Newsgroups: rec.audio.tech
Subject: Re: PC soundcard to stereo receiver connectivity issues
References: 
"SmarSquid"  writes:


> I need some help with an aggravating system imperfection. The
> TV/VCR/DVD/STEREO RECEIVER components of my A/V system interface with my PC
> sound card through the use of 2 sets of 6' RCA to stereo mini plug cables
> (it's a loop - one cable carries signal from the receiver to the sound card,
> and the other sends signal from the sound card to the receiver).
> 
> With all this interconnectivity, a loud buzzing/humming noise is present in
> the system. 

Sounds like a very typical ground loop problem.
For more information on them and how to solvel them take a look at 
my groundloop document which can be found at
http://www.epanorama.net/documents/groundloop/index.html

> If I remove the cable tv signal from the ANT IN of my VCR (which
> I use as the cable tuner), the noise is eliminated. If, instead, I remove
> the mini plugs from the PC sound card (or, conversely, remove the RCA jacks
> from the receiver), the noise is also eliminated.

Those experiments revelal that there are two effective methods how 
this noise can be removed:
1. Isolationg antenna connection
2. Isolating audio connections to PC

You can ue either one of the methods.
I would recommend antenna isolator using antenna signal  isolator.

> Typically, I have dealt with this problem by connecting a set of
> transformers (300 and 75 ohm) between the cable tv signal and the ANT IN of
> the VCR. This is low-tech, but traditionally gets the job done. 

This is one way to build antenna isolator. 
This works well if you use transformers which provide isolation 
(not all 300 and 75 ohm provide that).

> I recently
> relocated, though, and using this same procedure to deal with the same
> problem in my new location, I am now experiencing different results. Though
> the buzz is still removed, the cable TV signal becomes degraded as it is
> processed through the transformers and most channels are unwatchable. To
> make myself clear, the cable tv signal is perfect if connected directly to
> the TV and VCR with no intermediary transformer processing.

It seems that the problem is caused by the signal level reduction. 
This kind of transformers cause signal level loss. In your earlier 
case the cable signal might have been at high enough level originally 
so that soem attenuation did not cause problems. But in your new place, 
the signal level was lower, so that it was originally high enough 
to just get good picture, but when attenuated somewhat gives unusable results. 
You can compensate the signal loss caused by the isolation system 
by installing a small antenna signal amplifier between your 
antenna outlet and the antenna isolator. This usually helps.

> My first attempt at a new solution was to amplify the cable signal before it
> hit the transformers. A 14 db boost did indeed make a noticeable
> improvement, but it is still way off the mark. 

Sounds strange...

> I called a cable technician,
> who ran several tests and assured me the problem is not the cable signal
> strength.

Maybe it is then the quality of your isolators.

> This leads me to believe that perhaps I need higher quality (or shorter)
> cables between my receiver and sound card, or maybe higher quality
> transformers between the cable signal and VCR. Any ideas? 

Higher quality transformers sounds like a good idea to me. 
High quality transformers and well shielded isolator sounds 
good to me.

>  I need to look at
> cabling/filtering/shielding options as opposed to upsetting the overall
> environment by moving everything around and whatnot.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/