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SDV FAQ

Discussion in 'TiVo Series3 HDTV DVRs' started by bdraw, Jul 3, 2007.

  1. Dec 26, 2007 #901 of 2401
    ah30k

    ah30k Active Member

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    OK, I think I understand the problem. Luke, you are thinking HFC tech that was originally introduced in the 90s. I am thinking of the systems which connect nodes to the master headend using IP over fiber. This system I am referring to allows many MPEG streams to be simultaneously sent out over the IP mulicasts around the fiber ring. At each node, IP multicasts are joined into and then the IP MPEG streams are modulated onto the local plant. This is how SDV will work well. All of the MPEG streams are available on the fiber ring but only those that have current requests from the node will actually be modulated onto the plant.
     
  2. Dec 26, 2007 #902 of 2401
    ah30k

    ah30k Active Member

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    By the way, when did I insult you?
     
  3. Dec 26, 2007 #903 of 2401
    SCSIRAID

    SCSIRAID Active Member

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    I believe that is pretty close to what I said..... Frequency translation may or may not be considered modification.... probably not. 500Mhz isnt an 'optical' frequency and has to be modulated or frequency shifted to be sent over optical. From what I have read, the node demods downstream data from the fiber and ships it downstream on the coax. It also gathers the upstream data from the coax and modulates and ships that upstream on the fiber. This says that all the transport stream formation and QAM moduluation is done upstream from the node.

    I dont know 'How' the node demods downstream data but it would seem that frequency translation would be a likely choice. Attempting to encode it further would seem to be more difficult.
     
  4. Dec 27, 2007 #904 of 2401
    lrhorer

    lrhorer Active Member

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    It's trivial on a per-channel basis, and the differential cost for each additional channel is nothing at all until they fill up a 10G fabric and have to light up another switching module in their switch. The switch itself costs less than $250,000. Each 10G card costs less than $85,000, and can hypothetically carry 640 1080i HD channels. In practice, they can probably only manage about 500 HD channels, but I've never actually seen one fully loaded, myself. If it were all SD, that would equate to over 3000 channels. The local CATV system hauls in over $1 million dollars a day just for San Antonio. A loaded switch costing $500,000 represents less than a half a day's revenue, and the expenditure is capitalized, so it doesn't hit the expense budget.

    Even my company, which is much, much smaller than Time Warner Cable, doesn't have a heart attack at spending $500K on a key infrastructure component like that. Just recently the folks upstairs approved capital projects I submitted totalling $1.2 million dollars for expanding our local operation. Companies are required to publish their capital expenditures, and if you look at TWCs' or Comcast's financial statements, I'll bet they both laid out more than $1 billion each in capital expenditures the past year, or at least close to it, if not more.

    The same QAM modulator is used whether the channel is SDV or linear digital. There is no additional cost for SDV modulation, because it's the same.
     
  5. Dec 27, 2007 #905 of 2401
    lrhorer

    lrhorer Active Member

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    No, it's a switched protocol, essentially identical to Ethernet switching on your home LAN if you have one. The main switch at the headend sends each packet to whatever nodes require the packet in question. Large systems like those here in San Antonio use smaller intermediate headends, but the paradigm is the same. If a particular data stream is not in use by a particular intermediate headend, it doesn't get sent to the intermediate site. Only the digital streams needed by the node are sent to the node.
     
  6. Dec 27, 2007 #906 of 2401
    Luke M

    Luke M Member

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    But there's no reason to have a 1:1 relationship between broadcast channel modulators and nodes. That would be redundant, since you can modulate once and split the signal.
     
  7. Dec 27, 2007 #907 of 2401
    dswallow

    dswallow Save the Moderatоr TCF Club

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    But the problem is one QAM channel contains multiple subchannels of programming, the specific combination of which may and will change depending on what the subscribers on the node are tuned to. The actual combining of multiple channel datastreams and modulation on a specific QAM channel has to be performed at the node level.

    The MPEG encoding of each channel can be performed once for the entire system.
     
  8. Dec 27, 2007 #908 of 2401
    Luke M

    Luke M Member

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    That is only for switched channels. The question is why such a capability would be needed for broadcast (not switched) channels, which are the same for every node.
     
  9. Dec 27, 2007 #909 of 2401
    dswallow

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    That was what we were discussing in terms of all channels should be switched... it's quite likely in a node of only 150 to 500 homes that at any given time even some otherwise popular channels won't be tuned. Bandwidth is bandwidth and anything less than 100% 24-hour-a-day, 52-weeks-a-year demand for a channel represents utilization savings available to the cable operator.
     
  10. Dec 27, 2007 #910 of 2401
    mikeyts

    mikeyts Stream Warrior

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    Let's remember that not everything is switched. In Austin, they supposedly switch 175 (of 265 listed) linear SD digital channels and 8 linear HD channels (out of 28 listed) onto 8 QAMs, about 14 fewer QAMs than it would take to send all those channels everywhere all the time. This still leaves 90 SD digital channels and 20 HD ones that aren't switched (and which are all probably too popular--requested by someone at every node all the time--to be useful to switch).

    EDIT: I started composing this before the previous two were posted.
     
  11. Dec 27, 2007 #911 of 2401
    mikeyts

    mikeyts Stream Warrior

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    From that Multichannel News article about the use of SDV in Austin:
    It really isn't worth going after every little scrap of bandwidth for channels that are unused by anyone for 5 minutes here or there throughout the day. Being greedy about it doesn't buy you much but, as we're all well aware, will antoganize people trying to use your system via unidirectional DCR devices.
     
