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Tivo HD Pixelation Troubleshooting

Discussion in 'TiVo Series3 HDTV DVRs' started by Chimpware, Aug 3, 2007.

  1. Nov 11, 2007 #361 of 1597
    mchief

    mchief Morey

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    Sep 10, 2005
    Alexandria, VA
    Just to add some more confusion. I am currently running a THD with 9.2J and have seen some macroblocking and sound drop outs. My Hitachi HDTV also has a SA cable card. My THD has a SA MCard. Using split screen, I am able to put the THD on one screen and the TV cable card on the other. I am seeing the same macroblocking on both screens. Apparently my problems are the cable or cable card (Comcast). The signal strength is 98-100, so I'm suspecting the broadcast.
     
  2. Nov 11, 2007 #362 of 1597
    Phantom Gremlin

    Phantom Gremlin New Member

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    Jun 20, 2002
    Tualatin,...
    Your post is very informative. But it also has the tone of "apologizing" for TiVo. As I indicated in this posting the ANSI spec says a receiver should accept a signal anywhere in the range of -12 dBmV to +15 dBmV. If the TiVo HD can't do it, then it is a P.O.S.

    I need to go back and re-read the ANSI spec. Maybe there's a section in there about going to the local Radio Shack and buying an in-line attenuator. Just in case.
     
  3. Nov 11, 2007 #363 of 1597
    SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    Based on what I have heard, I believe there is more going on with FIOS than meets the eye. The action that needs to be taken to get good results is counterintuitive. The kind of things that have to be done for FIOS dont have to be done with typical copper cable. Instead of automatically declaring Tivo guilty, perhaps a look at FIOS would be another path to consider. As to Tivo's tuner capabilities, with TWC, at one time I was driving the S3 with a +10 signal and saw no adverse effects.

    Taking another path... Does anyone have any technical info on the implementation of FIOS? Are they doing a block conversion on a chunk of optical spectrum to create the 54-860 Mhz spectrum? I cant envision a bunch of individual QAM and NTSC modulators in the ONT. If they are block converting... controlling the cumulative jitter would seem to be a challenge.
     
  4. Nov 11, 2007 #364 of 1597
    richsadams

    richsadams Active Member

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    Jan 4, 2003
    As you said, -4.
     
  5. Nov 11, 2007 #365 of 1597
    wbradney

    wbradney New Member

    79
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    Mar 3, 2003

    This is worth a try, but it's not a panacea. I've had the pixellation on FIOS on and off since the beginning of August, after having 5 months of zero pixellation from March - August. Nothing on my system changed in August to cause the problem. I've had numerous visits from Verizon, and the signal has been attenuated with a variable attenuator until the signal disappears completely, and the pixellation was reproducible on the same fixed set of channels at any signal strength.

    Verizon IS working with TiVo on this -- according to their last communication with me on my trouble ticket, they're sending a senior tech with specialized monitoring equipment out to me shortly. That guy will apparently work directly with the TiVo engineers to troubleshoot the issue.

    I know of a couple of other people in various parts of the country who have similar problems.
     
  6. Nov 12, 2007 #366 of 1597
    AbMagFab

    AbMagFab What happened, TiVo?

    3,638
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    Feb 5, 2001
    Then return it.

    Alternatively, if you want to get it to work, add in-line attenuation to get the SNR down to 31/32.
     
  7. Nov 12, 2007 #367 of 1597
    TimAtkins1

    TimAtkins1 New Member

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    Nov 7, 2007
    I've attenuated my signal down so SNR is around 32 - no more pixellation or macroblocking or RS incrementing.

    Because I could not locally purchase attenuators, I took the signal down about 11db by running the signal serially through 3.5db and 7.5db splitters. Other than being inelegant, is there is any issue with using multiple splitters as compared to dedicated attenuators?
     
  8. Nov 12, 2007 #368 of 1597
    SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    Leaving all those unused taps open might allow egress or ingress and lead to interference... but there is always the old saying... if it aint broke, dont fix it. The engineer in me would want to do it with proper attenuators... or at least terminate the open outputs of the splitters.
     
