OTA HD vs Mpeg4 HD

Discussion in 'DirecTV TiVo Powered PVRs & Receivers' started by buzzmc1, Dec 22, 2005.

  1. Jan 26, 2006 #61 of 118
    newsposter

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    Ok so I guess i can infer that the SD pictures (90% of what is on in any given day) when watched through the Mpeg4 HD channel, will look just as good as when I watch the SD stuff OTA then? And also they will look better than the SD locals on DTV too right?
     
  2. Jan 26, 2006 #62 of 118
    newsposter

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    interesting points indeed. I guess the one certainty that it seems everyone will agree on is that a perfect OTA signal is by definition the highest possible quality we can ever receive since there is no 'middleman' to deal with. Can anyone refute that?

    I followed most of the 2nd paragraph above except when you said
    Is it because a 10mb mpeg2 is like a 5 mb mpeg4 but if the mpeg4 is 10mb (same size as mpeg2) then it's really 2x the quality of mpeg2 and thus equivalent to a 20mb mpeg4 stream? ---can u tell im lost?
     
  3. Jan 26, 2006 #63 of 118
    bdlucas

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    That's true (although it wasn't the issue I was addressing in my posting that you quoted nor the issue that the person I was responding to was talking about).

    However, it is possible that the re-compressed signal will be only negligibly worse, that is, that the difference won't be noticeable. As an analogy, if you open a JPEG compressed image and edit it and then re-save it, in theory you've lost some quality, but in practice you often won't see a difference unless you do it many times. Some are reporting that they don't see a difference between the re-compressed DirecTV signal and OTA.
     
  4. Jan 26, 2006 #64 of 118
    newsposter

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    You just answered my question above, thanks....Can you tell i'm reading posts in order instead of all at once :) Also Thanks for the affirmation in my decision to spend over 400 bucks to get my OTA in perfect working order. I probably would be unhappy with the directv HD signals for locals given the facts above in these posts(and problems i've read about in the HR20 with pixels). Though I'm sure anyone going from mpeg2 directly to mpeg4 without ever seeing an antenna outside is probably going to be happy. But something tells me going from ota to mpeg 4 i would not be very satisfied.

    The statement that even a perfect signal gets worse when you do 'anything' to it is really the key here. The virgin signal from a tower indeed sounds like the best thing in the world.
     
  5. Jan 26, 2006 #65 of 118
    SpankyInChicago

    SpankyInChicago New Member

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    In practice this certainly can't be refuted. In theory it COULD be possible to receive a better signal than OTA, but this would require that DirecTV (or the cable company) get access to the OTA signal before it has been compressed from the original uncompressed HD signal into the MPEG to ready for air signal. Of course on a network-based show sent to a local affiliate the show might already have been compressed prior to being sent to the local affiliate.

    Imagine an input stream X. If you were to compress X using MPEG2 at Y bit rate and compare it to X compressed using MPEG4 at Y bit rate, the resultant MPEG4 stream would look superior to the resultant MPEG2 stream. In other words, all things being equal (input source, output bit rate, encoder quality, etc.) MPEG4 results in superior picture quality than MPEG2.
     
  6. Jan 26, 2006 #66 of 118
    SpankyInChicago

    SpankyInChicago New Member

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    Sorry that I misundertood your the intent of your original post.

    I will say, however, that if you've worked with many JPEG images it doesn't take much to make a RAW image from your digital cam look bad by a few iterations through JPEG. And the problems obviously manifest themselves much quicker with moving images as opposed to still images.

    But, you are correct. It would be possible for a recompressed image to be visually the same quality as the original compressed image, though, obviously, it would not technically be the same quality. The factors, of course, are wildly varying, and the bottom line will always remain that OTA MPEG2 is likely always going to be the "best" picture quality.
     
  7. Jan 26, 2006 #67 of 118
    newsposter

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    Isn't there some way the geeks can measure the signal coming into our homes? :)
     
  8. Jan 26, 2006 #68 of 118
    bdlucas

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    There are are several. Two general categories:

    1. Subjective: which looks better, A or B? Do you see a difference between A and B? In some sense this is the only relevant measure. Ideal is a double-blind test under controlled viewing conditions. Anecdotal reports here and elsewhere are suggestive but should be viewed with skepticism.

    2. Objective: for example, PSNR, which is essentially a measure of the average pixel-by-pixel numerical difference between corresponding R, G, and B values.

    On both measures MPEG-4 as a technology has been shown to be capable (given a good encoder) of the same quality as MPEG-2 at about half the bitrate of MPEG-2. But as the discussion here indicates that's only part of the story as to whether MPEG-4 from DirectTV will look as good as OTA MPEG-2.
     
