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RE: HD Picture

Discussion in 'DIRECTV HD DVR/Receiver Discussion' started by aziz, Nov 29, 2012.

  1. aziz

    aziz Cool Member

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    Oct 11, 2008
    Folks:

    This might have been discussed before, I am just wondering why the HD picture from my HR20 can't seem to keep up? I was watching one of the Transformers movies and I saw some artifacts, stuttering, and pixelation on fast or busy scenes. I tried changing the format from 720p to 1080i but it did not seem to make any difference. I have a 120 Hz TV. I know D* only transmits at 60 Hz for most of its channels. Personally, I prefer watching a movie from a Blu Ray. What gives?:confused:
     
  2. jimmie57

    jimmie57 Hall Of Fame

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    Try turning the 120hz feature OFF.
    I watch the stock market all the time in the daytime. For a couple of years I could not figure out why the letters and numbers that crawl across the screen were jerky. I turned the 120hz off and now they are smooth as silk.
     
  3. RBTO

    RBTO Legend

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    Apr 11, 2009
    I think you're referring to the "smooth motion" feature which can cause stuttering (jerky motion) for some video, but what Aziz is referring to (pixellation and artifacts) is due to something else. (I personally hate "smooth motion" and do turn it off).

    It could be an HR20 hard drive access problem, or it just might be the provider itself. If there's any kind of glitch in the transfer of the signal from the provider (and it goes through a lot of different paths), it will show up as pixellation or mpeg artifacts.

    Do the breakups show up on a recording (can you play a segment back and still see them)? If so, chances are, they came in that way. If it's a one time thing and doesn't show up when you play back the DVR recording, it may be the hard drive in your HR20. It could be going bad or just have some bad spots on the platters. In that case, you need to see if it gets worse, and if it does, you need to replace your unit, or if you own it, replace the drive (the latter is best if you want to keep the HR20 which is a pretty good machine with a built-in OTA receiver). There can be some other HD causes, and some of the experts here might have some suggestions.

    I have an HR20 and see breakups all the time, but they aren't frequent at all, and therefore aren't objectionable. Most all are provider or Directv and I don't worry about them.
     
  4. paulman182

    paulman182 Hall Of Fame

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    The bit rate of satellite delivery is very low compared with Blu-ray.

    Considering how much lower it is, it does quite well.
     
  5. Laxguy

    Laxguy Honi Soit Qui Mal Y Pense.

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    Dec 2, 2010
    Winters,...
    How often are the objectionable artifacts appearing?
    Did you turn off the "motion smoothing"?

    As others have noted, sometimes it's in the broadcast before DIRECTV® gets it, sometimes in the transmission, sometimes it could be your HDD in the receiver. And it's this latter phenom that should be watched for replacement.

    Please post back with how this is coming along for you!
     
  6. FussyBob

    FussyBob Legend

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    Jan 11, 2009
    So is the bit rate of a 1080P movie less if I watch it as a direct feed from the satellite as compared if I download it from the internet as an On Demand?

    If so, what are the bit rates for each mode?


    Bob P.
     
  7. aziz

    aziz Cool Member

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    Oct 11, 2008
    Right now I have the 120 Hz feature set to OFF from Standard. I think it was probably the provider. The last time I noticed that bad was a long time ago when I was watching 30 Days of Night. All the other HD channels at that time were fine!
     
  8. Laxguy

    Laxguy Honi Soit Qui Mal Y Pense.

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    Winters,...
    Glad to hear that.

    Smooth sailing- er, watching!
     
  9. Dec 2, 2012 #9 of 10
    TomCat

    TomCat Broadcast Engineer

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    It could be the other things mentioned, but odds are it is due to the low (relative) bit rate in HD delivery. Changing the format does not help, because the problem is in the source, not in any sort of decoding process, although to be completely accurate, even though the problem is in the source (due to bit starving there is not enough information left for the encoder to make accurate guesses about what the missing information should consist of and the macroblocks can't update fast enough because they are left waiting for better data that never comes) the pixellation actually manifests at decode precisely due to the incoming relatively low bit rate.

