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Problem with snow on certain channels

Discussion in 'Electronic Design' started by Doc, Feb 2, 2005.

  1. Doc

    Doc Guest

    Okay, I'm probably the last person on Earth who doesn't have cable t.v.

    I found that by getting one of these WalMart antennas with the
    multi-position knob and running it through a signal booster along with a
    50-ft length of coax cable taped in a fairly tight loop, I can get very good
    reception on several channels. That booster works great. Even the PBS
    station that doesn't come in that well without it looks fine with it. I have
    the antenna signal run into a VCR via the coax in and going out to the tv
    via the stereo RCA connections for sound and the composite video out.

    However, there is one channel, the CBS affiliate in the area, that while I
    can get the picture to be generally sharp, there are these horizontal bands
    of snow that crawl up from the bottom. As one gets about 2/3 up, another
    starts. They're also accompanied by static in the sound. I've tried
    numerous positions, even tried taking the rabbit ears outside, even on the
    roof. No matter how clear the picture is otherwise, those horizontal bands
    and static won't go away. I tried turning off everything else in the house
    to see if something was causing it, to no avail. Tried going directly in
    from the antenna to the coax connector on the back of the t.v. to see if it
    made a difference, no difference. It only became an issue for the first
    time when there was something on that I wanted to tape the other day.

    Looking for insight as to what causes this and what can be done to eliminate
    it? (Besides getting cable/satellite etc.)

    Thanks in advance for all input. I don't have a background in
    electronics/broadcast etc. but don't hesitate to get technical. I can always
    do research to bone up.
     
  2. good reading on
    http://www.wrightsaerials.tv/articles.html
    see eliminating ghosts, bottom of page


    martin

    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
     

  3. Get a digital set top box. You can get these for $199.
    In am referring to a STB for free over the air digital TV
    (using your present antenna).

    Doug McDonald
     
  4. Those bands are usually caused by electrical arcing. That's an external
    source of interference - it's not happening because the CBS signal is
    weak, it's happening because something else is generating a signal on
    the same frequency. The interference source is much closer than the CBS
    station, so you probably won't be able to get rid of it by moving the
    rabbit ears.

    I wonder if AM radio reception is also unusually noisy? You may need to
    call your local electric utility and ask them to check for "leaky
    insulators" on the power lines.

    What city are you in? (that way we can judge how far you are from the
    TV transmitters and what channels the stations are on. That has a
    bearing on how to fix the problem.)

    Wild guess: the CBS affiliate is the lowest channel and the only one
    below channel 7?
     
  5. Tam/WB2TT

    Tam/WB2TT Guest

    The horizontal bands of snow are more than likely casued by some kind of
    arcing of the 60 Hz line. The TV vertical sync frequency is 59.94 Hz; so,
    the 60 Hz related noise slowly moves relative to it. You might be able to
    locate the noise source by walking around with a portable AM radio, tuned to
    a vacant channel. Check out the power line on the street, and neighbors
    houses.

    Fish tanks are notorious for causing this type of noise. Also, old oil
    burners.

    Tam/WB2TT
     
  6. Doc

    Doc Guest

    Actually, the NBC affiliate is the clearest channel on 2. CBS is 6, ABC is
    9, WB is 18, PBS 24, Fox 35.

    I'm in Deltona, which is south of Daytona Beach.
     
  7. That is likely being caused by RF radiation from the AC power lines.
    Today's
    TV's have great filtering, but years ago, ac hum leakage would cause what we
    called "hum bars" that moved up the screen, exactly as you describe. I'll
    bet
    that the TV station with the problem is Low VHF -- that is, one of the
    channels
    2 through 6, as most such radiated interference drops off at the higher
    channel
    frequencies. You may have an arcing insulator on a power line, a bad
    transformer
    or some item in a neighbor's house. Most power supply circuits in
    electronic
    equipment draw current only on the voltage peaks, so the switching-on of
    diodes and regulators will reflect little spikes back into the power line at
    120 Hz, just a hair over twice the TV field rate, so you get two per screen
    and they move up because of that hair's difference.

    Three easy things to try:
    Install an AC line filter; a surge/spike suppressor is NOT the same
    thing.
    Ground the TV's chassis if the TV has only a 2-prong plug. (This used
    to
    be verboten in the bad old days of hot-chassis sets -- no more.)
    Try taking your long-cable rabbit ears away from the set on a
    *horizontal*
    line -- that is, away from the house, just not *up*, since there are
    wires
    in the attic that could be radiating power line hash.

    Final point: In spite of your best efforts, maybe you didn't turn off
    enough
    in the house. The furnace power supply, garage door opener, appliances
    with digital clocks in them -- all these and more have electronic power
    supplies that draw current only on the peaks. A dimmer in your neighbor's
    house could be the culprit. Some computer power supplies are real stinkers.
    Welcome to my nightmare!

    "Sal"
    (a former EMI engineer)

    PS: You're going to love digital when you make the jump. Doug is right.
     
  8. Well, I was way off: I was guessing you were in Charlotte<grin>...

    If you're in Deltona, you're *really close* to the channel 2 tower. (a
    mile southwest of Orange City) Channel 6 is somewhat further south,
    near Bithlo. Channels 6 and 9 are actually on the *same* tower - but
    the higher frequency makes channel 9 much less susceptible to this kind
    of interference. Anyway, that explains why you have problems with
    channel 6...

