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Why is an HDMI signal causing interference with digital TV

PostPosted: Thu Nov 23, 2017 6:27 pm
by shabia
For this basically question, that looks so basic, right? But I'm not sure I can understand correctly. 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 https://en.wikipedia.org/wiki/HDMIand 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)
Thanks all guys who taking for your time to read.

Re: Why is an HDMI signal causing interference with digital

PostPosted: Fri Nov 24, 2017 10:45 am
by McGee2021
Well, this doesnt seem like it should be on the forum, but my best solution to the problem would be to shield the cable with some lead sheathing, but since that hasnt been available in years, just wrap the cable in some tinfoil, tie a copper wire around the foil, and ground the wire.

Re: Why is an HDMI signal causing interference with digital

PostPosted: Fri Nov 24, 2017 4:35 pm
by Robonz
The fact that rotating your antenna makes no difference indicates your wiring is suspect. Rotating the antenna should increase or decrease the signal which in turn should overcome noise. I would make sure all your antenna connections are good.

A reason the HDMI signal is interfering is quite often due to the cable being open circuit somewhere. It sounds like the TV signal is coming in through the cable and not the antenna. This would also let the HDMI signal come into the cable too

I hope that helps

Cheers
Keith

Re: Why is an HDMI signal causing interference with digital

PostPosted: Fri Nov 24, 2017 6:33 pm
by ppppenguin
Judging by the language, the OP is in the US. In the UK and most other countries terrestrial digital TV uses COFDM modulation which is pretty immune to a lot of interference. The US uses 8-VSB which I believe is more prone to interference.

This seems like a pretty standard case of RFI. Probably harmonics of the HDMI signal directly interfering with the received signal. Hence try usual cures such as ferrite rings and/or clamps on the HDMI cable. Also make sure it's a decent HDMI cable. I don't mean some exotic snake oil costing $$$$, just not the very cheapest which may be badly made. Try another HDMI cable anyway. Unplug the HDMI cable from the TV; does the problem go away?

Possibly use a masthead amplifier so that the off-air signal at the TV's input is strong enough to overcome local interference. Not that an amp near the set is not useful - you need to amplify the signal before it gets close to interefence.

Re: Why is an HDMI signal causing interference with digital

PostPosted: Sat May 05, 2018 10:02 pm
by n1ce1ce
I have mentioned the Chromecast Audio a few times as a viable streaming option. Its internal DAC is pretty good, and I think good enough for 16/44 reproduction. Right now, I am using it with my :arrow: Quad electrostats and I have no reason to complain. But if you want the best from it, a first rate external DAC might use the digital output for just possibly a better sound.

Re: Why is an HDMI signal causing interference with digital

PostPosted: Sun May 06, 2018 6:07 pm
by Dave Moll
ppppenguin wrote:Unplug the HDMI cable from the TV; does the problem go away?


It would appear so...

shabia wrote:I tried unplugging the HDMI cable from the TV...and voila the content returned immediately, crystal clear.