Cable programming. What kind of compression they using?

Discussion in 'Archived Threads 2001-2004' started by Galvin, Apr 10, 2002.

  1. Galvin

    Galvin Agent

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    For movies mainly. Some movies look good and some real bad.

    It can't be real DVD, because DVD doesn't have flaws that I see all the time when I watch movies on cable mainly Showtime since it's all I have.

    Flaws include, one part of the image floating while another is not. For exmaple in one movie, a characters face one side shadowed the other side not. one side of his face around the eyes was floating and not moving with the rest of his face.

    In other things like scene with a table, the table might jitter while the rest of the room does not.

    Distortion it's real bad with some movies, the newer ones not as bad. Night time scenes with shadows, colors don't blend, it's almost looking at a high color photo in 16 colors. If you can get the idea.

    Overall I imagine the cable companies take the movies and compress them so they can fit more on a disk. Problem is you lose picture quality in the process. I even see this on Pay per view and it really sucks at times.

    I wanted to know if cable networks are doing anything about this.

    Thanks
     
  2. Matt Pasant

    Matt Pasant Second Unit

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    I learned this in college and heard this from industry people, but I believe the rule of thumb with digital cable is 12:1 compression.
     
  3. Derek Miner

    Derek Miner Screenwriter

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    What are you watching on? Digital cable? Analog cable? C-band satellite? It could make a difference.
     
  4. Todd Hochard

    Todd Hochard Cinematographer

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    Typically, digital cable allots 2-3Mbps for stuff like shopping channels, around 4 for the "regular" stuff, 5-6 for HBO/Showtime, and 5-6 (sometimes slightly higher) for PPV.

    Generally, they use MPEG-2 just like DVD. Trouble is, it's real time, single pass encoding, done "on-the-fly," so there aren't any sort of variable-bit-rate, multi-pass tweaks that are done for DVD.

    The best approximation of what's going on can be done by looking at what TiVo or Replay does to the signal at various quality levels. On these boxes, high quality runs about 6Mbps, and standard/low quality is about 2Mbps. That's the best do-it-yourself way of looking at it that I can come up with.

    I've noticed that Showtime (all channels) always seems to be lower quality than HBO. The exception is HD- when Showtime does real 1080i, it's done right!

    Todd
     
  5. Galvin

    Galvin Agent

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    Digital Cable. But like you said it's not as a good picture. Hell, even videotape is clearer than some of the movies they show. Too bad we pay all this money for digital and the picture is no better except for mainly the shows, like Stargate, Jerimiah, etc.
     
  6. Martin Fontaine

    Martin Fontaine Supporting Actor

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    Yeah, the Non Variable Bitrate is correct, since the feed has to be constant...

    One thing that puzzles me, when I change channel, the screen is black during the change but as soon as it locks into the new channel, the image appears sort of like in blocks from top to bottom (It usually starts at some random position, goes to the bottom of the screen and then starts to the top then back to where it started) Kinda like one of those transition effects you can use in PowerPoint. Then after this is finished, sound kicks in. It doesn't bother me or anything, I'm just wondering why it's doing this.

    The way I understand MPEG compression, is that there are Key Frames (Also called GOP in MPEG Encoder programs) which are basically the whole image in a JPEG-Like format, typically twice per seconds, they they have a sub-key frames (Also called Sub-GOP) which are more information but not a full refresh I think, typically every 3 or 4 frames and the rest are just changes from the previous frame.

    If that was the case, wouldn't the image appear in one shot when I change channel? The same problem occurs when reception is bad (Like when a bird lands on a dish) where there is random garbage appearing and then it gets cleaned-up a few seconds later in the same way.

    Any experts know why it does this?
     
  7. Michael St. Clair

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    It all varies...some digital cable is compressed less, as low as 6:1...less than digital satellite compresses.

    Also, sometimes the compression is at the head end...sometimes the signal is compressed before the cable company even gets it...in certain cases you may see typical MPEG-2 compression artifacts on ANALOG channels.

    My digital cable looks good (better than current DISH), but not as good as it can look.

    Frankly, nothing but OTA and BUD and HDTV looks great on a big screen TV. Everything else that is digital (DirectTV, digital cable) is too compressed. Watch a giant screen on a BUD feed sometime and you might get sick and angry.
     
  8. Galvin

    Galvin Agent

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    Too bad HDTV is still in the 1000 dollar range. HDTV with high prices still considered a luxery to me.

    When I can buy an HDTV set that at least as big as my 21" tv for under 300 bucks i'll get one.
     
  9. Michael St. Clair

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    Galvin,

    Those of us with HDTVs still suffer through terrible picture quality for lots of our programming...less than half of what I watch is broadcast in HD.
     
  10. Robert Cranwell

    Robert Cranwell Stunt Coordinator

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    For Digital Cable the compression can be anywhere from 6-1 for premium channels HBO,Showtime,etc. To 12-1 for Discovery, Sundance, and Encore channels. HDTV is a 256Qam of 2-1 compression.

    Your still using a 6mhz channel for the compression formula,so I don't get the mbps rule comes from? Alot is based upon compression for action movies and black & white programs, so it is determined upon programing.

    Rob
     
  11. Wayne Bundrick

    Wayne Bundrick Cinematographer

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    Martin, there are three types of MPEG frames. Intracoded or I-frames are complete pictures like a JPEG. The other two types, P-frames and B-frames, contain motion data. However, P-frames and B-frames can also contain blocks of fresh intracoded picture data as needed. What you could be seeing is your decoder displaying these blocks as soon as they arrive instead of waiting for the next I-frame. There is a strategy called progressive refresh which intentionally includes intracoded blocks in P-frames and B-frames, so that over a sequence of several P-frames and B-frames the whole image has been refreshed before the next I-frame arrives.
     
  12. Todd Hochard

    Todd Hochard Cinematographer

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  13. Michael St. Clair

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  14. BobV

    BobV Second Unit

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    A couple of points...
    Now, my comments come from the digital satellite side of things, and from Canada, so take these comments with a grain of salt, but it should be pretty similiar.
     
  15. Todd Hochard

    Todd Hochard Cinematographer

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  16. Shayne Lebrun

    Shayne Lebrun Screenwriter

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  17. BobV

    BobV Second Unit

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  18. Michael St. Clair

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    Then they are using QAM64, or they are leaving extra space for now. QAM264 provides double the bitrate of terrestrial HDTV per 6mhz channel.
     
  19. BobV

    BobV Second Unit

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