Why is an HDMI signal causing interference with digital TV

Anything not specifically related to NBTV, but at least of some technical nature that might be of interest to NBTV members. Items for sale and links to retailers do not belong here.

Moderators: Steve Anderson, Dave Moll

Why is an HDMI signal causing interference with digital TV

Postby shabia » Thu Nov 23, 2017 6:27 pm

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.
shabia
"Fester, enough of the light-bulbs!"
 
Posts: 1
Joined: Thu Nov 23, 2017 5:58 pm

Re: Why is an HDMI signal causing interference with digital

Postby McGee2021 » Fri Nov 24, 2017 10:45 am

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.
John Logie Baird was obviously the man who sowed the seeds but did not reap the harvest.
User avatar
McGee2021
Just nod and pretend you understand me
 
Posts: 227
Joined: Wed Aug 12, 2015 9:41 am
Location: Tourist Trap Town

Re: Why is an HDMI signal causing interference with digital

Postby Robonz » Fri Nov 24, 2017 4:35 pm

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
User avatar
Robonz
Evil Genius
 
Posts: 137
Joined: Sat Aug 12, 2017 7:15 pm
Location: New Zealand

Re: Why is an HDMI signal causing interference with digital

Postby ppppenguin » Fri Nov 24, 2017 6:33 pm

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.
Jeffrey Borinsky www.borinsky.co.uk
User avatar
ppppenguin
Research Scientist
 
Posts: 40
Joined: Fri Nov 04, 2016 5:30 am

Re: Why is an HDMI signal causing interference with digital

Postby n1ce1ce » Sat May 05, 2018 10:02 pm

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.
n1ce1ce
"Fester, enough of the light-bulbs!"
 
Posts: 1
Joined: Sat Apr 28, 2018 2:16 am
Location: Australia

Re: Why is an HDMI signal causing interference with digital

Postby Dave Moll » Sun May 06, 2018 6:07 pm

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.
User avatar
Dave Moll
Just nod and pretend you understand me
 
Posts: 357
Joined: Tue Feb 27, 2007 9:11 am


Return to Off Topic

Who is online

Users browsing this forum: No registered users and 3 guests

cron