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

Stephen Worthington stephen_agent at jsw.gen.nz
Mon Oct 14 14:23:59 UTC 2013


On Mon, 14 Oct 2013 11:02:34 +0200, you 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).

No, that is not correct.  Lots of people miss that for an aerial to
work it also needs an earth.  So the aerials connected to the tuners
will have an earth path somewhere through the tuner.  If they were
connected only to your PC's tuners the earth paths through those
tuners would likely be common enough so that there was not a ground
loop.  But they are connected to the TV too, and that is on the other
mains circuit.  My 50 Hz audio hum from my amplifier only finally went
away when I unplugged all my DVB-T and DVB-S tuners from their
aerials.  It is possible to isolate aerials using inline capacitive
isolators on both legs of the aerial that pass the TV frequencies and
not the low frequencies - that is what I had to add to my system.  But
that was for fixing audio hum problems, which is likely not what you
are fighting.  Unless the hum is big enough to affect the power supply
in your imon device.

And unless you are using balanced outputs or optical to the audio
system, then there is almost certainly an earth path through it as
well.  Or is your audio going via the HDMI cable, then from the TV to
the audio system?

But ground loops are usually quite difficult to fix, and less likely
to be the cause of your problem, so I would try fixing things that are
easier to do first.

>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.

Unless that minimum length just happens to be the right wavelength to
pick up a bad signal!  It is quite a while since I last was doing this
sort of thing, but if I remember correctly, the shield needs to be
connected at only one end, otherwise you will be creating another
ground loop.

>Also possibly swapping out the isdn cable for good quality cat5-6 mi ght help with pd drop to the imon lcd.

That would be my first suspect as the cause of your problems.  I have
never seen an ISDN cable, but my guess is that it does not have much
current carrying capacity.  If you have to use cat6, use as many cores
of the cable as you can for each of the power and ground connections,
but maintain the pairing of power and earth in each pair.  Cat6 is
better than most signal cables for carrying power, but not really
designed for that.  So if you have to run new cable, how about a
shielded DC power cable?

> 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.

Yes, it probably needs to have the signal boosted in both directions.
It is certainly worth trying hubs at each end if you have or can
borrow another hub that works in the setup.  That is the second thing
I would try as it is easy enough to do and a good suspect for the
cause of your problems.

>Thabnks again for your input!
>
>Marius
>
>
>
>  


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