phelum
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Location: Sydney, AUS

RPi4B loses Wi-Fi when DVI screen connected

Thu Jul 29, 2021 10:41 pm

Hi,
I'm using an RPi4B with 5.10 and buster. When I connect a Samsung 940MG TV with DVI input I lose the Wi-Fi link.

If I use the supplied micro-HDMI to HDMI cable with an HDMI-DVI adaptor at the TV end, the TV does not recognise the connection. In this case the wifi link still works.

If I use the supplied micro-HDMI to HDMI adaptor at the RPi4 and an HDMI to DVI cable, the TV does display the console image but I lose the wifi link. If I switch the TV off (soft-off) the wifi link is restored. If I leave the TV on and disconnect the HDMI link the wifi does work so I doubt it is an RFI problem.

The supplied HDMI cable is thinner than other HDMI cables and I'm guessing that it does not contain all the wires and this is why it will not work with this TV. Does anybody know if this is the case ?

More importantly, does anybody know how an HDMI connection can kill the wifi ? Or how I can investigate this problem? Although the wifi is dead the Pi is still running and will respond to the USB-connected keyboard. ifconfig does show the wifi has no connection.

Thanks,
Steven

drgeoff
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Re: RPi4B loses Wi-Fi when DVI screen connected

Thu Jul 29, 2021 11:36 pm

For certain resolutions the clock frequency of HDMI signals is in the same band as WI-Fi. Have you tried changing the Wi-Fi channel on your router or using 5 GHz rather than 2.4 GHz?
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phelum
Posts: 100
Joined: Thu Jul 17, 2014 7:05 am
Location: Sydney, AUS

Re: RPi4B loses Wi-Fi when DVI screen connected

Fri Jul 30, 2021 4:44 am

I'll try this tomorrow when no-one is using the network. But I doubt it will help because there is a RPi3B in close proximity and it is unaffected even when I replace its monitor with this TV/monitor. I'll also try to get a micro-HDMI to DVI cable so I can test without an adaptor at either end.

Cheers,
Steven

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Gavinmc42
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Re: RPi4B loses Wi-Fi when DVI screen connected

Fri Jul 30, 2021 4:50 am

I think some have reported RF leaky Micro to HDMI adapters.
Cheap cables also have issues.

Have not had problems with the Pi cables.
I do use a HDMI to DVI adapter on the DVI monitor end not the Pi.
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phelum
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Joined: Thu Jul 17, 2014 7:05 am
Location: Sydney, AUS

Re: RPi4B loses Wi-Fi when DVI screen connected

Fri Jul 30, 2021 5:22 am

Gavinmc42 wrote:
Fri Jul 30, 2021 4:50 am
I think some have reported RF leaky Micro to HDMI adapters.
Cheap cables also have issues.

Have not had problems with the Pi cables.
I do use a HDMI to DVI adapter on the DVI monitor end not the Pi.
The micro-HDMI to HDMI cable might have a problem because when I use it with an HDMI-DVI adaptor at the screen end the monitor doesn't work at all. I'll try this with another DVI monitor to test this. The cable is the one that came with the RPi4 package kit.

Re the Wi-Fi: Although I lose the connection whenever I attach the monitor (via the micro-HDMI to HDMI adaptor and the HDMI-DVI cable), I can still do "iw wifi scan" (my udev renames wlan0 to wifi) and it still finds the network. If I try "ifup wifi" it reports "already configured". If I try ifdown and then ifup I get the DHCP probe lines but no response. So it looks like a software issue because I can't get an address but the "iw wifi scan" can still see the network.

Cheers,
Steven

phelum
Posts: 100
Joined: Thu Jul 17, 2014 7:05 am
Location: Sydney, AUS

Re: RPi4B loses Wi-Fi when DVI screen connected

Fri Jul 30, 2021 11:22 pm

I've tested my Pi4 with a different DVI monitor and it works fine with both cable combinations.

I've just tested my Pi3B with this Samsung TV/monitor using the HDMI cable with the HDMI/DVI adaptor at the TV end and like the Pi4 I get the display but lose the Wi-Fi connection. So it looks like there is something strange with this TV/monitor and the Pi doesn't like it.

The DVI socket on the TV is marked DVI/HDCP. Is HDCP different from HDMI ?

My reason for wanting to use this TV here is that it has speakers whereas none of my other monitors do.

Thanks,
Steven

drgeoff
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Re: RPi4B loses Wi-Fi when DVI screen connected

Sat Jul 31, 2021 9:36 am

Are the monitor and the TV running at exactly the same resolution and frame rate? If not, read my earlier reply again.
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phelum
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Joined: Thu Jul 17, 2014 7:05 am
Location: Sydney, AUS

Re: RPi4B loses Wi-Fi when DVI screen connected

Sat Jul 31, 2021 10:21 am

drgeoff wrote:
Sat Jul 31, 2021 9:36 am
Are the monitor and the TV running at exactly the same resolution and frame rate? If not, read my earlier reply again.
Definitely both are 1280x1024 and I assume both are 60fps (how do I confirm this ?). The Pi3 and Pi4 are side by side near the TV and only the one that is connected to the TV is affected. If it is an RFI problem I'd expect both to be affected. The Wi-Fi does reconnect (after a while) once the TV is disconnected. I've also tried the TV connected to a number of non-RPi ARM devices and have no problems. I do have a Pi2B and will try with it.

drgeoff
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Re: RPi4B loses Wi-Fi when DVI screen connected

Sat Jul 31, 2021 6:03 pm

phelum wrote:
Sat Jul 31, 2021 10:21 am
drgeoff wrote:
Sat Jul 31, 2021 9:36 am
Are the monitor and the TV running at exactly the same resolution and frame rate? If not, read my earlier reply again.
Definitely both are 1280x1024 and I assume both are 60fps (how do I confirm this ?). The Pi3 and Pi4 are side by side near the TV and only the one that is connected to the TV is affected. If it is an RFI problem I'd expect both to be affected. The Wi-Fi does reconnect (after a while) once the TV is disconnected. I've also tried the TV connected to a number of non-RPi ARM devices and have no problems. I do have a Pi2B and will try with it.
1280 by 1024 is an very unusual resolution for a TV.
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phelum
Posts: 100
Joined: Thu Jul 17, 2014 7:05 am
Location: Sydney, AUS

Re: RPi4B loses Wi-Fi when DVI screen connected

Sun Aug 01, 2021 12:16 am

drgeoff wrote:
Sat Jul 31, 2021 6:03 pm
1280 by 1024 is an very unusual resolution for a TV.
My apologies for the late-night and incorrect post. The correct size is 1440x900 and I'd just been playing with some 1280x1024 screens.

I've just been testing with my Pi2B (USB wifi dongle) and can get the link to fail if the Pi is very close to the TV. It's easy to get the initial DHCP probe to fail and harder to get the link to dropout once established.

Unfortunately it's all a bit academic now. I don't get the audio link because the TV is DVI not HDMI and the video link falters if the Wi-Fi network is busy enough. So I'll leave this possibly futile project for the moment.

Thanks for your help.

-- Steven

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