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Need help from A/V gurus

Discussion in 'DIRECTV HD DVR/Receiver Discussion' started by acheriff, Apr 9, 2011.

  1. Apr 9, 2011 #1 of 23
    acheriff

    acheriff Mentor

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    I am mirroring the output from an HR24 to an SD TV in my kitchen. I have the composite out from the hr24 going into an RF modulator and then a very long run of rg6.

    Intermittently, I get faint diagonal or wavy interference lines. Not really hum bars. The hr24, rf modulator, and an appletv are all plugged into the same sure protector.

    What should be my troubleshooting steps?

    I'm thinking first replace the coax connector plugged into the modulator?
     
  2. Apr 9, 2011 #2 of 23
    spartanstew

    spartanstew Dry as a bone

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    What rf modulator?

    Did you have to assign it a channel on your display? If so, try using a different channel.
     
  3. Apr 9, 2011 #3 of 23
    acheriff

    acheriff Mentor

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    It's a radio shack RF modulator that allows assigning either channel 3 or 4. I believe the interference is unchanged on either channel, but I'll confirm.

    I'm also wondering if there is a possible shielding problem with issues related to the proximity of the modulator either to the HR24 or the surge protector.

    The connections are a bit of a rat's nest and the little bit of pulling and tugging and re-positioning that I've done of the various components has not clearly identified the issue.
     
  4. Apr 9, 2011 #4 of 23
    mdavej

    mdavej Hall Of Fame

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    I have about a 200 ft run from my modulator and no issues. It's probably a cable/connector problem. I'd remake each end first, then check cable integrity if possible (no kinks or breaks). I don't think proximity or the surge protector is an issue, but you can temporarily bypass the surge protector and see. I don't think the surge protector buys you anything in this case anyway since the coax is all internal at this point. Also do a channel scan on your tv so it gets a good lock on the channel 3 or 4 frequency.
     
  5. Apr 9, 2011 #5 of 23
    TomCat

    TomCat Broadcast Engineer

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    It could be one of two things, both signal ingress issues:

    1) poor shield integrity on the cable, and/or poor connectorization technique, which amounts to the same thing
    2) exceptionally strong interference

    Or maybe a little of both.

    Remember that garden-variety consumer RG-6 is typically only shielded to 75%. If your RG-6 and connectors are sound (which minimizes problem #1 and indicates problem #2), you may want to replace the cable with quad-shield coax, which is usually about 97% shielded. That is what cable TV folks use at their headends, where there are a lot of stray and strong RF signals floating around.

    The bad news is that may not be a fix; if interference is bad enough, it can be radiating directly into the chassis of the equipment, and will be problematic regardless of how good the cable and connectors are (since it takes an alternate path)

    There are a lot of interference candidates for Ch 3 and 4, including cabs, ham radio, and God-knows-what-else (use Ch 4 to minimize ham interference). I used to be a headend/sweep tech for cable TV on Galveston island in Texas, and they have a 26-boat Vietnamese shrimping fleet there which all use unlicensed 2-way radios with illegal linear amps on them--talk about signal ingress issues.

    Another fix could be a small RatShack house amp placed ahead of the cable. If you raise the level about 10 dB or so (careful, too much will overload the tuner) that lowers the ratio of ingress interference proportionally by 10 dB, which is usually significant. A $8 variable attenuator and a larger amp will allow you to fine tune this (raise the RF level until it just is below overload, which gives the best interference rejection ratio).
     
  6. Apr 9, 2011 #6 of 23
    TomCat

    TomCat Broadcast Engineer

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    That's a good call; sometimes these things are not shielded well and that can be a path for interference to get in.

    Not likely necessary; you either pick up the frequency or you don't; there really is no in between, and there really is no "bad" scan of a channel. A channel scan simply "listens" for a signal on every single available channel in order, and if it does not see one it moves to the next channel. If it sees a signal over a certain threshold it writes that channel to a lookup table that can tune the physical channel when the virtual number is input. When done, you have what amounts to a "favorite channel" list in the STB (of all the potential frequencies out there, these ones have active channels on them). If you don't get a "good" scan, that channel will simply not be receivable at all, because it will not have been added to the lookup table. The only reason to rescan is if a channel you know is out there, comes up missing. It will not impact your interference issues one way or another.
     
  7. Apr 9, 2011 #7 of 23
    carl6

    carl6 Moderator Staff Member DBSTalk Club

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    The symptoms don't really sound like a ground loop problem, but that is also a possibility. Make sure you've got good grounding on all items involved. Ground loop is more likely to manifest itself as horizontal or diagonal bars scrolling on the screen.
     
  8. joed32

    joed32 Hall Of Fame

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    May sound dumb but when I get those wavy lines I move the modulator around, flip it over, twist it, whatever, and the picture clears up. If you find a more permanent answer let us know.
     
