DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 6.5 Interlaced scanning
forms a complete
picture – the frame – from
two fields, each comprising
half of the total number of
scanning lines. The second
field is delayed by half the
frame time from the first. This
example shows 525 lines.
I detailed spot profile in Image
structure, on page 43.
It is confusing to refer to fields as
odd and even. Use first field and
second field instead.
RCA trademarked the word
Proscan, but RCA – now Thomson –
confusingly uses that word to
describe both progressive and
interlaced television receivers!
1
2
...
262
525
263
t t+ 1 ⁄ 59.94 s
Interlaced scanning
I have treated the image array as a matrix of S AL by L A
pixels, without regard for the spatial distribution of light
intensity across each pixel – the spot profile. If spot
profile is such that there is a significant gap between
the intensity distributions of adjacent image rows (scan
lines), then scan-line structure will be visible to viewers
closer than a certain distance. The gap between scan
lines is a function of scan-line pitch and spot profile.
Spot size can be characterized by spot diameter at 50%
intensity. For a given scan-line pitch, a smaller spot size
will force viewers to be more distant from the display
if scan lines are to be rendered invisible.
Interlacing is a scheme by which we can reduce spot
size without being thwarted by scan-line visibility. The
full height of the image is scanned with a narrow spot,
leaving gaps in the vertical direction. Then, 1⁄ 50 or 1⁄ 60 s
later, the full image height is scanned again, but offset
vertically so as to fill in the gaps. A frame now
comprises two fields, denoted first and second. The
scanning mechanism is depicted in Figure 6.5 above.
For a given level of scan-line visibility, this technique
enables closer viewing distance than would be possible
for progressive display. Historically, the same raster
standard was used across an entire television system, so
interlace was used not only for display but also for
acquisition, recording, and transmission.
Noninterlaced (progressive or sequential) scanning is
universal in desktop computers and in computing.
Progressive scanning has been introduced for digital
television and HDTV. However, the interlace technique
remains ubiquitous in conventional broadcast television,
and is dominant in HDTV.
56 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
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