[mythtv-users] Humming Noise coming from LCD tv

[Simon Hobson]

Mike Thomas <mt3 at pfw.demon.co.uk> wrote:

> Eventually I put my computer in a separate
> room and used a CAT 6 HDMI extender. It was then I realised what a
> mistake I had made... DVI and HDMI extenders seem to severely restrict
> the video modes you can use, and quite independently they also bugger up
> the already useless EDID from the TV. Hours of fun.

It depends a lot of what you use.
A good quality extender will use two Cat5e/6 cables and won't limit your resolutions or block the back channels. Any extender using only one cable will restrict something as there simply aren't enough connections to carry everything without some compromise.

If you look at the HDMI specs, you need 3 pairs for the video, then several more for the other signals. With only 4 pairs, then "everything else" has to be muxed onto one pair and you won't have anything for the back channel. With 8 pairs available, this isn't a problem.

In the past I've used VGA extenders which didn't have any ground connection for the sync signals unless you used screened cable, and the video signals were transformer coupled onto a pair each. I had to double up one side of the red* channel to use as a ground as it "nearly worked" but the screen was "jittery" when it missed sync pulses in the noise once the PC and screen weren't on the same power circuit.
* I used the red channel as I figured it would be the least visible if I introduced a bit of noise onto it.