Dot Crawl.....Cause and Cure!! (1 Viewer)

[DavidMich]

I think I know what this is.
My questions are simple:
What causes this?
and
How do you get rid of it?
I have the contrast turned down, but I still see this effect every now and then. (Not very often).
Would an ISF calibration cure it?
Please help, as I am mystified on this one....Thanks!!

 

[RyanDinan]

Hi David,

Dot crawl is caused by the "imperfect" separation of the luminance (Black and White info) and chromiance (color info) in a composite signal. A comb filter built into the TV is used to separate these two signals. There are several types and qualities of comb filters, such as 2-line, 3-line, and 3D digital comb filters. The later is the best of the bunch, as it is far more precise, however, not perfect. Dot crawl tends to happen when luminance info is mis-interpreted as chromiance info, and the "rainbow" swirls are caused by the opposite.

An ISF calibration will not help the accuracy of the comb filter in your set.

However, if you are able to use S-video or component connections to your set (where the color and black & white info are sent separately), you will avoid all these problems. Keep in mind that the source device must be originally stored in one of the above-mentioned signals for this to be true. For example, DVD is stored as component video - So using component or S-video cables would avoid any separation problems, as the signals are kept separate from the begining.

Laserdisc on the other hand, is stored as composite, so the signal must be separated - either by the laserdisc player itself (if it offers S-video output) or by your TV. Since most newer TV's have better comb filters, it's better to use a composite conneciton from the laserdisc player, as it is most-likely older, and not as accurate.

Hope this helps!

-Ryan Dinan

 

[NickT]

Ryan has explained it very well. The only thing I will add is to stress the importance of the source. S video is not automaticly better. I have an S video output on my digital cable converter, but I use the RF or composite outputs because the comb filter in my TV is much better than the one in the cable box.

 

[John Royster]

I'll second Nick's observations. I have severe dot crawl using svideo out of my digital cable box. I'm assuming this means the comb filter in the cable box is pretty bad. What I am describing as dot crawl, is tiny dots crawling in the borders between solid colors...like red text on white background.

So, I use composite from the cable box with my mits 65819. Go figure.

 

[MickeS]

I use s-video from the digital cable box, because it removes the dot crawl for me better than with a composite connection. So I guess David just needs to try and see what works best for him.

 

[Allan Jayne]

For digital cable you may need to run both composite (for analog channels) and S-video (for digital channels) between the box and the TV.
Using S-video, digital channels that were never composite at any time earlier in their life will not have to suffer going through any comb filter.
Video hints:
http://members.aol.com/ajaynejr/video.htm

 

[John Royster]

Allan,

Is what I described called dot crawl?

 

[Kevin P]

Dot crawl tends to happen when luminance info is mis-interpreted as chromiance info, and the "rainbow" swirls are caused by the opposite.

Actually, it's the other way around. A comb filter misinterpreting luminance info as chroma will cause rainbow swirls (color artifacts); misinterpreting chroma as luminance causes dot crawl.

I remember having a TRS-80 Color Computer in my younger years, and since the video was clocked based on the chroma carrier frequency, you could display alternating white and black lines/dots on the screen and the TV's comb filter would interpret it as color (either red or blue, depending on which columns you made white or black). Pretty neat stuff. Sort of the ultimate color artifacts, but you could take advantage of them.

KJP

 

[DavidMich]

I really appreciate all the info!
I now feel enlightened!!
I am using Svideo from the satellite box, and component from the DVD player. It only happens very rarely, but as we all become perfectionists, I was trying to figure out how to eliminate it etirely.
I've got to tell you, that the Pioneer set is terriffic!!
Anyone looking for a RPTV needs to check them out!!!