Re: reading from cassette

[Don Bruder <dakidd@sonic.net>]

In article <20030831204429.21879.00000286@mb-m04.aol.com>,
 mjmahon@aol.com (Michael J. Mahon) wrote:

> 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.

As purely anecdotal evidence, I've cured *EVERY SINGLE* noise problem 
I've ever encountered when doing audio work by the simple method of 
making sure that both pieces of gear are solidly grounded *TO EACH 
OTHER*. Whether that meant finding the ground plane and attaching a wire 
to that on the computer, then figuring out a way to attach the other end 
to the ground (such as it sometimes is, especialy in today's cheaply 
made mostly-plastic junk...) of the amp/deck/whatever, or just using a 
good set of cables with a dedicated ground wire running alongside, this 
has fixed the problem for me every single time, without fail. 

It's not a question of the ground being "isolated", per se, (A concept 
which is practically the diametric opposite of what anything other than 
a protective, or "earth" ground is supposed to be - ground in the 
non-earth-ground sense is supposed to be a common-to-all-components 
reference level for the entire device. That level might be 0 volts, or 
it might be 50, or 90, or even 1500 volts. But it remains "ground" so 
long as it is the "common" level for the device. As such, "isolating" it 
defeats the entire purpose) but a question of ground truly being 
"ground" *FOR BOTH PIECES OF GEAR*. If you can measure a voltage 
difference between the two items' ground planes, you're going to have 
noise. No ifs, no ands, no  buts, you're going to have noise. The one 
with the higher "ground" level is going to (unsuccessfully, usually) try 
to draw the other item up to it's level, while the "other" item is 
trying to drag the higher-level item's ground down to match its idea of 
ground. If you "force" both items to agree to a single level by 
electrically bonding their ground planes together, the noise goes away. 
In my experience, that's a 100% certainty.

I've measured 40 volts of difference between the "grounds" of two 
different pieces of audio (in this context, I'm using "audio" to refer 
to *ANY* source or consumer of a desirable audio signal, be it computer, 
TV, radio, turntable, or whatever, not neccesarily strictly "dedicated 
to making noise" hardware) hardware in the past. The signal was so noisy 
as to be almost unusable for any purpose, let alone approaching 
"audiophile" quality. Placing a wire between the two items' "common" 
ground (not earth/safety ground - that's a whole different kettle of 
fish that may or may not be related to the "common" ground, depending on 
the exact situation) killed the noise completely, presumably by forcing 
both systems to agree that one level was ground. 

Is it voodoo, or is it science? I really don't care... it works, or at 
least, it has worked, repeatedly, for me. As long as "ground" in both 
machines is at the same level, whether that level is 0VDC, +4VDC, or 
even oscillating from -50 to +75 volts, as was once the case, the noise 
"magically" ceases to exist.

-- 
Don Bruder -  dakidd@sonic.net <--- Preferred Email - unmunged, SpamAssassinated
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