[mythtv-users] Humming Noise coming from LCD tv

[Mike Thomas]

On Fri, 4 Apr 2014 11:24:51 +0100
Simon Hobson <linux at thehobsons.co.uk> wrote:
> 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.

On Fri, 4 Apr 2014 06:37:54 -0400
Raymond Wagner <raymond at wagnerrp.com> wrote:
> On 4/4/2014 5:46 AM, Mike Thomas 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.
> 
> They shouldn't.  DVI/HDMI extenders are basically nothing more than a 
> signal amplifier that boosts the signal from STP DVI/HDMI cable to
> UTP CAT5.  The only reason one might give you problems is if it
> doesn't bother passing back the DDC channel, but then depending on
> your graphics driver, you could just feed it your own pre-captured
> EDID block.

Dear Simon and Raymond,

Thank you for your interesting replies. I confirm that the HDMI
extenders I have used so far do indeed use just one CAT 6 cable.

At the time I investigated them I could not see any difference in
the advertised specifications. The manufacturers and suppliers simply
didn't answer my questions in a meaningful way. Whilst I know this is
generally a bad sign, it seems to be the norm in computers these days.

I wholeheartedly agree with your point about the signalling rate
requirements, but the comments about the DDC are just not true for my
extenders. My extenders do send EDID back to the computer. The entries I
see are correct for some modes and reflect those output by my monitors,
but most of the entries appear to be synthetically derived. The
generated entries generally cause the monitor to reject a signal
which meets their specs, but not always. I am interested in using the
proper EDID block. Perhaps you could provide a reference?

There appear to two styles of extender on sale: the balun type you
describe and the 10 gigabit Ethernet type. These latter extenders make
the claim of high resolution over impressive distances whilst
simultaneously providing a 100Mbit Ethernet connection. I would dearly
like to replace my existing extenders with superior models of whatever
type, but I am unwilling to take the plunge on these expensive devices
without some confidence they might work.

I am trying to drive a small number of HDMI sources over 60 meters at
1920x1080 at 50/60Hz and also drive a DVI source over 20 meters at
1920x1200 at 50/60Hz. I am using ProSignal's CAT 7 patch leads and CAT 6+
fixed wiring as this is the best I can find at the moment. Despite
the mismatch in the specification my use of CAT 7 patch leads has
certainly improved matters.

Do either of you have any experiences of good or bad extenders which
you would care to share?

With many thanks.

Mike.