SD PQ: OTA vs. MPEG-2

Discussion in 'DirecTV TiVo Powered PVRs & Receivers' started by DallasDave, May 12, 2006.

  1. DallasDave

    DallasDave New Member

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    Aug 31, 2004
    I've heard on this forum that OTA HD PQ is superior because D* compresses their HD programming to save bandwidth on the satellite…does D* do this with SD programming, too? If not, what would cause the noticeable difference in PQ between the OTA SD picture and the satellite SD MPEG-2 picture? I'm making the comparison by toggling between a local station (via satellite) and same station via OTA with output set at 480i…and the OTA picture (even at 480i) is noticeably sharper. Is this a fair comparison?

    Some of the satellite stations are really poor. I'm a transplanted Chicagoan, and Cubs broadcasts on WGN are really awful (in addition to the Cub's play itself). Any explanations and/or suggestions? My satellite signal readings are in the 90's, and so I don't think signal strength is a problem. The OTA antenna is diplexed into the satellite feed going to the HR10-250.

    Any info/suggestions are appreciated.
     
  2. Guindalf

    Guindalf Now with added dalf

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    D* compresses all their signals to conserve bandwidth. There is nothing you can do to change it.

    As for signal strength, it makes no difference. Because it's a digital signal, you either receive it or you don't. As long as you are receiving a solid signal, it will be the same quality if you're in the 90s or the 70s.

    With the OTA signal, strength DOES matter, so if you're getting a good signal from the transmitter, it may well be a LOT better than the D* reception.
     
  3. AccidenT

    AccidenT New Member

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    Oct 25, 2004
    South...

    The OTA signal he's referring to is digital as well, so that signal strength isn't any more or less important than satellite signal strength.
     
  4. videojanitor

    videojanitor Member

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    Dec 21, 2001
    There SHOULD be a huge difference in quality for the two -- the difference can vary from "just a little better" to "outrageously better" depending on the particular station. Here's why:

    1. DirecTV SD local channels are tremendously compressed -- the bit rate is on the order of 2 to 3 Mb/s. To compress that hard, some filtering of the image is required -- this removes high frequency information, which makes the picture soft.

    2. Although some stations feed DirecTV with fiber direct from their studios, many (dare I say, most) are picked up off-the-air with an antenna and a tuner. That's a composite analog signal, which will then be filtered and compressed way down.

    3. OTA SD goes through the same "pipe" as HD, so it is not filtered. This means that up to ~19 Mb/s is available to send that signal -- in reality, much less will be used, especially if a station multicasts, but it's still a far higher rate than DirecTV can provide. Also, many stations have component digital switching in master control, so you may be getting a component digital picture via the OTA SD path.

    There are a lot of variables here, but the bottom line is, if a station has a good upconverter and is not otherwise monkeying with the signal, the OTA SD signal should look vastly superior to DirecTV SD locals.
     
  5. DallasDave

    DallasDave New Member

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    Aug 31, 2004
    Thanks...great explanation...
     
  6. TyroneShoes

    TyroneShoes HD evangelist

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    I think that would be highly unusual. I don't know of a MC switcher that uses such a format. While it might be common in certain equipment islands within a TV station, such as porting video from an older digital VTR into the system, virtually all modern transport of digital SD video is by 270 mb serial digital interface or SDI, commonly referred to as "601", which uses a single 75-ohm coaxial cable.

    But the quality would be similar. Most video paths in a typical TV station are also converted to and from analog and SDI, sometimes many times over. While that sounds problematic, if done properly it really isn't. The end result typically has fairly uniform quality.

    The difference between a direct fiber feed and receiving SD OTA as the first step in backhauling it to a DBS uplink site is, on the other hand, significant, or can be (though won't really be an issue for HD LIL). That, and the regular high compression applied to LIL SD channels is certainly going to degrade them significantly from what can be seen of OTA SD on a DT channel, regardless if the original SD video was analog or not, or if the MC switching is digital or not. Many stations that support upconverted SD into their DT channel still use analog MC switching for SD, and many do not, and the difference there is really not significant. But I doubt that very many if any are using component digital switching in their master control.
     
