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HD channels

Discussion in 'DIRECTV General Discussion' started by miedwards72, Apr 28, 2007.

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  1. Tom Robertson

    Tom Robertson Lifetime Achiever DBSTalk Club

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    Small correction: MPEG2 and MPEG4 are variable compression algorithms whereby the compression can range from totally loss-less to extremely lossy. Somewhere in between the eye can start to see the difference and somewhere below that point, many TV stations still think they are getting away with HD.

    Cheers,
    Tom
     
  2. techntrek

    techntrek Godfather

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    Apr 26, 2007
    True, they can be loss-less (some online sources say otherwise), but in reality DirecTV uses them in a very "lossy" mode. Evidenced by the blocking in dark areas when the picture fades to black before a commercial. I started seeing this suddenly about 2 years ago so I'm guessing that's when they flipped the switch?
     
  3. techntrek

    techntrek Godfather

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    Edit: I did more research and now conclude that MPEG-2 and 4 both are always lossy so I'm retracting some of my last post. They work by dropping some chroma information and dividing the redering into blocks (this agrees with my comment above about seeing the blocking during fades), MPEG-4 does additional compression partly by dividing the picture into more blocks and adding prediction alorithms that span more frames so more "guessing" is done. In both cases even with the lightest compression applied it doesn't appear its done in a loss-less way. There is a lot of conflicting info out there.

    So to compare it to still picture formats, it appears neither MPEG2/4 are like PNG (or ZIP files), which are loss-less. When you uncompress you get every bit back out. MPEG2/4 are much more like JPEG, where many bits are permanently lost and various levels of compression are possible. The lowest compression levels can appear loss-less to the eye at first glance but are not.
     
  4. sarfdawg

    sarfdawg Legend

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    Jan 21, 2007
    All I can say after reading that is HOLY CRAP! :D
     
  5. finaldiet

    finaldiet Hall Of Fame

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    Never had a problem with local 2, but this weeks CSI:Miami looked bad. When I checked my recordings of 24,bones, and other CSI'S recorded monday, they all looked great on same channel after. Something must have been changed on monday, 4/30.
     
  6. n3ntj

    n3ntj Hall Of Fame

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    Lancaster,...
    I said the same thing..
     
  7. texasbrit

    texasbrit Well-Known Member

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    Aug 9, 2006

    Your comparison with JPEG is probably a good one. Both MPEG-2 and MPEG-4 are lossy compression algorithms, designed to get the very high bandwidth HD signal into something that can reasonably be transmitted and recorded. These links http://en.wikipedia.org/wiki/MPEG-2 and http://en.wikipedia.org/wiki/MPEG-4 are useful in getting a basic explanation.

    MPEG-4 was developed with one objective being to achieve the same quality as MPEG-2 but with smaller file sizes and data rates. Because the original signal for HD locals is an MPEG-2 one, converting it to MPEG-4 involves various subtle opportunities for losses. Certainly the resulting MPEG-4 signal can't produce a better picture than the MPEG-2 signal. I have never seen any technical analysis of the conversion of an MPEG-2 signal to MPEG-4, particularly a real-time conversion such as is involved with DirecTV. Software for converting files (such as when creating a DVD) has the opportunity to make several passes at the conversion and can get greater compression efficiency as a result.
     
  8. cantfish2much

    cantfish2much AllStar

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    Feb 5, 2007
    ... and someday the rest of us will be able to get D* HD locals to compare.
     
  9. heisman

    heisman Icon

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    Feb 11, 2007
    At least we know al-qaeda's next target.
     
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