<span style="font-size:13px; line-height:1.6em">Thanks all for the very interesting discussion on fire-risk, rules governing conduits and ground loops, as well as the general discussion of my problem.</span><br /><br />Before I inline post the rest of my comment. I'd just like to clear one or two things up:<br /><br />I am aware of the pronciples of conduiting power cable, of inductance, amperages, heat generation between the paralell cabling and of the fire risk involved. Our mains voltage here is 230V (2x115AC - unlike the UK which is a shifted AC at 1x240+0). We have very strict rules and very robust, shielded mains installation cable as most of our housing is wooden. My conduit does not and will not contain mains voltages. In fact there is a contact near the computer which is on the next spur of the same circuit as the rest of my entertainment system if earth loops should prove to be a problem. I did some earth loop testing yesterday and did not pick up any prob
lems. My house circuit breakers also contain earth voltage trippers so I can safely say that earth leaks are at an absolute minimum.<br /><br />The maximum voltage running through the conduit is 12vdc and the amperage is also failrly low although I may have underestimated the current draw of the LCD backlight (which I think is linked to the problem).<br /><br />Rest of comment inline posted.<br /><br /> <blockquote>>What I hadn't calculated at all was that the computer and the power to the rest of the system ARE on separate mains circuits now - deliberate to minimise the risk of power being cut to the computer - and I didn't think about potential problems with earth loops.<br />>However, the only earth to earth connection is over hdmi (to the TV).<br /><br />No, that is not correct. Lots of people miss that for an aerial to<br />work it also needs an earth. So the aerials connected to the tuners<br />will have an earth path somewhere through the tuner. If
they were<br />connected only to your PC's tuners the earth paths through those<br />tuners would likely be common enough so that there was not a ground<br />loop. But they are connected to the TV too, and that is on the other<br />mains circuit. My 50 Hz audio hum from my amplifier only finally went<br />away when I unplugged all my DVB-T and DVB-S tuners from their<br />aerials. It is possible to isolate aerials using inline capacitive<br />isolators on both legs of the aerial that pass the TV frequencies and<br />not the low frequencies - that is what I had to add to my system. But<br />that was for fixing audio hum problems, which is likely not what you<br />are fighting. Unless the hum is big enough to affect the power supply<br />in your imon device.</blockquote><blockquote style="margin-left: 120px;"> </blockquote><div style="margin-left: 40px;"><span style="font-family:arial,helvetica,sans-serif">Thanks for the pointer. In fact my system has no areals at all
as the only external signal is through IP (fibre to the house then cat6 to the STB). The MythTV capture device is a Hauppauge HD-PVR. Granted. There is a loop through the USB shielding to the Hauppauge - HD-Fury - HDMI - HDMI matrix switch/splitter - STB -TV and back to earth but this has not changed with the new placement.</span><br /><br /><br /> </div><blockquote>And unless you are using balanced outputs or optical to the audio<br />system, then there is almost certainly an earth path through it as<br />well. Or is your audio going via the HDMI cable, then from the TV to<br />the audio system?<br /> </blockquote><span style="font-family:arial,helvetica,sans-serif">Audio is through HDMI as far as the TV, then toslink to the amplifier (the only way for consistently correct sync. with A/V content). For audio only content</span><span style="font-family:arial,helvetica,sans-serif"> I do have a tosl</span><span style="font-family:arial,helvetica,sans-serif">ink
connection from the PC direct to the preamp</span><span style="font-family:arial,helvetica,sans-serif">.</span><span style="font-family:arial,helvetica,sans-serif"> I have zero hum on all of my audio channels.</span><blockquote><br />But ground loops are usually quite difficult to fix, and less likely<br />to be the cause of your problem, so I would try fixing things that are<br />easier to do first.<br /><br />>Good point with the cables acting as antennæ. I think I should certainly be looking at good quality, double shielded cables of a minimum length for purpose.<br /><br />Unless that minimum length just happens to be the right wavelength to<br />pick up a bad signal! It is quite a while since I last was doing this<br />sort of thing, but if I remember correctly, the shield needs to be<br />connected at only one end, otherwise you will be creating another<br />ground loop.<br /><br />>Also possibly swapping out the isdn cable for good quality cat5-6 mi g
ht help with pd drop to the imon lcd.<br /><br />That would be my first suspect as the cause of your problems. I have<br />never seen an ISDN cable, but my guess is that it does not have much<br />current carrying capacity. If you have to use cat6, use as many cores<br />of the cable as you can for each of the power and ground connections,<br />but maintain the pairing of power and earth in each pair. Cat6 is<br />better than most signal cables for carrying power, but not really<br />designed for that. So if you have to run new cable, how about a<br />shielded DC power cable?</blockquote>I did start to test yesterday. Bringing the upstairs components down to the cellar, unthreading the conduit and using the same cables in free air. The conduit is not the primary cause of the problem as the IR signals are still exactly as borderline as they were in the sitting room. Increasing the cable length (with signal cable) for the LCD power cord does worsen the problem. Unfortunately h
ot-swapping the cable gives kernel panics so I have to reboot the system for each test of a new cable and I didn't get as far as a successful test before my wife wanted to use the TV ;-). I think I'm going to abandon signal cable altogether and go for good quality, shielded low voltage cable<blockquote><br />> I'll start by trying the same cables but in free air, to rule out conduiting problems, then shorten one cable at a time to see which one is having problems with the extra length. I might also try TWO posered USB hubs, one at each end of the usb connection, acting as repeaters so that I get the strongest possible signal through the cable.</blockquote>With yesterday's results I'm moving USB a little lower down the list of culprits, although I'm also upping the quality of the USB connections and have found a second hub to act as a repeater if I still have problems after loosing teh ISDN cable.<blockquote><br />Yes, it probably needs to have the
signal boosted in both directions.<br />It is certainly worth trying hubs at each end if you have or can<br />borrow another hub that works in the setup. That is the second thing<br />I would try as it is easy enough to do and a good suspect for the<br />cause of your problems.<br /> </blockquote>Thanks for all your comments.<br /><br />Cheers!<br /><br />--Marius--