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POE filter in an apartment

Discussion in 'TiVo Coffee House - TiVo Discussion' started by Kerwin51580, Sep 25, 2012.

  1. CoxInPHX

    CoxInPHX COX Communications

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    Phoenix, AZ
    Does your Cable Co have retail stores? They should have them probably free.

    Or (several w/ free shipping)
    http://www.ebay.com/sch/i.html?_sacat=0&_nkw=moca+filter&_frs=1
     
  2. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    I'd put it on the input side of Splitter 2. Since both TiVos are connected to that splitter they'll be able to talk just fine and then there is no chance the POE filter will interfere with your cable modem. (it shouldn't anyway, but some people report slight drops in cable modem speeds after installing a POE filter)

    Dan
     
  3. lrhorer

    lrhorer New Member

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    San...
    It can be anywhere before the root splitter. If I read his topology correctly, the root splitter is splitter #2, so anywhere before that should be good.

    That is utter nonsense. A reflection would be deadly to the MoCA network. A reflection only doubles the signal amplitude at the reflection point. Everywhere else all you would be doing is producing ghosts - seriously bad ones. If a node is, say 15 meters from a reflection, then it would receive a particular bit in the bit stream at time t = 0, then it would receive the very same bit again at t = 0.05 microseconds later, or at 100Mbps or 5 bits later. The resultant signal would be nothing but a string of garbage consisting of adding together every bit in the stream with the bit that was transmitted 5 bits earlier. What's more, the two carriers would collide, causing alternating nodes of virtually zero signal and a carrier almost twice the level all the way down the line.

    The filter terminates the signal. It does not reflect it to any extent, as best the manufacturer can mange given the price of the filter.

    This is also incorrect, although not quite as badly. By definition, those carriers represent intentional signals, not noise. They could hypothetically interfere with something on your neighbor's system - most likely their MoCa system. but by definition they are not noise.
     
  4. lrhorer

    lrhorer New Member

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    San...
    Not. It's a high pass filter, not a low pass filter. Upstream frequencies on the CATV system are 5 - 40 MHz. Downstream frequencies are 55 to either 550, 750, 850, or 1000 Mhz. MoCA will be above that.
     
  5. lrhorer

    lrhorer New Member

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    San...
    It depends on the filter. Most probably play it safe and block 1000MHz +. Some might block frequencies as low as 850 MHz, but doing so risks losing video channels on some CATV systems.
     
  6. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    Nevada
    If MoCa uses MIMO like Wifi N then the ghost would actually improve the signal. Not sure if it does though, haven't read up on how exactly the MoCa standard works.

    Dan
     
  7. unitron

    unitron Active Member

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    semi-coastal NC
    A reflection on a cable may strengthen an analog signal if the reflection is in phase with the original, but the chances of that are perhaps only one out of 360.

    Terminate cables and transmission lines properly and save the phase shift for Hammond B-3s and Leslie cabinets.
     
  8. CoxInPHX

    CoxInPHX COX Communications

    2,350
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    Jan 14, 2011
    Phoenix, AZ
    So Cisco is putting out misinformation:
    http://www.cisco.com/en/US/prod/collateral/video/ps8806/ps9095/ps10163/7016817.pdf
    Features
    ● Prevents interference between adjacent subscriber homes that use MoCA technology
    ● Minimizes MoCA energy on the cable TV feeder
    ● Acts as a reflector to increase the MoCA signal level within the subscriberÂ’s home
     
  9. Kerwin51580

    Kerwin51580 New Member

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    Sep 20, 2012
    New York...
    Got mine of off ebay for about $8 with free shipping.
     
  10. lrhorer

    lrhorer New Member

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    San...
    Apparently so. It would not be the first time. Such nonsense is rife throughout industry, and Cisco is no exception. I suggest the last place anyone should seek technical information is an advertisement. Poorly supported statements, glossing over details, and even out and out lies are the norm for advertising. Most ads, even for a lot of technical companies, are written by people who have never even taken an engineering course or any physics at all, and haven't a clue how the item they are hawking works.

    All of that is true, and perfectly implied by what I said.

    Utter nonsense. In fact, because of its very nature, MoCA is very susceptible to problems caused by impedance mismatches. The first thing anyone who experiences issues with their MoCA system should do is make sure every port on every directional coupler is properly terminated. Note there is an excellent chance (in fact it is almost certain) the S/N of the MoCA signals will increase significantly, perhaps even drastically, which may be what the ad was trying to convey. In their technically deficient minds, this might have been what the advertising folks understood to be the case. Even if it were ultimately true, the increase in signal would not help, at all, since it is the S/N which determines the throughput, not the absolute signal level, and a signal increased by a simple physical reflection has precisely the same S/N as a properly terminated one. What's more, doubling the signal (even if it did somehow help the S/N, which it doesn't) would only result in an absolute maximum gain of 3 dB. Properly terminating the line, however, could easily result in an increase of S/N of more than 20dB, which is huge.
     
  11. lrhorer

    lrhorer New Member

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    San...
    That is specious. First of all, by definition MIMO requires radiators that are spatially diverse, which would require dual ports on the MoCA adapters. MoCA does indeed employ spread spectrum modulation which makes interference from reflections less impactful to signal levels in a certain sense, but it does not mitigate the fact that the reflected signal arrives later in time than the primary signal. If the reflection is close to the radiator, then it can help, yes, although in a coaxial system usually not. The fact the carrier is spread out in frequency and not entirely dependent upon any single small section of the signal means the awful spectral response generated by the reflection is not an issue for the receiver, and the sections that are received have a higher S/N, giving better throughput and fewer errors. If the space-time displacement of the reflected signal results in a phase angle in the payload of greater than 90 degrees, however, it will tend to completely destroy the information in the bit stream. Only if the level of the demodulated ghost is typically at least 6dB lower than the demodulated primary can the ghost ordinarily be electronically rejected for phase angles larger than 90 degrees.

    To put it a different way, again assuming a 100 Mbps payload, if the reflection is less than about .75 meters from one of the nodes, then the signal presented to that node will have up to a 3 dB higher signal level than if the line is properly terminated. Any node further than that away, however, will receive a horribly degraded bitstream from any node other than the one right by the reflection. Indeed, if the node is 1.5 meters from the reflection, then it is likely no other node on the system will be able to receive anything at all from that node for a 100Mbps payload. If the nodes drop the bit rate to 50 Mbps, they should all be able to talk once again. Note even the node that is half a meter or so from the reflection won't benefit much, however, since while the signal level may be higher, the S/N won't usually be.

    The bottom line is the last thing anyone wants in any coaxial transmission system and most especially in a MoCA system is reflections.
     
  12. lrhorer

    lrhorer New Member

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    San...
    I know the horse has already left the barn on this one, but for future reference, there is no need to power anything down. Disconnecting the inoput cable won't send anything into a permanent tizzy. Of course, while it is disconnected, you won't get any cable service for the duration, but as soon as it is reconnected, everything will come right back up.
     

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