  9. Jan 26, 2006 #69 of 118
    jcricket

    jcricket New Member

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    Theoretically yes. But (someone more technical can correct me if I'm wrong) it's possible D* could compress the MPG4 feeds as much as they do the MPG2 feeds, resulting in crappy SD, HD, etc.

    Assuming D* allocates enough bandwidth to the MPG4 feeds, HD should look close to as good as OTA HD, and SD program broadcast over those MPG4 feeds should look close to as good as OTA SD.

    Your SD that you get via the MPG2 feeds from the existing satellites will probably still look like crap until D* stops compressing so much (i.e. hell freezing over). What's more likely is that as channels start broadcasting in HD, D* will put up MPG4 feeds of those channels, which they will hopefully keep enough bandwidth for.

    I think that's why so many people here are iffy on the idea of getting HD channels (esp. locals) from D*. They worry that even though they're in HD, and being broadcast with MPG4 compression (saving 1/2 the bandwidth), D* will still find itself crunched and compress things even more.
     
  10. Jan 27, 2006 #70 of 118
    newsposter

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    Everyone thanks for the insight. it's exactly what I've been looking for and explained in terms I can understand. I also know a lot more people will be interested in the content of this thread.

    One thing that took me longest to understand is post 65 , last paragraph. But I think I got it down to: if the same picture is crammed into the same space, mpeg4, by definition, is better than mpeg2. It's appears the code/mechanism or whatever, produces a better picture even with the same amount of space taken up. (plz correct me if i'm wrong :))

    Also while OTA is a more 'virgin' signal, you may not notice the difference with MPEG4 unless you are a discriminating viewer. So the picture can be technically, but not visually, superior.
     
  11. Jan 27, 2006 #71 of 118
    bdlucas

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    That's right, assuming a high-quality encoder. This matters because MPEG-4 (as does MPEG-2) owes much of its efficiency to detecting moving scene elements and encoding them by sending instructions to the decoder that say "take that part of the image from the previous frame, move it over here, and apply this correction". There's no single correct way to do this encoding, but the better the encoder is at finding such moving scene elements the smaller the "corrections" will have to be and so the less space the encoding takes up. Much of the improvement in MPEG-4 over MPEG-2 is due to a more flexible vocabulary (encoding) for describing moving scene elements.

    Since for a given encoding scheme (e.g. MPEG-2 or MPEG-4) there's a tradeoff between image quality and amount of space taken up, the flip side of your statement is that the same image quality can be achieved by MPEG-4 as can by MPEG-2 while using "less space" (fewer bits) for the MPEG-4 encoding, by about a factor of two.
     
  12. Jan 27, 2006 #72 of 118
    AbMagFab

    AbMagFab What happened, TiVo?

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    Not entirely true. What people are forgetting is that TV's are getting better every day.

    The newest round released late last year support the full 1080i resolution. This means that HD compression is more obvious.

    It's analogous to how SD on an HD TV looks worse than on a non-HD TV. Well, compressed HD on a full 1080i TV looks worse than on a 720/768p HD TV (which is what most people have today).

    I've got a 1080p 60" Sony XBR1, and DirecTV HD compression is extremely noticable. I haven't cancelled my HD package yet (as many people have pointed out, we don't have an option), but the HD picture quality is poor on a 60" (or greater) 1080p set. (It's probably not as noticable on a 720/768p set).

    It more than a "discriminating viewer", it's what you're watching on.
     
  13. Jan 27, 2006 #73 of 118
    newsposter

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    so how good a pic am i getting on my 2005 1080i crt rptv? :) I'm pretty happy but maybe i dont know what i dont know.
     
  14. Jan 27, 2006 #74 of 118
    AbMagFab

    AbMagFab What happened, TiVo?

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    CRT's are analog, and so it's a very different picture. Generally speaking, the newer digital 1080i's, especially the SXRD/D-ILA's are considered better, perhaps much better than the CRT's.

    The CRT's weren't really 1080i, in that they weren't displaying every pixel. The digital 1080i's display each and every pixel. But to some, this is an album/CD debate in terms of the overall experience.

    Bottom line - the 1080i CRT's won't show the picture issues as much as the 1080i digital sets, but that's because they don't have as detailed a picture.

    More to the point, the providers need to stop messing with the pictures. FIOS, at least in my area, is supposed to have an amazing picture, substantially better than DirecTV (see reports on AVS).

    (Now let the analog freaks come and flame away...)
     
  15. Jan 28, 2006 #75 of 118
    newsposter

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    They darn well better be better for 2-3X what i paid for mine! Until sxrd or plasma is around 1800-2000, no way would I spend more on the digital displays until all the world is in HD.

    SD was paramount concern for me (read the horror stories of lcd and plasma trying to get a decent pic) and I think the general consensus is that unless you have sxrd or other high end gear, SD sucks on digital tvs. So I went with a nice big crt at a cheaper price than i paid 8 yrs ago with my other big screen. Love technology!