  9. acheriff

    acheriff Mentor

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    Thanks for all the replies--they are all very helpful.

    I put a new compression connector at the termination by the modulator and separated the rg6 run from the power run to the surge protector.

    Despite my high hopes, the interference persisted.

    On a whim, I tried the original suggestion of changing the distribution channel on the modulator to channel 4---complete resolution of the interference!

    Anyone with a plausible theory?
     
  10. harsh

    harsh Beware the Attack Basset

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    Something that attacks channel 3 isn't usually going to attack channel 4 and vice versa. It may have something to do with local broadcasts (pretty rare any more) or an appliance that hums at just the right harmonic.
     
  11. houskamp

    houskamp Active Member

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  12. carl6

    carl6 Moderator Staff Member DBSTalk Club

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    Outside rf interference on channel 3 frequency leaking into the system. Analog tv channel 3 uses 60 to 66 MHz in the VHF-Low band. The amateur 6 meter band is 50 to 54 MHz, so not a likely source of interference. TV channel 2 is directly below, at 54 to 60 MHz (do you have a local channel 2 station?). Land mobile radio starts around 72 MHz, again far enough away that it's not a likely source of interference.
     
  13. acheriff

    acheriff Mentor

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    Oddly, I don't believe I have any local broadcast on channel 3. I do have locals on channels 2 and 4.

    I'll monitor over the next few days and post back.

    Thanks again.
     
  14. TomCat

    TomCat Broadcast Engineer

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    If the signal is strong enough, it can intermodulate with other signals creating a second-order or even third-order product. Those can be nearly at any frequency outside the normal band of operation, although theoretically they are the heterodyne product (one signal's frequency minus or plus another signal's frequency) of two frequencies.

    Signal overload in the output stage a source can easily create interference products outside its normal band, this from operating in the non-linear region of the transfer curve, which can produce spurs and other interference products on a number of frequencies all at the same time. This is why linear amps are illegal, as overdriving them can interfere with signals in a number of other bands.

    While these are not as likely as a signal designed to use the same frequency (just as harmonics are usually not as strong as fundamentals) it means you can not rule out a source frequency simply because it is not "operating" in the channel of interest, as it may be contributing to a harmonic or other product.

    Another issue is the IF frequency (40-46 MHz approximately for NTSC TVs). If a signal or a second or third-order product of a signal that falls in that band is radiating into the TV at this frequency it may manifest as interference. The FCC once licensed 43.5 MHz for pagers, and tested it with a continuous carrier signal for 3 weeks with no complaints. Once they turned it over to paging companies which then modulated that carrier, it created strong and obnoxious interference on every TV for a mile or two. They eventually had to pull the license and abandon that frequency, but our phones at the cable TV company would not stop ringing until they finally did.

    For the gentleman that "moves the modulator around" until the problem goes away, that means that either you moved it around until the source stopped keying the mic at that particular time, or (more likely) there is a bad connection, which means the shield integrity is compromised when the cable connector is held in a certain position.
     
  15. hasan

    hasan Well-Known Member

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    "This is why linear amps are illegal, as overdriving them can interfere with signals in a number of other bands."

    Eh?

    If I don't operate my linear amplifier in the linear region, then I'm going to get a notice from the FCC (at least in non-cw modes, and 99% of those running RF amps are indeed running them linear.) The exciter RF amps are also run in Class B or AB1 (both linear), sometimes even Class A.

    Could you please provide the context for your comment above, as I'm completely confused. It appears to be exactly backwards, linear is a legal requirement from the FCC, and the allowable distortion products are specified in the rules.
     
  16. raoul5788

    raoul5788 Guest

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    Your locals on 2 and 4 aren't really there anymore with the switchover to digital.
     
  17. harsh

    harsh Beware the Attack Basset

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    That's not necessarily the case. Several stations have gone to lower frequency assignments. I have a channel 4 less than a mile from my house.
     
  18. gphvid

    gphvid Godfather

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    Sounds like a signal strength issue. I would put a signal amplifier in the line to boost the signal along your rg6. My original DirecTV install needed those between the dish and my box. A long cable run could be robbing you of appropriate strength.
     
  19. mdavej

    mdavej Hall Of Fame

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    OP already solved his problem by changing the modulator to a different channel.

    We're now speculating why he had interference on the other channel. It really doesn't matter. That's why modulators and VCRs always have at least 2 channels. This problem was solved back when modulators were invented.

    Time to close the thread?
     
  20. acheriff

    acheriff Mentor

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    Aug 13, 2007
    To close the loop, I have remained "interference-free" since changing the channel on the modulator.

    While it was ultimately a simple fix, I did appreciate the education provided in the thread--it will no doubt come in handy for my next misadventure.
     

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