Re: reading from cassette

[mjmahon@aol.com (Michael J. Mahon)]

Don Bruder replied:

>In article <hPr4b.240101$It4.113796@rwcrnsc51.ops.asp.att.net>,
> "desiv" <desiv@attbi.com> wrote:
>
>> So, why would the recording of the MP3 to the tape (intermediary) not have
>> the noise from the PC's ground?
>> 
>> (I am not an electronics guy, so that's a real question.  I'm not trying to
>> be funny..)
>> 
>> Is that a type of noise that the Apple would pick up but a recording
>usually
>> doesn't???
>
>No, that's a type of noise that won't be present (on any audio 
>equipment) if the hookup is done correctly. A correctly cabled audio 
>hookup from a computer to an audio device will have no noise such as 
>what I think you're probably referring to (the "hummmmmmmmm" that is 
>sometimes heard). If it does, then something about the wiring between 
>the computer and the audio device is wrong - more than likely, grounding 
>between the devices is faulty - and correcting that fault will eliminate 
>the hum completely.
>
>For lots more details (especially if, as it seems, you're having that 
>problem), check out the various audio newsgroups and look for 
>discussions about "Ground loops" and how to cure them.

These problems are indeed caused by "ground loops", or, more
precisely, current flowing in the ground wire(s), producing a
voltage drop across the resistance of the ground wire that is
added to the signal.

The problem is that computers are not "audio equipment" and
frequently do not have grounds as well isolated from noise on the
AC line (not just 60Hz noise, but often harmonics of 60Hz
caused by very non-linear loads, like switching power supplies.

As a result, a computer-to-computer connection which would
produce good results between audio equipment, may produce
excessive noise.

Other means can be used to isolate the signals (thus decoupling
the grounds) or the ground can be made very low resistance to
reduce the voltage drop, but these are more difficult and less
reliable than simply using an isolated cassette deck as an
intermediary (not connected to both computers simultaneously,
of course!).

-michael

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