    And the reason you only see it on fast motion is because MPEG that uses P and B frames is an inter-frame process, meaning that the predictive and bi-directional predictive frames depend on the information from adjacent frames and I frames to help construct them. Since B and P frames are difference frames from the I frames, it takes few bits to create them when the difference between frames is low, such as in a static shot, and it takes more bits, bits that are not always available at low bit rates, to create them when then difference between frames in the group of pictures is great, such as when there is high motion.

    Some MPEG is short-GOP or I-frame only, which is intra-frame compression, and does not have this problem, but needs a much higher bit rate. But MPEG4 AVC for DBS is long-GOP inter-frame, and highly compressed. Which means what you are seeing is, sadly, par for the course.

    The result is that the decoder has enough information when motion is low to reconstruct the P and B frames, but not enough information to accurately guess at the missing info in adjacent frames when motion is high, with the result being much greater artifacting when there is high motion. IOW, the amount of artifacting is below the visual threshold (not really noticeable) on low motion, and above the threshold (noticeable) when motion is high.

    The reason I mention it is this: if you can pin the artifacting down to scenes with high motion only, and do not notice it on scenes that have low motion or are static, that is your smoking-gun evidence that it is caused by bit starving at the encoder due to being severely compressed, which is about a 95% chance of being the culprit.

    Watch the Victoria's Secret Fashion Show this week for the prime annual reminder of why MPEG is overcompressed for delivery; all of the glitter and ticker tape and whip pans and flash-bangs make MPEG artifacts stick out like a sore thumb, and it is broadcast on CBS, where the affiliates are squeezing the main signal to make room for extra subchannels when March Madness comes around. It gets a little bit better every year, but back in 2004 or so it was one hour-long glitch.

    It sort of takes a trained eye to make this distinction of problems on motion only (but any eye can be trained if you just know what to look for), because at these bit rates there is another sort of artifacting that appears on close inspection, and that is color banding or contouring on static shots, usually visible in skys or pastels. High compression along with a 4:2:0 color space (rather than 4:2:2 or 4:4:4 which you find in pro video at the production level) and 8-bit encoding accounts for that, because 8-bit encoding has large steps in brightness or 1/4th as many quantization levels as 10-bit (232 vs. 1066) which makes the rounding errors three times larger by comparison, and 4:2:0 aggravates that, especially if the video is noisy to begin with.

    OTA honestly isn't much better, probably not even noticeably better, but a good Blu-Ray will be encoded at 3 or 4 times the rate DBS uses, so will not have the motion artifacting issue, at least most of the time.
     
  10. TomCat

    TomCat Broadcast Engineer

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    Aug 31, 2002
    The bit rate for DBS HD averages around 7.5 Mbps. Downloaded files do not have a fixed bit rate in the same manner, or do not suffer from the same problems as much (motion artifacts) because to create the file does not depend on being under a bit "ceiling". IOW, they can take the care to use more bits where they need them, because they do not have to create the file in streaming real time.

    That said, bit rates are still usually low because they still need to be able to be downloaded at a practical rate; no one will wait for 6 hours to download a 2-hour movie at 20 Mbps internet speeds, and the larger the file is the quicker you reach your MB cap and get throttled by your ISP.

    One of the ways to mollify this problem is to use 1080p24 delivery. Fewer frames, fewer bits needed. Also, progressive compresses easier at low bit rates than interlaced content does, so most content downloaded is 1080p24. The trade-off is a low frame and flicker rate, and in that case you may want to turn 120 Hz back on (some like it, some don't; it wouldn't be there if at least some of us didn't consider it a valuable feature) because most TVs that have this will recreate new frames in between the transmitted frames, raising the effective frame rate and removing most of the flicker and judder seen at 24 fps.
     

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