[mythtv-users] Audio - hiss (Buzz)

[Dave Hofstra]

On 3/14/06, Scott Croft <scott.croft at cox.net> wrote:
>
> On Mar 12, 2006, at 8:01 PM, Scott Croft wrote:
>
> > Further to my post below I have found some additional information.
> > It would
> > seem to be a "grounding" type issue I think.  When I plug the patch
> > wire
> > into the line in of the sound card there is no hum.  If I pinch the
> > free end
> > of the patch wire with my fingers I can cause the hum.  I then
> > attached an
> > old headphone jack to the line in in the onboard audio card.  With
> > the wires
> > exposed I experimented with the audio in.  If I ground the center
> > audio wire
> > with my fingers or something other than the CPU case I can create the
> > hum/hiss in the right or left channel.  Grounding the wire to the
> > CPU case
> > dose not seem to cause the hum.  Grounding the "ground" wire from
> > the left
> > or right wire dose not seem to cause a change in the hum.  I am not
> > much
> > good with audio so anyone have any suggestions?  Thanks in advance
> > for any
> > input.
> >
>
> Are you saying that you have hum *until* you plug the patch cord into
> the the line in ?
>
> Is this hum what you initially described as "hiss"?
>
> Everything else you say is more or less normal, touching the "hot"
> input wire will normally cause hum, your body is acting as an antenna
> and picking up the ambient 60-cycle buzz in the atmosphere around
> you. Shorting the input to the case (ground) will normally eliminate
> most if not all output.
>
> What you are proving is that your audio amplifier seems to be working
> more or less normally, but if you have hum with nothing connected to
> the input which goes away when you connect a cable to the line in I
> would look for a bad input connector (loose/cold solder joint etc.)
>
> I also note that you initially said you had PCM turned up and line in
> down ?? For the setup you describe this would be wrong, you would
> want PCM down or muted and line in turned up, the TV card in your
> case is no different from the iPod as a sound source.
>
>
>
> Yes it is more of a Buzz/hum and not a hiss, I guess I did not use the
> right
> audio term!
>
> The hum is not present at all until one of two things happen.  I
> short/ground the left or right channel going to the line in, or I plug the
> sound card line in to the TV card line out with the patch cable.  After
> reading and asking question I agree with you that when I touch or ground
> the
> line in I should here the buzz.  The problem is that I get the same
> results
> when I connect the TV card to the line in on the onboard audio (also get
> the
> TV audio).  Both left and right channels have the buzz.
>
> I think that I tried the mixer settings you describe (think I read
> something
> to the same effect) and could not get any sound at all with settings like
> that.  Not sure about this.
>
> Since my last mail I have added a PCI sound card and disabled the onboard
> audio in the bios.  This works with no trouble.  Not the solution I
> wanted,
> but it works.  I am using all the same settings and hardware with the
> exception of the onboard audio.  I guess this tells me that my brand new
> onboard audio is broken, or I have some type of grounding problem with the
> PC??
>
> Again thanks for the input, it is much appreciated.  I am moving on to get
> the remote control working, and the s-video output to fill my entire TV
> screen.
>
> Scott
>
>
>
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>

I am no audio expert, but if it is truly a hum you are describing, I might
think about ground-loop problems.  Both of my machines with analog output
have this problem.  You can buy a ground loop isolator at an electronics
store, or try using a 3prong --> 2prong adapter to cut out the ground loop.
I have used the 2 prong adapter method on both of my computers and my
subwoofer and it solved all  my 'humming' problems.

Good Luck...!

Dave Hofstra
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