Newsgroups: rec.audio.tech
Subject: Re: HUM Line Level Signal, HELP!
References: 
"Good Music"  writes:

> HELP,
> 
> I have a Hum which only occurs when I connect the 1/8" Audio ("Speaker")
> Output of my PC sound card to the Line Level RCA Input of my VHS vcr.
> 
> It is a really bizarre situation: I can connect the "speaker output" jack of
> my PC sound card to any amp or cassette deck and it works fine with no hum.
> Also , I can connect any other audio device (stereo CD player, stereo
> receiver FM output, etc) to my VHS vcr sudio input with no hum.
> 
> But when I have the "Speaker" output of my PC sound card running to my
> stereo amplifier, then as soon as I connect the "tape out" RCA of my stereo
> to the "audio in" (left channel = MONO) of my HiFi VCR, not only does hum
> start coming through the TV speaker, but the hum "backs up" back into my
> Stereo Amp, and my STereo speakers start humming!
> 
> I tried bypassing my stereo and connecting the "speaker" out of my PC sound
> card directly to the VCR audio in, but that still creates a hum.  Connecting
> any other device (CD player, turntable preamp, receiver FM output) to the
> input of my VCR works fine with no hum.

Sounds like a typical groundloop problem. 
Your PC is grounded to mains outlet ground. 
Your VCR gets grounded to the antenna cable ground. 
Thos two grounds have some voltage difference in them 
and the AC voltage between them can be heard as the 
humming noise.

Test my theory: Disconnect the VCR from antenna outlet 
(disconnect everything from wall antenna) and the humming 
should go away!
 
> So, I read an article about "unbalanced lines" (i.e. RCA type stereo
> connectors), and it says, certain pieces of equipment just have
> "incompatible" grounds (???? what?  I thought all grounds were the same!)

In ideal world all the grounds would be same. 

In a real world the different grounds can easily have some 
potential differences (from fraction of volts up 1 volts, 
but can be higher) because of the noise coupled to the 
wiring that makes the connection to the common ground. 
The noise can be coupled to this wiring in many ways: 
inductive coupling from nearby power wires/transformers, 
capacitive coupling from power wires, RF interference, 
leakage current from equipment together with groundign wire 
resistance cause voltage drop, voltage losses in wiring parts 
where the functions of neutral and ground are shared by 
same conductor (this kind of arrangement is used in 
some parts of electricla distribution system).

> Here's the article:
> http://www.whirlwindusa.com/tech01.html
> 
> They only solution they state is  to "use balanced lines". 

Balanced line is a good devices against very many noise problems 
with audio connections. It usually helps also in the ground loop 
problems when the equipment used are well made (there are some 
audio devices with poorly designed balanced connections).

> Then it goes on
> to explain how the balnced push-pull works, and how you can "lift" the
> ground at one end.  (Incidentally , I tried "lifting" the ground at one end
> for my Sound-Card-to-VCR connection, and that only produced MUCH MORE hum,
> although the audio signal was still audible...)

Lifting grounds on unbalanced connections generally does not help 
at all or help situation very rarely. 

> SO PLEASE HELP me figure out, how do I get sound from my PC (in the form of
> a .WAV file) into the audio input of my VCR?  I don't have money to buy a
> lot of pro audio gear and it seems I shouldn't have to... the PC has a sound
> card with a line level "speaker out" that works to my stereo, so there
> should be some way I can "fix something" to make it work to the audio in of
> the VCR.

Read my groundloop documentation from  
http://www.epanorama.net/documents/groundloop/index.html
It described the problem and gives tips to solve the problem.

The problem solving part for especially for this case can be found at 
http://www.epanorama.net/documents/groundloop/home_solving.html


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