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

[marcus hall]

On Mon, Oct 14, 2013 at 11:50:24AM -0400, Jay Ashworth wrote:
> ----- Original Message -----
> > From: "Marius Schrecker" <marius.schrecker at lyse.net>

> > 2. The problem is the crowded conduit. I'm getting inductance problems
> > which are causing noise.
> > I need to split up the cable runs with max one or two cables per
> > condiut ( a pain in the *** as I'd need to rip the cellar ceiling
> > apart (again).
> 
> Not inductance so much as crosstalk, depending on what else is in that
> pipe. Power cabling is the only think I would expect to actually
> cause a problem; the jacket thickness should be enough to protect you
> otherwise.

Well, the way that crosstalk gets into a wire is through inductance, that
is, the magnetic field generated on one circuit couples inductively into
another circuit.

USB, however, being a differential circuit (D+ and D- wires) should be
relatively immune to inductance problems, since the induced signal appears
on both wires nearly identically, so the difference between the two wires
is mostly unchanged by the induced signal.

That said, though, it's still recommended that signal cables stay at least
a foot from power cables, and try to cross each other at right angles.
Running parallel for any significant distance is increasing the problems.

> > Does anyone know about the electromechamnics of what I'm doing and can
> > they shed some light on where I should start changing things?
> 
> Well, what's the fill percentage on that conduit?  For power, you don't
> want to go over about 60% or so -- and I can't imagine you *could*
> with preterminated LV cables; you couldn't get the plug though the pipe.

Running signaling cable and power in the same conduit makes me worry a lot
about code violations in many ways..

Certainly if the conduit is overfilled, there may be serious concerns about
heat dissapation from the wires carrying significant power.

All of the wires in the conduit certainly *must* be rated for the highest
voltage carried by any conductor in the conduit.

Termination of the various conductors at either end of the conduit must
be done to code..

etc.

Marcus Hall
marcus at tuells.org