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

[matthew.garman@gmail.com]

On Wed, Jan 11, 2006 at 04:58:36PM -0500, f-myth-users at media.mit.edu
wrote:
> 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).

How bad is "bad"?  :)  I mean, there's definately noise between the
computer and the receiver.  Will I hear random bits being flipped (I
assume that noise will cause SOME flippage)?  Although I suppose
there's some kind of parity or error correction built in to the
protocol...

> supposedly the -only- source of ground interconnection, then going
> to an optical interconnect might help, but that's a fragile
> solution.

What makes it fragile?  Is it because of the point above, that I
might be close to enough bit flipping that I hear garbage?

> 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

I *think* it's new.  If it's not, then it's definately worse than it
used to be.  And yes, there were *major* changes---I got a new TV,
and got rid of the VCR and DVD (mythtv makes them obsolete).

> 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.)

Hmmm... it seems like my sound card is unusually quiet.  Part of the
reason I even noticed this problem is that I have to turn up the
receiver louder than I do for anything else when using my HTPC
(mythtv, xmms, whatever).  Even with PCM cranked, I still have to
turn the volume more than I usually do.

> (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]

In my embedded systems class in college, my professor had this great
analogy of why you ground signal and power separately: think of
indoor plumbing, and how the toilet output is generally routed
separately from every other drain.  In the case of a backup, you
DON'T want stuff from the toilet coming up!

Thanks!
Matt

-- 
Matt Garman
email at: http://raw-sewage.net/index.php?file=email