What does a line doubler do?

Discussion in 'Archived Threads 2001-2004' started by TreyP, Feb 10, 2002.

  1. TreyP

    TreyP Stunt Coordinator

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  2. MichaelGomez

    MichaelGomez Stunt Coordinator

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    Normal TVs have horizontal scan lines. They go in even and odd scan rates. Another words lines 1,3,5,7 etc then 2,4,6,8 etc. A line doubler does this in 1/2 line increments. I think that the doublers in HDTV convert to scanning the lines in order. Anyway, in a generic sense the LD scans 2 times as many lines and this makes the picture better. There are less visible scan lines.

    Mike
     
  3. Allan Jayne

    Allan Jayne Cinematographer

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    The simplest line doubler outputs each scan line twice. It has to emit the entire scan line twice, not just the first half of each scan line twice. Practically speaking the line doubler has to convert the video to digital, do its work, then convert the video back to analog to go out the component video jacks or, if built into the TV, feed the picture tube.
    Meanwhile the TV or monitor has to accept this video with twice as many scan lines per second (be 480p capable for NTSC or 576p capable for PAL), otherwise the line doubler is of no use.
    For regular interlaced video, there are 30 sets (fields, or half pictures) of odd scan lines and 30 sets of even scan lines alternating per second. After doubling, whether the successive video frames occupy the same 480 (NTSC) scan line positions on the picture tube as non-interlaced video or is staggered slightly between frames resulting in 960 unique scan line positions is up to the TV.
    The line doubler together with a progressive scan capable (non-interlaced) TV yields a picture with much less flicker. For regular interlaced video, the odd scan lines start to fade while the even scan lines are being drawn and vice versa. The keen eye will see this as noticeable dark "gaps between scan lines". Since the electron beam comes around twice as often during progressive scan, the phosphors don't have a chance to fade.
    Much improved picture quality is had if the doubler does more than just output each scan line twice. For example sometimes (when that part of the screen has stationary subject matter) that part of intervening (odd or even) scan line should be taken from the next interlaced field while other times (when that part of the screen has moving subject matter) that part of the intervening scan line should be the blend of the scan line above and the scan line below. The line doubler has to do lots of processing to get it right, and it has to store (buffer) several fields so all the material is on hand when the progressive video frame is being built.
    Still better performance is had if the doubler recognizes the 3-2-3-2 repeating pattern when the video (60 fields per second) was produced from 24 frame per second movie film. It is desirable that the progressive scan video frames each be constructed from video fields that match, and given this repeat pattern (called 3-2 pulldown) the doubler can find matching fields if it looked for them.
    More:
    http://members.aol.com/ajaynejr/viddoubl.htm
     

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