1. Join Now

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

525/625 How?

Discussion in 'Televisions' started by S, Jul 16, 2001.

  1. S

    S
    Active Member

    Joined:
    Jul 9, 2001
    Messages:
    237
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +8
    Can somebody please explain how my television copes with the different numbers of vertical lines present in 525/60 and 625/50 images?

    As I understand it, the resolution of a television is determined by the number of holes in its shadow-mask. So, assuming a one-to-one vertical mapping, my UK-spec television will have the holes arranged in horizontal lines stacked 576 high.

    When playing back a 525/60 image (with 480 lines of image) does my TV retain the same vertical line spacing (but then wouldn't the image be offset and/or noticeably smaller) or does it attempt to scan the full screen depth but in 480 passes instead of 576 (but then wouldn't the mismatch with the shadow-mask be noticeable)?

    Or are modern televisions more like computer monitors where (I think) the shadow mask contains many more vertical lines than there are scan lines?

    Suggestions anyone?

    Thanks

    S
     
  2. S

    S
    Active Member

    Joined:
    Jul 9, 2001
    Messages:
    237
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +8
    Oops! Bit of a mix-up with my axes.

    How about:

    "Can somebody please explain how my television copes with the different numbers of horizontal lines present in 525/60 and 625/50 images?

    As I understand it, the resolution of a television is determined by the number of holes in its shadow-mask. So, assuming a one-to-one vertical mapping, my UK-spec television will have the holes arranged in horizontal lines stacked 576 high.

    When playing back a 525/60 image (with 480 horizontal lines of image) does my TV retain the same vertical spacing (but then wouldn't the image be offset and/or noticeably smaller)? Or...does it attempt to scan the full screen depth but in 480 passes instead of 576 (but then wouldn't the mismatch with the shadow-mask be noticeable)?

    Or are modern televisions more like computer monitors where (I think) the shadow mask contains many more horizontal lines than there are scan lines?"


    After rummaging around a bit it turns out that my TV has an aperture grill and not a shadow-mask so there are no holes.

    Given that my TV displays 525/60 encoded data over the full screen depth, I can only reach the conclusion that the vertical resolution must be coarser than for the same image in 625/50 resolution (i.e. R2 DVDs must offer a better still picture than the equivalent R1 DVD) but I'm confused as to why this doesn't seem to be an issue with most given the minutiae to which some other factors are considered.

    Is it simply a case that 525/60 is good enough or are there some subtleties to the MPEG-2 encoding process which means that the extra information on a 625/60 disc is largely redundant?

    I'll stop there since this seems to have turned into a DVD rather than TV-related query but any information would be gratefully received.

    Thanks

    S
     
  3. Gary H

    Gary H
    Standard Member

    Joined:
    Jul 13, 2000
    Messages:
    41
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    8
    Ratings:
    +9
    Hi S,

    This is not a very technical explanation :) but this is what happens on my set:

    Full screen NTSC material is displayed with NTSC number of horizontal lines ( I'll leave out the numbers since I don't remember off the top of my head ) using the same vertical height screen as PAL material.

    Since NTSC uses less scanlines ( insert correct numbers here ;-) ) small horizontal 'line gaps' ( told you it wasn't technical ) appear between the visible scanlines.

    On my TV ( Tosh 3377DB - 33" 4:3 CRT ) these show up as 'annoying' horizontal dark lines all down the picture.

    For this reason I never buy 4:3 or non-anamorphic NTSC DVDs - they look awful !

    Anamorphic NTSC suffers from the same problem ( at least technically ) but because the visible image is 'squashed' vertically ( my TV has a 16:9 mode in case you were wondering ) - ie all the scan lines now occupy only the centre section of the screen they are 'much' closer together and so the gaps are 'almost invisible' to the human eye.

    That said, I will always choose a PAL anamorphic DVD over an NTSC one since ( although you cannot seem to actually see the gaps between the scanlines on anarmorphic NTSC ) the PAL version seems ( to me ) to be 'sharper'.

    Again - apologies for the lack of 'technical' explanation but I think that explains what you will see when viewing different types of NTSC material on a PAL TV set.

    This effect is obviously exacerbated the larger the screen you have.

    Cheers - Gary

    [ 20-07-2001: Message edited by: Gary H ]
     
  4. robk

    robk
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    To add to this reply..If my understanding of it is correct, a CRT isn't made up of pixels since it is an analogue device. The screen is made up of numerous vertical wire like-elements in a repetitive RGB sequence all the way across the screen. It is the electron gun that "draws" the horizontal lines down the screen. The vertical resolution is determined by the accuracy of the electron gun and the depth of the screen. Therefore in an NTSC 525 line image it simple leaves a slightly larger gap between subseqent scans. (These are visible on large screen TV's)

    On the other hand horizontal resolution must therefore be limited by the design of the tube (and the quality of the source)

    (Or maybe I'm just getting the wrong end of the stick)!
     
  5. robk

    robk
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    I meant "draws" horizontal lines ACROSS the screen!!
     
  6. S

    S
    Active Member

    Joined:
    Jul 9, 2001
    Messages:
    237
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +8
    Thanks for the replies.

    I've been trying to convince myself that the lack of extras (generally speaking) on R2 DVDs might in some way be compensated by the better picture quality. Unfortunately I don't think I've got a case.

    Yes, the vertical resolution is less but I don't really notice it (probably because my TV has an aperture grill rather than a shadow mask).

    On the other hand I reckon R1 discs probably have better horizontal resolution on the basis that something has to give in the MPEG compression algorithm (assuming that the 'target' size for compressed frames is the roughly the same for both formats).

    R1's only weak-point would appear to be this business of 3:2 pull-down and that's nothing that progressive scan won't sort out.

    Ah well....
     

Share This Page

Loading...