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

Karl Newman newmank1 at asme.org
Mon Oct 14 13:48:09 UTC 2013


On Mon, Oct 14, 2013 at 2:02 AM, Marius Schrecker <marius.schrecker at lyse.net
> 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
>

I was going to suggest trying high-quality cables, but you've got that
covered. Also, USB hubs are not all the same. If you're going to buy
another one, try a different brand/model and first try it in place of the
existing one.

Karl
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