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Determining cable tv/internet signal levels

Discussion in 'Electronic Repair' started by Dan, Mar 16, 2006.

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  1. Dan

    Dan Guest

    I am having difficulty with my cable provider (comcast). Through the
    cable box, a number of channels exhibit tiling, audio dropouts, or are
    completely black. When I view these same channels directly on my
    television (to the limit of its 99 channel tuner) they come in, albeit a
    bit fuzzy. In addition to this, my internet speed rarely approaches the
    paid for 8 meg, typically it's 5 meg or less. I'm wondering if this is
    a signal level issue. I have a 60 mhz oscilloscope. Can I use this to
    directly measure the signal at the cable outlet, and if so, what levels
    should I see? Comcast has been less than stellar in its response to
    these troubles, maybe if I can measure the signal levels I can at least
    determine if the problem is a low signal to the house, or originates in
    the distribution (cables, splitters, associated connections) internal to
    the house.


  2. Jamie

    Jamie Guest

    check your splitters and all connections.
  3. Bill Jeffrey

    Bill Jeffrey Guest

    Dan -

    This may or may not be relevant to your situation - I can't tell without
    more info.

    If you have high speed cable Internet (which it sounds like you do),
    your cable modem measures signal strength, and you MAY be able to read
    the actual measurements. For example, I have a Motorola Surfboard
    modem, and whoever is out there at will tell me
    the signal levels at the modem, both uplink and downlink.

    You can also go to and run some error-rate tests.
    These don't tell you the signal level directly, but a high error rate
    can be very indicative of low signal levels. It is quite sesnitive,
    right around the threshold level.

    In my neighborhood, Cox cable gives me very good service, but they
    really don't want to muck with the neighborhood-wide signal levels. I
    understand why - there is a delicate balancing act between signal levels
    and interference/distortion. If they crank up the Internet signal, it
    produces interference in some of the cable TV channels. If they crank
    up the cable TV channels, it produces interference in other channels.
    Etc etc. Last time I called the Cox tech out, he agreed that the
    incoming Interent level was too low at my location, and the solution was
    to install a bi-directional amplifier right at the service entrance to
    my house. This boosted the incoming Internet signal (but not the TV
    channels) coming into my house. He set the amplifier to simply "bypass"
    on the outgoing Internet signal. The neighborhood is happy, since
    nothing happened out there. I am happy, since 6 dB more downlink signal
    solved all my problems. Good solution!

  4. Bill writes good stuff. I'll just add that Comcast is very responsive to
    the these issues and you should first try their technical support. Has
    always worked for me and at no extra charge.
  5. By the way, I always request broadband technical support ... maybe that's
    the difference.
  6. DaveM

    DaveM Guest

    I agree. When I first got Comcast internet service, I had intermittent
    service... slow login, sometimes unable to login, disconnects, etc. I called
    Comcast and the tech they sent out fixed the problem on the first visit.
    The original installer had run an extension to my computer from the back of my
    main TV, where the cable was split to the rest of the house. The run from the
    pole to my TV set is probably 175 - 200 feet when all the turns and bends are
    accounted for. The tech looked at the signal levels, put a splitter in the line
    at a point closest to my computer, and ran a separate line to my computer. He
    said that increased the signal at my computer by about 3db. Haven't had a
    problem since (except for when hurricanes knocked it out for over 2 weeks).
    In retrospect, I probably need to replace all of the RG59 in my house.. it's
    about 25 yrs old by now {:>)

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Never take a laxative and a sleeping pill at the same time!!
  7. Dan

    Dan Guest

    Bill et al- thanks for the replies and Bill especially for the very
    useful site referral, I'd never seen that one before. I do have a
    Motorola modem (SB5101). The downstream measurements are 711 mhz (so
    much for the 60 meg scope ;-/ ), SN ratio of 34.9 dB & power level of
    -6.7 dBmV. Upstream is 23 mhz, 36.5 dBmV. If you can give me any
    insights into the implications of these numbers, I'd appreciate it.
    Also, what are they referring to on the page under "LOGS"? I see they
    go back a month, are these stored in the modem? There are quite a few
    entries reading "SYNC Timing Synchronization failure" of various types,
    is this a problem?

    As to calling comcast, I have repeatedly, they've sent out 3 guys, the
    first 2 essentially did nothing, the last "jerry rigged" (to use his
    term) the connector at the box end, saying the buried cable was of a
    type they no longer support, and that most "techs" don't know how to do
    this. This was 2 weeks after the initially "installation", despite
    talking to several supervisors & an individual at their engineering
    department & asking them to at least run a temporary line until the
    repair could be effected, since I need the Internet to work from home.
    I was told in a phone message from engineering that though I had NO
    service, a temporary line "is not required". I don't doubt they may be
    more responsive in areas where there are alternatives to their service,
    that's limited here, especially for true high speed Internet (6-8 meg,
    as opposed to .7-1.5 meg DSL), and in my experience their customer
    service has sucked. On my most recent contact, I told a "supervisor" if
    something wasn't done once and for all I'd have no choice but to cancel
    my account. Without skipping a beat, she replied "would you like me to
    go ahead and put that cancellation through now?" What an attitude.
    Finally she said she'd "make a few calls & get back to me". That was
    about 10 days ago, during most of which time I was on a business trip,
    and I've heard nothing. I can't wait until there IS an alternatve to
    the bastards. My previous cable company in another state was
    Wideopenwest, I nver had a problem with them, though I'm sure there are
    those with a different experience. Anyway, after replacing a wall jack
    yesterday & cleaning up all internal connections involved the tv portion
    is satisfactory (for the moment, knock on wood) but Internet speed (I do
    use is still typically 60% of what it should be.

    Thanks again all for the helpful replies.

  8. Just for reference, here are my numbers for a SBV5120:

    Frequency: 603 MHz
    S/N: 36 dB
    Power Level: 5 dBmV

    Frequency: 33.024 mHz
    Power Level: 30 dBmV
  9. Dan

    Dan Guest

    Thanks Tony, so your power level is 5, not NEGATIVE 5? (mine was -6.7)

  10. Bill Jeffrey

    Bill Jeffrey Guest

    And here are my numbers
    579 MHz
    SNR = 34
    Power = -5
    19.599667 MHz
    Power 44 dBmV

    I think the power level of -5 is OK. I seem to remember reading
    somewhere that it should be above -10, and sure enough when mine was -10
    was when I was ahving the trouble. The amp I spoke about is what raised
    it to -5, and now I have no trouble.

    Did you dig down into They keep changing the page
    layout on the opening page, and it looks like they have reached a new
    high in uselessness. But if you go down to and start looking around, it is a lot
    more useful. In particular, I came across an entire section on Cable
    Modem and Wiring Issues, and it touches on signal strength issues.

    There is an entire saection on Comcast, one on Cox, one on Adelphia, etc.

    Good luck.


    Sorry that you are having trouble with Comcast ... that is not typical ... I
    wonder what is different in your case? I'd suggest going to the top with a
    letter or two. I find that letters have much more impact than emails and
    phone calls.
  12. Dan,
    Yes that is +5 dBmV
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