Information Technology Reference
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the after glowing of the layer of phosphorus increases the effect of the inertia. This
advantageous technological side effect makes interlaced video content look better
on a classic CRT screen than on a modern LCD or plasma screen. The interlacing
technique results in a gross bandwidth reduction of 50%. However, this comes at
the expense of visual artefacts associated with interlacing, including interline twit-
ter, line crawling and field aliasing [5].
The higher spatial resolution of 1080i seems to be an advantage but this is rela-
tive. First of all, vertical low-pass filtering is typically applied on interlaced video
in order to suppress interline twitter [5], effectively reducing the vertical resolu-
tion in 1080i. Secondly, it is known that the optical resolution of a human eye lim-
its the ability to distinguish details to an angle of approximately 1 arc minute
(1/60th of a degree). Within this angle, details cannot be distinguished. It is typi-
cally assumed that in an average living room, the viewing distance for a television
set is about 2.7 m [6]. Simple geometry then shows that a screen must be about 85
cm high to allow the distinction of two neighbouring lines in an image of 1920 by
1080 pixels. This corresponds to a screen with a diagonal of 1.75 m or 68”. This
type of screens are currently too cumbersome, heavy and expensive for the aver-
age consumer. In contrast, a vertical resolution of 720 lines for 720p corresponds
to a screen with a diagonal of 50”, which is more realistic.
Another advantage of 720p is that it offers a better line refresh rate (50-60
frames per second times 720 lines versus 50-60 fields per second times 540 lines
for 1080i). This results in a better viewing experience for fast moving content [7].
Motion blur and jitter are less observed with 720p than with 1080i. The fact that
LCD and plasma screens are inherently progressive further contributes to the ap-
peal of 720p. When offered interlaced video content, such screens have to convert
it to progressive content before displaying. The quality of this conversion depends
on the complexity and the processing power, and thus the price of the built-in
conversion chips. Professional solutions cost about 10.000 Euro per system. Evi-
dently screen manufacturers are forced to use much cheaper, less efficient devices.
For these reasons, existing 1080i video content should be converted to 720p by
professional equipment at the broadcast stations prior to distribution, instead of re-
lying on less efficient de-interlacing mechanisms built into current television sets.
Another advantageous factor for 720p is the content representation efficiency.
1080i25 content consists of 1920 x 1080 x 25 = 5184106 pixels per second while
720p50 content consists of 1280 x 720 x 50 = 4608106 pixels per second, corre-
sponding to a difference of more than 10% in favour of 720p. Moreover, subjec-
tive tests by EBU indicate that compressed 1080i25 content needs 20% more
bit-rate than compressed 720p50 to obtain a comparable visual quality [3].
The above arguments indicate that 720p is the best HD format for recording,
production and distribution. However, 1080i has some practical advantages over
720p. A first advantage is the better support for 1080i in current production sys-
tems and in (tape-based) cameras, mostly because the format has already been in
use for some time in the United States and Japan. For the same reason there is a
larger availability of 1080i content. For example, HD recordings of the last Olym-
pic Games were distributed in the 1080i format. A second advantage is the ability
to use identical time codes in 1080i as in SD. Time codes are essential for