  12. Dec 27, 2007 #912 of 2401
    ah30k

    ah30k Active Member

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    The smaller you made your node sizes the more edge QAMs you will need. There is no free lunch when it comes to bandwidth savings. Edge QAM vendors are salivating at the thought of widely deployed SDV.

    You will also need more return path demodulators as the message traffic increases significantly over just VOD.
     
  13. Dec 27, 2007 #913 of 2401
    dswallow

    dswallow Save the Moderatоr TCF Club

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    "640K ought to be enough for anybody." -- Bill Gates, 1981 :p

    It is always a matter of conflicting requirements... it's not unreasonable to expect cable companies will want to provide at least the first few tiers of basic channels in full to all subscribers just to avoid the inevitable complaints from unidirectional device owners who want to have access to channels. Inevitably, though, those same device owners will also want some SDV-based channels, so they'll be upset with you anyway.

    It's much like the "need" to keep any analog channels around today. Look what we can do with the bandwidth taken up by one analog channel now and justifying using them for analog channels becomes more and more difficult.
     
  14. Dec 27, 2007 #914 of 2401
    mikeyts

    mikeyts Stream Warrior

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    SDV just does not buy you enough on channels that are popular. Any SDV channel runs the risk of being requested when there is no bandwidth for it and the more popular the channel, the less acceptable that risk becomes.

    I can't find it now, but I read one study online in which it was found that no more than 34 of the least popular 200 channels of a set of 300 was ever tuned concurrently on any last-mile segment of the cable system under study. Those are the kind of efficiencies that make SDV worthwhile.
    Those are fairly easily justified--there are hundreds of millions of analog-only tuning televisions in use in the US today (my housemate uses one and until a year ago, I owned a 13-y/o 36" tube which is still in heavy use by the person that I gave it to). The cost of reclaiming that bandwidth now is the cost of distributing digital tuning devices to be attached to those televisions, and many of the owners of those televisions aren't interested in having a separate tuning box, because space is at a premium in the situation where the television is used (little sets on cramped bookshelves and hung under kitchen cabinets and sitting on workbenches in garages). Some people just don't want to have to deal with a separate device at all. (Someone needs to come up with a box that can simultaneously "tune" 75 digital basic/extended basic channels and blend them together as NTSC channels on coax--we about at the point where it can be done, but whether cheaply enough for this app is another matter).

    Every year, more and more of those televisions are replaced with digital tuning ones, so the problem gradually diminishes and eventually it will make sense to dump analog cable. It's likely to happen before it's convenient for everyone. The FCC has recently mandated that local television will have to continue to be rebroadcast by cable in analog until 2012 (even though the broadcasters themselves will be prohibited from putting it on the air in analog form :rolleyes:). That decision doesn't affect even all of the 20 channels that you get in basic cable though, much less extended basic.
     
  15. Dec 27, 2007 #915 of 2401
    dswallow

    dswallow Save the Moderatоr TCF Club

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    MicroTune has a chip (the MT2131) that, in volume, is under $3 that can tune analog, digital and QAM. It wouldn't be all the hard to imagine creating a device with a bunch of those coupled with NTSC modulators (or, I suppose, ways to recombine multiple streams into a QAM modulated channel). The main stumbling block would be dealing with encryption on all those channels simultaneously.
     
  16. Dec 27, 2007 #916 of 2401
    bicker

    bicker bUU

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    Famous last words in this industry, I suspect. :D
     
  17. Dec 27, 2007 #917 of 2401
    mikeyts

    mikeyts Stream Warrior

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    A couple of years back, Toshiba demonstrated simultaneous decoding of 48 SDTV streams (read from an HDD--see this) on the Cell processor, developed jointly by them, IBM and Sony and used most famously in the Playstation 3. Toshiba's interest in the processor was for general purpose multimedia content crunching in A/V devices; at this point, they're manufacturing the lion's share of them, having bought Sony's factory and picked up their PS3 demand as a supplier. I don't know what additional load decrypting the streams adds.
     
  18. Dec 27, 2007 #918 of 2401
    Luke M

    Luke M Member

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    http://www.broadlogic.com/tpix.htm
     
  19. Dec 27, 2007 #919 of 2401
    dswallow

    dswallow Save the Moderatоr TCF Club

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  20. Dec 27, 2007 #920 of 2401
    mikeyts

    mikeyts Stream Warrior

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    Very cool. Apparently BroadLogic demonstrated a "residential gateway" based on the chip at a CableLabs conference back in August (see this). A nice thing about this is that you'd only need one of them in a house--just put it in the attic (or wherever your cable comes in) and connect it between the incoming cable and the outlets in the rooms where you were using analog tuners. Any outlets to which you'd attached digital tuners and/or cable modems would need to stay on a clean line, but that should be easy enough.

    I wonder if these gateways could be made addressable, allowing the addition of subscription SD digital content tiers like the "Digital Sports Tier", and "Digital Lifestyles Tier", as well as the premium channel tiers (HBO, Showtime, Cinemax, Starz), most elements of which are still SD? That could actually enhance service sales to analog-only or analog-mostly households.

    In that article I linked, it gives a CEA estimate that there are an average of 3 analog televisions still in use per household. They are being replaced by digital televisions at a decent pace (estimated 16 million shipped this year), but it's gonna a while yet.
     

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