[mythtv-users] Audio - hiss

[Brian Wood]

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.