC broadcast is limited to 4.2 MHz, so
horizontal resolution is limited to 332 “TV lines.” In
576i systems, there are 78 TVL/PH per megahertz of
video. Most 625-line PAL broadcast systems have bandwidth
roughly 20% higher than that of NTSC, so have
correspondingly higher potential resolution.
Viewing distance
Pixel count in SDTV and HDTV is fixed by the corresponding
scanning standards. In Viewing distance and
angle, on page 8, I described how optimum viewing
distance is where the scan-line pitch subtends an angle
of about 1 ⁄ 60 °. If a sampled image is viewed closer than
that distance, scan lines or pixels are liable to be visible.
With typical displays, SDTV is suitable for viewing at
about 7·PH; 1080i HDTV is suitable for viewing at
a much closer distance of about 3·PH.
A computer user tends to position himself or herself
where scan-line pitch subtends an angle greater than
1 ⁄60 ° – perhaps at half that distance. However, at such
a close distance, individual pixels are likely to be
discernible, perhaps even objectionable, and the quality
of continuous-tone images will almost certainly suffer.
72 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
Eq 7.1