Looking for help with comcast cable signal attenuation

Discussion in 'General Electronics Chat' started by gte, Aug 2, 2011.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Hi,

    My on demand stopped working after comcast upgraded their firmware on my cable box as some sort of guide upgrade.

    They can't get out here for a week and after checking the status screen on the box, they confirmed the signal strength was too strong, which was what was causing my on demand to fail.

    Any ideas how I can temporarily attenuate the signal so I can fix my on demand until they get out here? I was thinking of trying a 100 foot cable in between the wall and the cable box. In case anyone wants to know, the status screen of the box says :

    Tuner 1 is 439.750 mHZ and has -12 dBmV (and it will fluctuate up to -20 dBmV)

    They did not tell me what a good baseline signal should, but it looks like
    100 feet of rg6 will cause 4.3db of loss at 400mHz ... is this my easiest option, or is there a better option?

    Thoughts? Thanks for reading.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ahh, -12dBmV is actually pretty low. Maybe they thought you were telling them it's at 12dB? The techs usually shoot for the signal being around 0dB.

    Did you add any splitters or other hardware between where the drop comes to your house (should be by the power meter) and your cable box?

    There should be at most one splitter between the drop and your TV, and one -10dB tap if you have or have been wired for a cable modem; sometimes a tech will use a 2-way splitter if they don't have a -10dB tap handy.

    Check to see if your coax is kinked anywhere. The RG-6 that is used should never be bent into a radius less than 1.5", or a 3" circle. If it's been bent more than that, its' electrical properties have been ruined. Lots of folks bend the coax right where it comes into/out of the cable box or into the TV or VCR/DVD player.

    Oh, if you've installed a VCR or DVD player or any other similar equipment between the incoming cable and the cable box, that's likely what's causing the problem.

    150 feet is about the maximum distance that RG-6 can be run from the main cable system (whether on a pole or underground) to a house. If it's further, the tech has to use RG-11, or the signal attenuation will be too great, particularly in the higher frequencies.

    The techs who maintain the main lines align the signal amplifiers so that the higher frequencies are much stronger than the lower frequencies (called "twist" in the cable biz); so that by the time the signal gets to the point of use, it's about even across the spectrum.

    If you add another 100 feet of cable, your signal quality will go down the tubes.
     
  3. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Thanks Sgt, I will check for splitters as it's been 4 years since I wired up the house, I do not remember. I do know am not passing through a vcr or dvd player.

    I don't know what they thought or if they misunderstood, but the conversation was via a chat client, so I will cut and paste it for clarity in case that makes a difference.

    I googled the error I'm getting (error 14) and found this information, one post at 06-05-2011 11:21 AM mentions a transmit signal being too high or something.

    http://forums.comcast.com/t5/Video-On-Demand/On-Demand-Error-14/td-p/852469


     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, anyway - if you really want to reduce the signal, you could try to pick up some decent splitters. A 2-way splitter will reduce your signal by 3dB-3.5dB. 3-way splitters usually have one port that's 3dB-3.5dB down and two others that are ~7.5dB down. Anyway, every 3dB is a half power point. -12dB is 1/16th the signal strength of 0dB. The advantage of using splitters instead of long runs of coax to attenuate signals is that you won't upset the "twist" using splitters. However, I think going below -12 will get you nothing but poor signal quality; if you're on digital only now, it'll make your screen look like a jerky robot.

    It's hard to tell a good splitter from the outside. The poor quality splitters have very small toroidal transformers inside that get magnetized if there are any spikes on the cable, which ruins their RF properties. They can be degaussed, but unless you happen to have a really good degaussing coil, it's cheaper to just replace them with good ones. The better splitters have larger toroidal transformers that will take more electrical punishment.
     
    gte likes this.
  5. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    A splitter is a better idea, I will try that. I'll report back if it works or not, and I will also report back whether the comcast person was confused about whether my signal was too strong or not. I wouldn't be surprised if you knew more about their signal strength then they did.


     
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