  7. videojanitor

    videojanitor Member

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    Dec 21, 2001
    Call me crazy, but isn't "601" component digital? :confused:

    The station I am working at right now does all of our MC switching in this format. All of our VTRs and servers are 601, as is our MC switcher, routing switcher, and everything else in the MC signal path. The only time it becomes analog is when it is run through an encoder to make NTSC to feed the analog transmitter. There are no analog conversions in the DTV path -- the 601 output of the MC switcher feeds the upconverter, and from there, the STL and the transmitter.
     
  8. TyroneShoes

    TyroneShoes HD evangelist

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    I would never call you crazy (I reserve that for some of our other friends here :D ).

    I guess we are really talking about the same thing then, and it could just be that in my little world the term "component digital" is never used, while in your little world it might be common. The term "component" usually refers to a parallel protocol, with discrete cabling for the separate "components" of a video signal. SDI is serial by definition, but handles the "components" separately as well, just packetized serially. Folks here refer to "SDI" and "601", but never use the term "component digital", although I guess technically they can be the same thing. Chalk it up to regional differences? I'll just assume it's my mistake for not recognizing "component digital" for being the same thing as SDI, and leave it at that. My apologies for confusing the issue needlessly.

    But that's really beside my point anyway, which is whether the signal at MC is SDI, component digital, or analog in master control, the quality of the signal is probably about the same, and whether it is one or the other this has little bearing on the final product, whether it be SD OTA on a NTSC channel, SD OTA on a DT channel, or SD DBS LIL.
     
  9. TyroneShoes

    TyroneShoes HD evangelist

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    Sep 6, 2004
    That is indeed the common way to do it, but typically the signal is converted to the analog domain and back once to several times before or even after reaching the MC switcher in most facilities, especially if you have EAS, NAVE, weather crawl CODI or other ancillary processing of the SD signal. And that would also apply to that signal eventually upconverted for DT.

    Even though we receive our networks as ASI and crossconvert the SD to 601, and use a 601 switcher (actually 6 switchers) and 601 servers and VTRs, all signals must go through this post processing, which means at least one D/A/D conversion. We also have remote antenna farms with analog fiber links and a primary analog production switcher for local production, so rarely does any SD signal remain totally in the digital domain, which is common for a facility that is midway in the process of changing from an analog facility to a digital facility--a description most TV stations can relate to.
     
  10. videojanitor

    videojanitor Member

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    Dec 21, 2001
    Well, glad we got that cleared up -- I thought I was losing my mind for a minute (which is entirely possible!).

    You are correct that inside the TV station, we always just refer to it as "SDI" -- however, I thought that would be confusing to say here, so I used the term "component," since people in the consumer world can relate to that.

    From my point of view though, component video is always preferable to composite -- I can see the difference from a mile away. When I look at our upconverted signal, it looks damned good, when we are airing something that has been component all the way from the source. Now I will grant you this this difference is probably lost on most people, but it is real, and just another factor that help make the OTA SD picture look that much better than the DirecTV SD transmission. But DirecTV's heavy compression is definitely doing the bulk of the damage -- on that I think we agree.
     
  11. videojanitor

    videojanitor Member

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    Dec 21, 2001
    I'm sure this varies by station, but here, everything downstream of the MC switcher is SDI. That includes the EAS, logo inserter, DekoCast, NAVE, etc. There is no analog in the air chain.

    On the input side, except for C-Band satellite feeds, virtually everything is SDI. Programs and commercials come in via Pathfire, DG or Vyvx, which all output SDI, and will display nice component pictures if they put them in that way at the other end. These get recorded (via SDI) to either Profile servers or Betacam SX tape -- the SDI output of those devices go to the MC switcher (SDI) and down the rest of the SDI chain. Thus, just to use one example, we can put an episode of "Everybody Loves Raymond" on the air, and it is has remained in component digital format from the syndicator all the way through to air. The difference is clear to me -- zero NTSC artifacts. It is sweet.
     
  12. TyroneShoes

    TyroneShoes HD evangelist

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    Yep. You'se is preachin' to the choir. Probably a good call on not just throwing out SDI without a lay explanation, I'm with you on that. I don't think composite even exists professionally anymore, although we have been known to dub DTV feeds for sports highlight clips and the like.
     
  13. newsposter

    newsposter Poster of News

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    Aug 18, 2002
    SE PA
    I'll keep this short and sweet Dave and not complicate it by technical lingo:

    OTA SD is so much better for me than directv SD that I record every single program possible OTA whether HD or SD (and of course it's approx 8x the space even for SD ota) . You can just sit there and even flip back and forth to see the HUGE difference.
     

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