Confused about RPTV Comb Filters

Posted by Allan [IP: 157.123.138.60] on January 28, 1999 at 12:04:34:

In Reply to: Confused about RPTV Comb Filters posted by Thomas Webb on January 27, 1999 at 17:17:54:

From the shopper's perspective:

I would look for a comb filter overtly advertised as at least "three line 2D" or (same thing) "three line adaptive". Still I would look at and evaluate the picture since even in the "3 line 2D" category quality can vary. Dot crawl along horizontal color boundaries is the most obvious deficiency of the least expensive comb filters. (Cartoons are not a good subject to evaluate dot crawl with when color patches are separated by black outlines.) Next look for "rainbow swirls" amidst closely spaced vertical lines, the less the better. Pinstripe shirts and picket fences are good subject matter to see this. Still better comb filters have little or no dot crawl up and down vertical color boundaries (side by side patches).

An RPTV is no more likely to have a particular kind of filter than a direct view TV. The comb filter does not improve reception of distant stations. When you are viewing an S-video source, notable DSS and DVD, the comb filter in the TV set is not called into play.

A two line comb filter is much more ordinary and I wouldn't bother with TV sets with just one of those. It is likely to leave a lot of dot crawl. If the number of lines is not specified, it is a two line. Two line comb filters don't have more than one dimension ("1D").

A "3D" comb filter is a big step up in price. At this time I think the visual experience of having a "3D" comb filter is not worth the added cost. Evaluate the picture yourself, particularly moving subjects. Again quality varies from one brand and model to the next. Also you have to be careful because sometimes the manufacturer skimps on quality of some parts of that comb filter so that, although still pictures look superb, moving subjects are surrounded by dot crawl.

Makers of "3D" comb filters claim that theirs outstrip all the rest but a few makers (Faroudja, Crystal Vision) of "2D" filters say theirs come very close to the best "3D" filters.

Ignore the word "Y/C". All comb filters belong to the family of circuits called correctly "Y/C separators". "Y" means luminance; "C" means color.

Ignore the word "digital" as it applies to comb filters. It is often used incorrectly or deceptively in advertising.

It is rare but possible for a comb filter to be out of adjustment so as to give a picture worse than on a TV set "without" a comb filter.

Other comb filter terminology:

"Glass" or "acoustic" and "CCD" refer to ordinary, relatively inexpensive, comb filters.

"User switchable notch filter" reduces dot crawl by sacrificing horizontal resolution, essentially doing what TV sets without comb filters do. However some excellent comb filters contain notch filters that are sparingly called into play automatically.

"1H" equals "two line" which is ordinary. "2H" equals "three line", much better. "Lines" (1H, 2H) and "dimensions" (1D, 2D, 3D) are separate topics. All comb filtering involves taking scan lines at least two at a time.

"Adaptive" means having at least two dimensions, or varying the filtering from left to right across every scan line. I think it is desirable to have. If neither the word "adaptive" nor the number of dimensions is specified, the comb filter probably has just one dimension, or an unchanging filtering formula which is likely to leave some color smearing along horizontal color boundaries that TV sets without comb filters don't leave.

• More on comb filters

Follow Ups:

• Confused about RPTV Comb Filters - ML 10:46:13 01/29/99 (1)
  • Confused about RPTV Comb Filters - ML 10:50:37 01/29/99 (0)

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