<br />On Monday, October 14, 2013 10:42 CEST, Stephen Worthington <stephen_agent@jsw.gen.nz> wrote:<br /> <blockquote><br /><br />Just a few thoughts of things that have bitten me in the past.<br /><br />There is serious potential for ground loops - is the PC on a basement<br />power circuit? And are the USB hub and the amplifier the audio cables<br />plug into plugged into the sitting room mains? Combine that with the<br />earth paths through the aerials into the tuners, and you could be<br />picking up a lot of hum. And there could even be serious voltage<br />differences between the basement and sitting room circuit's earths.<br />Put a meter between an earth coming up from the basement and the<br />sitting room earth to check for that.<br /><br />Long cables are also aerials, and can pick up all sorts of strange<br />signals when not properly shielded - not just from nearby cables or<br />equipment, but any transmitter they are the right wavelength to<br />
receive. Powerful nearby radio stations are easy to receive on a long<br />cable of a matching wavelength and their signal can swamp other<br />signals.<br /><br />My (tiny) experience with a long USB cable is that the voltage drop on<br />one is severe - you need to use cables with voltage regenerators to<br />get the device at the far end with enough voltage to work properly. Or<br />use a self-powered device at the far end, or a powered hub. Of<br />course, there are hubs and hubs - some USB devices do not like to be<br />on any hub, and some hubs work much better than others.<br /><br />The imon display presumably has a backlight, so is drawing a fair bit<br />of power. So check the voltage it is receiving at the end of its<br />ISDN/RJ45 power cable while it is drawing that power - it might be<br />enough for the backlight, but not enough for its USB transmitter to<br />pump out a valid signal back through a long cable.<br /><br />Problems like this are usually fixable
if you know what the cause is -<br />so you may need to be using an oscilloscope to look at your signals to<br />diagnose the problem.<br />_______________________________________________<br />mythtv-users mailing list<br />mythtv-users@mythtv.org<br />http://www.mythtv.org/mailman/listinfo/mythtv-users</blockquote><br /><br />Thanks Stephen,<br /><br /> Very good pointers to what to look at.<br /><br />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. However, the only earth to earth connection is over hdmi (to the TV). 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. Also possibly swapping out the isdn cable for good quality cat5-6 mi
ght help with pd drop to the imon lcd.<br /><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.<br /><br />Thabnks again for your input!<br /><br />Marius<br /><br /><br /><br />