  9. Nov 12, 2007 #369 of 1597
    TimAtkins1

    TimAtkins1 New Member

    11
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    Nov 7, 2007
    I terminated the open splits right from the start. Thanks.
     
  10. Nov 12, 2007 #370 of 1597
    AbMagFab

    AbMagFab What happened, TiVo?

    3,638
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    Feb 5, 2001
    The low-quality splitters might also screw with signal strength non-linearly, so you might find variability with some channels.

    You can buy in-line attenuators at places like smarthome, for like $2 each. They come in -3db, -6db, -10db, -20db, and I think some others.

    I bought a bunch and just fiddled with each Tivo until I got to the right SNR. I chose not to do it at the source, but for no particular reason other than I wanted as strong a signal as possible right to the destination. My "gut" felt better that way.
     
  11. Nov 12, 2007 #371 of 1597
    SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    How much 'wiggle room' did you end up with? Can you take out 2db of attenuation or add 3-4db of attenuation and still work OK?
     
  12. Nov 12, 2007 #372 of 1597
    AbMagFab

    AbMagFab What happened, TiVo?

    3,638
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    Feb 5, 2001
    It's pretty coarse, if that's what you mean. Taking out 3db vs. 6db often didn't make a ton of difference in SNR, so I tended to err on the more attenuation side of things.
     
  13. Nov 12, 2007 #373 of 1597
    SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    I was thinking of functioning properly vs SNR. So if you added or removed 3-6db of attenuation... would it still work 'ok' ie no pixelation or other 'problems'?
     
  14. Nov 12, 2007 #374 of 1597
    AbMagFab

    AbMagFab What happened, TiVo?

    3,638
    0
    Feb 5, 2001
    Ah, I see. The magic number definitely seems to be 31 SNR. Anything higher, and you get pixellation/increasing RS Uncorrected. 31/32 was as high as I could get. 32/33 pixellation was back.

    So attenuate until you hit that magic number!
     
  15. Nov 12, 2007 #375 of 1597
    lrhorer

    lrhorer Active Member

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    Aug 31, 2003
    San...
    That is the ANSI spec, and it means just what it says. The receiver should be able to receive a signal on a single carrier witin the -12 dBmV to +15 dBmV range with the specified bit errror rate (usually about 1 bit error per million or billion bits). This does NOT mean the receiver will be able to reliably receive any signal on any medium where the channel in question is within that range. Very few if any consumer grade receivers, for example, could manage to reliably receive a signal on one channel at 0 dBmV if the adjacent channels are both coming in at +25dB mV. Similarly, the amount of 2nd and 3rd order distortions in the receiver itself climb rapidly as the number of channels and the total input power increases. If the CATV plant in question is delivering a full +15 dBmV wall to wall on 170 carriers, then any receiver would croak. Indeed, that level of signal may be enough to actually damage or even destroy some receivers. It amounts to 0.36 watts, which is enough to make a 1/8 or even 1/4 watt resistor mighty hot on most circuit boards. Even if not, no receiver has a common mode rejection that high across the entire 50MHz - 1000MHz band, and those sorts of broadband levels will drive any receiver insane. It's a lot like trying to hear someone scream at you at a rock concert right in front of one of the stage monitors. No matter how loud they scream, you won't be able to understand them.

    So is there an answer? Yes, there is. An optimum signal level would be where the lowest carrier in the band is 3dB higher than the lowest level at which the receiver can reliably receive that same channel. Any lower and the chances the signal may drift down to the point whrere reception becomes an issue start to rise sharply. There is no benefit whatsoever to increasing the signal level beyond this point. Therefore the best situation would be to arrange for the signal on the lowest level carrier to be about -9 dBmV and no higher than about -6 dBmV. The spectrum ideally should also be fairly flat. If the highest level carrier is more than 10dB higher than the lowest level carrier, then it is not unlikely your CATV provider may have a problem on their plant. Have them verify this. If not, then a tilt compensator may be in order. Since in the event the situation probably derives from being way at the end of the CATV cascade and having a very long subscriber drop, it's likely there may be a residential amplifier involved, and many high quality residential CATV amplifiers have variable tilt compensators built in.
     
