1. Join Now

    AVForums.com uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Why does my comb filter better seem better than the broadcasters'?

Discussion in 'Projectors, Screens & Video Processors' started by David Mackenzie, Jul 21, 2005.

  1. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    OK, I *guess* this would go in this forum. It's less of a question about a specific product and more a general curiosity that maybe someone in here with experience in working in video can answer.

    Here goes. Why is the comb filter in my Sony LCD WEGA TV better than the ones that most broadcasters seem to have? All too often, I've watched TV shows that have been sourced from NTSC Composite masters, through my Freeview decoder (connected of course through RGB SCART). It doesn't irritate me, but I'm just really curious as to why I can still see a lot of visible dot crawl. The same goes for a lot of DVDs of American TV shows also taken from NTSC Composite, bought from both Regions 1 and 2 (The Simpsons is an example). My DVD player is connected through Component.

    Now when I connect the DVD player playing a DVD like the Simpsons box sets over the Composite video connection (Yellow RCA), the comb filter on my LCD WEGA does a much better job. Not only does it obliterate close to all of the dot crawl being added by using this connection method, but it removes most of what's present on the MPEG-2 file on the disc as well.

    Again it doesn't bother me but I'm curious, surely this shouldn't be right? I'm certain the LCD WEGA has a shockingly good comb filter in it, but shouldn't the studio equipment be better? Or is all of their equipment just a little more out of date?

    Wondering if any tech-heads can clear this up for me! Thanks!
     
  2. Ryu

    Ryu
    Active Member

    Joined:
    Dec 9, 2004
    Messages:
    251
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Ratings:
    +22
    I have to say I have also noticed this on my friend's Sony Wega. He isn't very AV minded and he had connected up his DVD player through the SCART socket but with the output on the DVD set to composite. Now even to my trained eye at first glance I couldn't tell if he had set it up correctly or not. I had to get real close to the screen and look at some red text to see any noticeble sign that it was actually processing a composite signal. Hats off to Sony and their comb filters.
     
  3. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    Yeah, it's incredible stuff. I remember plugging in my DVD player over Composite and setting the picture to NTSC, I actually sat and laughed because I thought it was some sort of joke. It works almost as well for PAL signals which seem harder to filter, but doesn't work very well at all for PAL-60 but luckily I don't use anything that sends out a PAL-60 signal.
     
  4. pwood

    pwood
    Active Member

    Joined:
    Dec 6, 2003
    Messages:
    2,479
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    66
    Location:
    Central Scotland
    Ratings:
    +136
    In simple terms, what exactly is a comb filter?
     
  5. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    It seperates the Brightness and Colour information from one another, because when you use Composite Video, they're both "smushed in" to a single cable. If the Composite signal is shown "as is" without any Comb Filtering, you get this with lots of dot crawl (NTSC example, PAL is a little nicer looking):

    [​IMG] [​IMG]

    Before and after

    Comb filters seem very much a part of American A/V talk, perhaps because on NTSC images, the dot crawl is a bit more pronounced. It's still there on PAL Composite pictures though, make no mistake, but it seems that most comb filters don't do as good a job cleaning it off PAL video. It's almost as if the crawling dots on PAL pictures are smaller and are harder for the filter to 'catch', but hopefully someone can give me a better explanation.

    In fact, most of the cheaper CRT sets sold in Europe don't seem to have comb filters at all - not even for NTSC.
     
  6. MartinImber

    MartinImber
    Active Member

    Joined:
    Apr 5, 2001
    Messages:
    3,851
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    71
    Location:
    Worcester
    Ratings:
    +21
    I don't think my TV has a comb filter
     
  7. Carl Ed

    Carl Ed
    Standard Member

    Joined:
    Mar 9, 2005
    Messages:
    54
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    6
    Ratings:
    +0
    Cheaper TVs use a notch filter instead of a comb filter. They soften the picture substantially.
     
  8. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    I think VCRs use notch filters, for obvious reasons. That's probably why I was surprised at seeing more dot crawl on LaserDiscs than video tapes.
     
  9. Chris5

    Chris5
    Active Member

    Joined:
    Mar 21, 2004
    Messages:
    1,161
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    63
    Location:
    Herts., uk
    Ratings:
    +122
    I don't think dot crawl is anything to do with combe filters. combe filters remove stripy colour interference patterns on bits of the picture that contain hi detail (hi frequency) information, you used to see it a lot on clothing (eg jackets, ties etc) The tv get confused by hi frequency b&w detail and incorrectly display it as colour information. combe filters filter out the stray hi frequency information to stop the effect. Dot crawl is caused by another problem (not sure what though) and always has been very prominent on Sony TVs. most modern tvs have very effective digital combe filters so that you don't see the problem much these days
     
  10. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    Chris, I think Comb Filters also do what you described (removing colour interference patterns/"rainbow" patterns) but their main promoted purpose is to remove dot crawl. In fact, I think that's why they're called Comb Filters - they remove the tooth-comb pattern otherwise known as Dot Crawl.

    More here: http://members.aol.com/ajaynejr/vidcomb.htm

    Not too sure about what you mean by dot crawl being most prominent on Sony TVs - I was actually browsing Fraser's TV department yesterday and the small Sony sets struck me as the best for this sort of thing.
     
  11. Dale Adams

    Dale Adams
    Active Member

    Joined:
    Jul 9, 2003
    Messages:
    175
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +15
    Comb filters are used for separating the combined luma and chroma components of a composite video signal. There are 2 common problems when the separation is not perfect (and it never is):

    1) High frequency chroma bleeding into the luma. This produces artifacts such as 'dot crawl' around sharp color transitions.

    2) High frequency luma bleeding into the chroma. The result of this is brightly colored rainbow-like artifacts around areas with high luma detail, such as herringbone jackets.

    - Dale Adams
     
  12. Chris5

    Chris5
    Active Member

    Joined:
    Mar 21, 2004
    Messages:
    1,161
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    63
    Location:
    Herts., uk
    Ratings:
    +122
    well....... I was half right :)

    Thanks Dale
     
  13. MartinImber

    MartinImber
    Active Member

    Joined:
    Apr 5, 2001
    Messages:
    3,851
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    71
    Location:
    Worcester
    Ratings:
    +21
    My only composite source is now my Sony Super Beta deck, everything else is RGB connected
     
  14. pwood

    pwood
    Active Member

    Joined:
    Dec 6, 2003
    Messages:
    2,479
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    66
    Location:
    Central Scotland
    Ratings:
    +136
    My Pansonic ES10 DVD recorder has this facility. I suppose its best turned off then as I'm recording from SKY box via RGB not composite. Can't say I notice a difference.
     
  15. GagHalfrunt

    GagHalfrunt
    Active Member

    Joined:
    Sep 1, 2001
    Messages:
    521
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Location:
    North Cotswolds
    Ratings:
    +3
    Interesting because I've always run my Laserdisc player via SVideo which would use the comb filter in the LD player.

    I wonder if the TV's filter is better?
     
  16. David Mackenzie

    David Mackenzie
    Banned

    Joined:
    Aug 28, 2003
    Messages:
    10,213
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    Glasgow, UK
    Ratings:
    +1,310
    It probably is. I compared my Pioneer CLD-2950's S-Video output vs connecting it through Composite, the Composite connection (TV's filter) looked a lot better with less "rainbow effect".
     

Share This Page

Loading...