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I do not have cable or satellite television so I rely on Over-The-Air digital programming (free) and Internet streaming services to provide entertainment.

A couple years ago, I purchased a video card for my desktop PC which provides simultaneous HDMI and DVI outputs -- the HDMI cable is connected to my television and the DVI connection is for my computer monitor. The outputs mirror each other so that whatever I see on the Desktop monitor is available on my 65" Panasonic TV whenever I have the TV's input set to the HDMI port that the computer's HDMI cable is connected to. This setup has worked amazingly well, but just his weekend I noticed an issue:

A few years back, one of the regional over-the-air broadcasting stations had their 2000 foot transmitter antenna tower collapse. Prior to the collapse, I never had any issues receiving their signal. Unfortunately, after the collapse, the broadcasting company decided to retain the same output transmission power, but built the replacement tower significantly shorter. Consequently, I thought I had lost those channels forever.

This weekend, I performed a re-scan of channels on my TV and to my surprise (and joy!) found that these lost channels were again available! The signal is not the strongest, but since the broadcast is digital, everything looks great.

So while I was watching the programming on these rediscovered channels, all of the sudden I heard my Desktop PC wake-up (the fans start buzzing, the speakers crackle for a brief moment.. very common since the computer is always running Skype in the background). Well as soon as the computer wakes up, I notice my television signal is no longer available. I thought that peculiar timing so I tried unplugging the HDMI cable from the TV...and voila the content returned immediately, crystal clear.

Any ideas why the HDMI signal is causing interference like this? I've done some googling and found similar complaints for Chromecast device used with over-the-air TV, but have not found the explanation as to why it happens?

Additional info:

The TV is connected to an external outdoor antenna on top of the house. The antenna can be rotated, but the direction of the antenna does not make a difference for this channel.

Other TV channels (stronger signal) are received just fine when the HDMI signal is connected to the TV

The Desktop computer and television are on different circuits, but wired to the same ground (no ground loops)

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closed as off-topic by duskwuff, Daniel Grillo, Voltage Spike, uint128_t, dim Jul 8 '16 at 8:47

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – duskwuff, Daniel Grillo, Voltage Spike, uint128_t, dim
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ It would be interesting if you could look up the frequency of the weak station that gets masked, and see if it is perhaps an odd multiple of one of the clock or sync signals in the HDMI signal. If your video card/driver will let you, slightly changing the video timing might move the interference off the frequency of interest. \$\endgroup\$ – Chris Stratton Jul 7 '16 at 3:05
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    \$\begingroup\$ Ground loop maybe? And considering this only came up after the tower collapsed, it may be their fault. Not properly calibrated. Or you need shielded cables? \$\endgroup\$ – Passerby Jul 7 '16 at 3:24
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    \$\begingroup\$ I would start by trying out shorter, better quality HDMI cable. \$\endgroup\$ – Dmitry Grigoryev Jul 7 '16 at 12:12
  • \$\begingroup\$ This is off topic electronics.stackexchange.com/help/on-topic \$\endgroup\$ – Voltage Spike Jul 7 '16 at 19:49
  • \$\begingroup\$ Excellent suggestions, I will give them a try. Thanks for the responses. \$\endgroup\$ – digitale Jul 8 '16 at 4:58
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ATSC is an all digital feed. As such math, can be used to compensate for weak and / or corrupted signals. This can be done to such an extent that the recovered data may be indistinguishable from the transmitted data. However, as soon as just enough data is lost to make the math not work... poof... the signal appears to just stop. Some ATSC decoders still try to display a picture. These pictures usually look pixilated and jerky.

It may be possible that you are receiving a weak signal. And that weak signal is susceptible to even minor interference up to the point when the tuner turns the signal into a digital stream.

A computer is loaded with clocks, signals and devices that generate all kinds of interference. But, there are all kinds of things you can do to mitigate interference. Make sure all your antenna cables are clean and that they make good solid contact. Make sure your computer case is secure and no panels are missing. Perhaps add a ferrite bead to the HDMI cable. Re-arrange your TV and computer to see if proximity is effecting reception. Avoid running noisy cables along side the antenna wire. Test if keeping cables separate improves reception. Using a larger antenna or one designed for higher gain.

You might use an RF amplifier to boost your signal before the ATSC tuner. In most cases is better to place the amplifier closer to the antenna so you do not amplify noise picked up along the way from the antenna to the ATSC tuner.

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    \$\begingroup\$ And then of course there is the option of seeing if you can install a larger aerial to try and increase the signal strength. \$\endgroup\$ – Tom Carpenter Jul 7 '16 at 3:48
  • \$\begingroup\$ @Tom Carpenter, another good point! Missed that one. I'll add your point to the answer. \$\endgroup\$ – st2000 Jul 7 '16 at 12:01
  • \$\begingroup\$ Thanks for the informative and well thought out answer. I hadn't thought of using an amplifier. \$\endgroup\$ – digitale Jul 8 '16 at 5:00

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