[mythtv-users] OT: USB inductance (or other) problems with external peripherals

[Zig]

On 10/14/2013 4:02 AM, Marius Schrecker wrote:
>
> On Monday, October 14, 2013 10:42 CEST, Stephen Worthington 
> <stephen_agent at jsw.gen.nz> wrote:
>
>
>
>     Just a few thoughts of things that have bitten me in the past.
>
>     There is serious potential for ground loops - is the PC on a basement
>     power circuit? And are the USB hub and the amplifier the audio cables
>     plug into plugged into the sitting room mains? Combine that with the
>     earth paths through the aerials into the tuners, and you could be
>     picking up a lot of hum. And there could even be serious voltage
>     differences between the basement and sitting room circuit's earths.
>     Put a meter between an earth coming up from the basement and the
>     sitting room earth to check for that.
>
>     Long cables are also aerials, and can pick up all sorts of strange
>     signals when not properly shielded - not just from nearby cables or
>     equipment, but any transmitter they are the right wavelength to
>     receive. Powerful nearby radio stations are easy to receive on a long
>     cable of a matching wavelength and their signal can swamp other
>     signals.
>
>     My (tiny) experience with a long USB cable is that the voltage drop on
>     one is severe - you need to use cables with voltage regenerators to
>     get the device at the far end with enough voltage to work properly. Or
>     use a self-powered device at the far end, or a powered hub. Of
>     course, there are hubs and hubs - some USB devices do not like to be
>     on any hub, and some hubs work much better than others.
>
>     The imon display presumably has a backlight, so is drawing a fair bit
>     of power. So check the voltage it is receiving at the end of its
>     ISDN/RJ45 power cable while it is drawing that power - it might be
>     enough for the backlight, but not enough for its USB transmitter to
>     pump out a valid signal back through a long cable.
>
>     Problems like this are usually fixable if you know what the cause is -
>     so you may need to be using an oscilloscope to look at your signals to
>     diagnose the problem.
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>
>
> Thanks Stephen,
>
>  Very good pointers to what to look at.
>
> 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.
>
>  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.
>
> Thabnks again for your input!
>
> Marius
>
My guess is your problems are most likely do to power/ground problems 
rather than crosstalk or poor cables.

You can minimize the ground loops by plugging in all your equipment 
power cords into one or two outlet strips that are plugged into a single 
outlet.  That means you also have to run a power cord through your 
conduit through the floor along with your signal cables, but in my 
experience, this doesn't cause a problem.

This could be tested by bringing all your equipment temporarily upstairs 
(or downstairs) and using all the same cables and bundling them in the 
same way, and then power all your stuff with a single wall outlet and 
see if the problem persists.  If the problem is gone, you can even test 
running power along side your signal cables to see if it causes problems 
- my guess is that it wont.
Ziggy
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