    Tam and Sal's suggestions are well-taken. (well, I suppose you probably
    don't have oil furnaces in Florida<grin>) Indeed, one co-worker once
    had a TV that interfered with itself! -- a RF filter on the power cord
    fixed that problem.

    The suggestion to try a digital box is also a good one. $198 at
    Wal-Mart, stores like Circuit City also carry them though I don't know
    if you can get one quite that cheap elsewhere. Connects to your
    existing TV **IF** it has direct audio and video inputs. (if not, I
    think you can buy an adaptor -- a "RF modulator" -- at Radio Shack for
    less than $50) You'll get an absolutely *beautiful* picture - none of
    the seasonal interference that I'll bet plagues channel 6 in late spring
    and early summer - and there's a good chance you'll get a few extra
    channels that aren't available with analog TV. There's no monthly fee,
    once you've bought the box that's it.
     
  9. I was an engineer at Ch 55 in Orlando in 1988 and Ch 6 always had a
    poor picture. Their signal was weak and there was a lot of noise in
    their chroma. I ran into their chief engineer at one ot the local parts
    houses one day and he was complaining about how bad NTSC was and that he
    couldn't wait to go digital. It was all I could do to keep from laughing
    in his face. The low budget Christian station (Ch 55) was running the
    old RCA TK46 cameras and had the cleanest live video of any station in
    the market. The entire engineering staff was two men to maintain four
    sites and our annual budget for the station was less that one tenth of
    what they spent. WE had a mix of very old equipment and state of the
    art but it all worked when we needed it. BTW, the Ch 55 tower is across
    the highway from Ch 2 in Orange city. Its 1749 feet and Ch 2 is around
    1200 feet. The last time ai was at the CH 55 tower five of the Orlando
    FM stations were on a single antenna array at about 1000 feet. The
    transmitter site was the cleanest layout and building I've ever worked
    in, in the broadcast industry.
     
  10. Jerry G.

    Jerry G. Guest

    What you described is usually caused by a local interference. This can be
    from a communications system that is emitting some harmonics that are near
    to the same frequency as the station you are trying to receive. It is also
    possible that there are two TV stations that are far apart, and you are
    about half way between, and they are on the same channel, or very close to
    each other's channels, or one is generating a harmonic to the other.

    Your best way to have proper reception, is to have a cable or satellite
    service. Using terrestrial service to have TV is not practical these days,
    unless the station is local to you. Your viewing of what is out there is
    very limited. Many of the TV stations are looking at dropping terrestrial
    service. This is very expensive to keep up, and it is very limited.

    --

    Jerry G.
    =====

    Okay, I'm probably the last person on Earth who doesn't have cable t.v.

    I found that by getting one of these WalMart antennas with the
    multi-position knob and running it through a signal booster along with a
    50-ft length of coax cable taped in a fairly tight loop, I can get very good
    reception on several channels. That booster works great. Even the PBS
    station that doesn't come in that well without it looks fine with it. I have
    the antenna signal run into a VCR via the coax in and going out to the tv
    via the stereo RCA connections for sound and the composite video out.

    However, there is one channel, the CBS affiliate in the area, that while I
    can get the picture to be generally sharp, there are these horizontal bands
    of snow that crawl up from the bottom. As one gets about 2/3 up, another
    starts. They're also accompanied by static in the sound. I've tried
    numerous positions, even tried taking the rabbit ears outside, even on the
    roof. No matter how clear the picture is otherwise, those horizontal bands
    and static won't go away. I tried turning off everything else in the house
    to see if something was causing it, to no avail. Tried going directly in
    from the antenna to the coax connector on the back of the t.v. to see if it
    made a difference, no difference. It only became an issue for the first
    time when there was something on that I wanted to tape the other day.

    Looking for insight as to what causes this and what can be done to eliminate
    it? (Besides getting cable/satellite etc.)

    Thanks in advance for all input. I don't have a background in
    electronics/broadcast etc. but don't hesitate to get technical. I can always
    do research to bone up.
     
  11. The OP does have a full complement of stations local to him. If he were
    in, say, North Dakota he might feel differently.

    Terrestrial service is impractical only if price is no object. What's
    cable these days, $30/month? (I suspect quite a bit more. I hear of
    plenty of people paying more than $100/month after premium channels.)
    That's *at least* $360/year, probably quite a bit more.

    You can buy a pretty nice antenna and a digital STB for $360, and
    they'll last more than a year. (+/- hurricanes.....)

    If you have to have those extra channels like HBO, Fox News, ESPN then I
    guess you need cable/satellite. There *are* those of us who don't find
    the extra channels worth the cost.

    I would not count on terrestrial transmission going away in the near
    future. A number of rather large firms have rather large investments in
    local TV stations, and will not surrender those investments without a
    fight. Terrestrial transmission continues to be the easiest way to get
    those local stations' programming to outlying cable systems.
     
  12. I'm sorry, Jerry, but for US analog TV, this is almost 100% incorrect:

    Harmonics do not appear as a pair of horizontal lines slowly moving up the
    screen

    Two (or more) stations on the same channel may produce a closely spaced
    pattern
    of hundreds of horizontal lines (co-channel interference), not two lines.

    Closely spaced channels do not interfere harmonically; harmonics are
    MULTIPLES.

    Terrestrial TV is quite practical and getting more so every day. Perfect
    digital
    pictures from 50+ miles are an everyday reality for many of us.

    Cable and satellite are superior to OTA (broadcast) only in raw numbers and
    maybe
    an immunity to certain forms of interference.

    Terrestrial service is increasing to meet a constantly growing demand.
     
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