  16. Nov 12, 2007 #376 of 1597
    lrhorer

    lrhorer Active Member

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    Aug 31, 2003
    San...
    This is not quite correct. It may be true for a CATV spectrum profile similar to yours the optimum signal level may be found when the S/N is about 32. This does not mean it will be optimum for everyone. The S/N is affected by a large number of factors and the optimum S/N depends on the total number of carriers, in-band flatness, noise level on the CATV plant, and the receiver sensitivity. The assumption your plant configuration is typical may or may not be true, and the typical plant profile may not relate to the CATV profile of any particular user. That said, they can try your recommendation as a starting point and refine it from there.
     
  17. Nov 12, 2007 #377 of 1597
    lrhorer

    lrhorer Active Member

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    Aug 31, 2003
    San...
    Actually, improperly terminated (i.e. loose) terminators will cause more ingress / egress than none at all, so make sure the terminators are tight. They need to be snug when tightened with a small wrench. It should not be possible to remove them with the fingers. Don't get too excited with the wrench, though, or you can break off a port.

    There is another big reason to terminate any spliter ports, however: flatness. You would be appalled to see what unterminated ports will do the the signal levels on the other ports of a splitter. In some cases it can cause an attenuation of over 10dB to some parts of the spectrum, or even possibly just to one or two channels. It's also not possible to predict very well which part of the spectrum will be affected or how much. It depends on the lengths of the attached cables.
     
  18. Nov 12, 2007 #378 of 1597
    lrhorer

    lrhorer Active Member

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    Aug 31, 2003
    San...
    Yes, there can be - maybe. Of course the mere fact there are more connectors in the equation means there are more single points of failure, and connectors are right up there in the list of things most likely to cause problems. That's not to say they will, but they can. If the connectors are of good quality and their installation is done properly, they shouldn't cause many problems.

    The problems they can cause include ingress and ghosting due to low return loss issues, especially if the connectors are bad or loose.

    In addition, while the splitters are said to increase the "flat loss" (properly termed attenuation), the fact is their response is not flat. Every splitter looses more at higher frequencies than at low frequencies. At 50MHz, a hihg quality 2 way splitter will lose very close to 3.5 dB between the input and output. At 1000MHz, the number may exceed 4.5dB. Multiple splitters will of course loose even more at the high end compared to the low end. Now if your subscriber tap happens to be fairly close to an amplifier, then this isn't going to have a significantly bad impact. Indeed, it may even make the signal better, because close to an amplifier the high frequencies are also going to be high in level, and multiple splitters will provide some welcome - although not really necessary - tilt compensation. Further towards the end of the subscriber plant the high frequencioes are going to be much lower in level already than the low frequncies, and the additinal equalization from the splitters can cause the highest channels to be too low in level.
     
  19. Nov 12, 2007 #379 of 1597
    Phantom Gremlin

    Phantom Gremlin New Member

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    Jun 20, 2002
    Tualatin,...
    Your explanation is interesting in general cable terms. However the discussion I was replying to was in the context of FiOS and an ONT on the side of the house. I would expect the ONT output to be balanced to within a few dB. Also hopefully we don't need to worry about things like "tilt" since in most FiOS installations the coax is very short (compared to a cable plant).
     
  20. Nov 12, 2007 #380 of 1597
    SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    It would seem a reasonable expectation... but is it met? Tilt could also come from the source itself or from off the shelf components with tilt compensation that isnt required. I certainly dont know.

    I would like to find a good explanation of how FiOS generates its 50-860 Mhz output. Block converter? It could also be interesting to see what comes out of the ONT above 860 and below 50Mhz... given the shouting next to the concert speaker line of thought.
     

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