    Plus the HDtivo does a nice job on SD when it's kicked up to 1080. Very noticable difference even to my wife when I forget to flip the switch from 480i (burning dvds)
     
  16. Jan 29, 2006 #76 of 118
    TyroneShoes

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    I'm afraid that is a false analogy. Digital artifacts and native resolution are two very different things that have little in common with each other and very little effect on each other. Increasing the native rez to 1080i will not necessarily make compression artifacts more visible than at lower native resolutions, and for the most part, does not do that in the least. This is due to the nature of DT compression artifacts, which are usually limited to pixellation and mosquito noise for DT reception, and primarily to pixellation on motion.

    If native resolution is the variable, then we have to look at how resolution might affect such artifacts. This applies to both types of artifacts, but I will use pixellation as an example. This sort of artifact manifests as blocks of pixels remaining on the screen for a split second longer than they should, as the decoder struggles to update them with new information in a timely manner. If an 8x8 pixel block is visible longer than it should be, that will be just as noticeable or visible when displayed on the screen as 1 to 1 from a 1080i or even a 1080p image, as it might when reinterpolated to 720 or 768 in the display.

    To be masked significantly, an artifact 8 pixels high or wide would have to be smaller than either 1080 native rez, 768 native rez, or 720 native rez can resolve, which means the native rez of the display would have to be 1/8th of 1080, or 240x135, or lower, for the artifact to not be resolved. Since even 1280x720 is orders of magnitude more resolved than 240x135, that would have no more effect on masking 8x8 macroblocks than 1080p rez, other than making the edges of the macroblocks a bit less defined, which is about as signifcant as determining what side someone's hair is normally parted on while they're standing in a hurricane.

    This is the exact flip side of the argument that 1080p resolution will not provide more actual visible resolution for an image than 720p will, if that image is not a high-resolution image in the first place. Out of focus? Bad telecine transfer? Cheap lens? 480i master? higher native display rez provides no advantage whatsoever, just like no HD display can make a NTSC copy of a classic Seinfeld ep ever look like HD resolution, no matter how much he might be the "master of his own domain".

    Bottom line, assuming insignificant resolution differences will make digital compression artifacts significantly more or less visible is a flawed assumption, and comparing this to the difference between resolution of images on SD or HD displays is like comparing apples to oranges.
     
  17. Jan 29, 2006 #77 of 118
    AbMagFab

    AbMagFab What happened, TiVo?

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    Math confuses me... And what looks good is what matters, not what a bunch of numbers say.

    I have a 768p 50" HD TV, and a 1080p 60" HD TV. The DirecTV HD looks okay on the 768p, and looks bad on the 1080p. The OTA HD looks great on both.

    Ergo, the better the TV, the more you'll see the effects of the DirecTV compression.

    Remember, DirecTV is recompressing a 1080i signal into something slightly more than a 720p signal. Even if we ignore the picture degredation, the reality is you won't notice much difference on a 720/768p set since it just won't resolve the picture in enough detail.

    However, a 1080p set will resolve beyond what DirecTV is compressing. So, when comparing a DirecTV HD signal with its OTA counterpart, the difference is plainly visible - DirecTV is ruining the HD picture.

    And most of the people posting likely have a 720/768p set, so they won't see much of a difference. BUT, that doesn't mean there isn't a difference. And as more people upgrade to 1080p sets, they will see what the rest of us see - a lousy HD picture from DirecTV, and great HD pictures OTA.
     
  18. Jan 29, 2006 #78 of 118
    dswallow

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    Unfortunately there's so many ways for the display owner to screw things up you can't always blame the bad picture on the source material; they need to come up with some automatic ways to calibrate the display to a standard. :)
     
  19. Jan 29, 2006 #79 of 118
    AbMagFab

    AbMagFab What happened, TiVo?

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    Quite true. I, of course, believe my sets to be well calbrated.

    Even so, if the OTA picture looks perfect, and the DirecTV picture looks poor, that has little to do with the calibration, since I'm viewing them on the same input.

    I have a great picture with OTA on both sets. I have a lousy picture from DirecTV HD on the better set, compared to the OTA picture. Ergo, the DirecTV picture looks worse, independent of calibration.

    Unless you know of some magic calibration that would only effect the DirecTV picture... ;)
     
  20. Jan 29, 2006 #80 of 118
    dswallow

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    THe most obvious thing from over-compression is the blockiness o of darks, so obviously raising brightness is aesthetically harmful. Unfortunately if you've got it calibrated properly for OTA HD over the same input, there's not a lot you really can do.

    It could be worse, though... you could live in one of those markets where every affiliate sends out several subchannels. ;)
     

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