Quote:
Originally Posted by andy
The dots in PAL also line up 180 degrees out of phase on adjacent lines. .
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It would have been good if they did, then PAL TV's could have implemented analog Y/C comb filters for sharper pictures since the 80's like the RCA Dimensia premium TV's since 1984
The NTSC developers foresaw this in the early 50s, but - 10 years later - the PAL developers screwed up - incorporating a 25Hz offset (to minimize dot crawl) resulting in an awkward 8-field 'Color Frame' sequence and really nasty artifacts like on those screen shots.
To comb filter a PAL TV, it requires expensive frame store digital processing chip-sets--not available until just before the phase out of PAL . Germany - where it started - ditched PAL for DTV a number of years ago.
Interesting link:
http://www.burnworld.com/dvd/primer/ntsc.htm
"Another important factor in choosing the new exact frame rate was to make sure that the color signal phase would be shifted exactly 180 degrees for each scanline. There are two reasons why this is important. First, the chroma signal does cause some distortion to older TV sets, especially those that were used at the time of the introduction of color TV and which didn't have notch filters to filter out the chroma information. In addition, early color tv sets (and newer cheap ones) suffer from imperfect luminance and chrominance separation, causing dots to appear near strong-colored edges. These dots are called creepy crawlies or, more commonly, dot crawl. They are particularly visible along vertical lines in the transmitted video, especially when SMPTE color bars are transmitted. The phase shift makes these dots non-stationary and thus reduces their visibility. The second reason to the phase shift is that it makes it possible to use a comb filter, which allows separating chrominance and luminance information with much better fidelity. While an exect 180 degree phase shift per scanline is not an absolute necessity for a comb filter to work, it makes implementation easier and also gives the best potential quality. This is a lesson that was later forgotten when developing the PAL color coding scheme. This probably didn't seem like a big omission at the time, since comb filters didn't become widely available in NTSC television sets before the 1980's (and, because of huge implementation difficulties, high-end PAL 100 Hz TV sets didn't get comb filters before the late 1990s). Nevertheless, the theoretical groundwork that made comb filters possible was there from the beginning"