[mythtv-users] Slightly OT: receiver input buzzing/humming

f-myth-users@media.mit.edu f-myth-users at media.mit.edu
Wed Jan 11 21:58:36 UTC 2006


    > Date: Wed, 11 Jan 2006 12:38:56 -0500
    > From: Steve Adeff <adeffs at gmail.com>

    > On Wednesday 11 January 2006 10:12, matthew.garman at gmail.com wrote:
    > > After I posted the message, however, I started reading about SPDIF
    > > and digital audio IO.  Looks like my 7NIF2 lacks the SPDIF header on
    > > the motherboard.  But I *do* have a Creative SoundBlaster Live in my
    > > pile of old components.  It's pretty old, but it does have digital
    > > out.
    > >
    > > I'm guessing that a digital cable would not be affected by grounding
    > > problems, right?  Or at least any grounding effects would not be
    > > audible?

    > optical connection yes, coaxial no, since coax still uses a ground which would 
    > link the two grounds together. I'd also suggest using a smaller ground wire, 
    > 14 or 16, so that if there is a ground fault it will want to travel down the 
    > components natural ground instead of flowing through the other component. As 
    > well, if you receiver does not have a ground prong then definitely ground it 
    > manually with a 12g to the wall socket ground!

Eh?  If he's sending digital information down the coax, then no amount
of ground hum will be audible at the receiver, until the hum is so bad
it starts flipping bits in the data stream (at which point, he'll hear
either silence or godawful artifacts, depending on the protocol---but
it sure won't be 60 Hz hum).

Sure, if the problem is ground loops, then anything which can
spuriously tie grounds together is a problem. But given the
randomly-interconnected nature of computer hardware (much less stereo
hardware), trying for a true single-point ground as it done in, e.g.,
lab instrumentation amps is infeasible.  If the coax is supposedly the
-only- source of ground interconnection, then going to an optical
interconnect might help, but that's a fragile solution.

It's ambiguous from the original poster's comment whether the problem
is new, or he's just started noticing it.  My guess, if the latter, is
that the shield on some cable got damaged, or something was recently
changed in the hardware configuration elsewhere (new component? new
cable?), and certainly replacing cables is the easier & quickest way
to debug that.  If he has another way of producing audio, I'd also try
substituting that at the end of the cable, once the cable is known
good.  (After all, it -could- be that some filter cap in the sound
board on the computer blew or is getting leaky; such things do
happen.)

While experimenting with tying chassis grounds together might be
interesting, not all components use that as their signal-ground
reference (though most do).  It might make more sense to try, just as
an experiment, putting all components on a single outlet strip, so you
know they're on the same phase and circuit.  (I've seen strange currents
induced on so-called power grounds when different phases meet through
equipment, especially in older structures with poor wiring [which is
one reason why certain lab and audio gear keep chassis ("power") and
signal grounds rigorously separate], and it's always possible that a
ground or connection somewhere has gotten ohmic and therefore there's
a voltage difference around that, in theory, can't exist.)


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