Re: Question about how RCA wires & headphone mini-jacks "work".


"infamis" <no.email@xxxxxxxxxxx> wrote in message
news:uoPHf.56059$PL5.15036@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <0QtHf.4317$bW.1094@xxxxxxxxxxxxxxxxxxxxxx>, "mc"
<look@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
When I think it needs to be:

audio_generating_source -> conductor wire A -> destination device ->
conductor
wire B -> audio_generating_source


That's what it is, and conductor wire B is the outer shield of the cable,
which is called "ground."

Is <called> ground or is it <connected to> ground?

The former. It is not connected to the earth.

I "think" (this thread has made be rethink a lot of stuff I thought I
understood) I understand what ground means in DC, or more specifically, in
a
car's environment. "Ground" in that sense usually means (e.g., is
synonomous
with) "the negative battery terminal" (unless you have a postivie-grounded
battery, which would indicate the positive battery terminal). For example,
the
conection of say a car radio would generally be:

Ground means ANY point in the circuit which WE CHOOSE to ALWAYS connect one
side of our voltmeter to. Ground has no physical reality. Ground is merely
the arbitrary choice of a point in the circuit. In audio, the outer shields
of all the cables are connected together, so we choose that point in the
circuit to designate as ground.


battery+ -> conductor -> radio -> conductor -> metal body of car (aka
"ground") -> battery-
which completes the circuit.

I understand "ground" in DC circuits to be synonomous with either the
postive or negative battery terminal.

In a car, yes. But "ground" means "arbitrarily chosen reference point" and
I can imagine circuits in which the most logical choice of "ground" is not
connected to either side of the battery. Admittedly, they are fairly
unusual circuits, such as double-ended power supplies built from a single
battery.

Audio systems do not have, or require, rods driven into the ground. Let
go
of that concept. That is not what "ground" means with an audio signal.
Let
go of that.

I have to figure out how to read old posts on usenet so I can re-read you
guys' advice over & over til I get it.

Skip the long digression about electrical power transmission lines, unless
you like long digressions.

I think the problem is simply that you are attributing too much meaning to
the word "ground."

One thing to cling to is this: Audio cables are each 2 conductors. The
inner wire is one conductor and the outer shield is the other one. Two
ordinary wires would work, but they would pick up more hum. To minimize hum
pickup, we make one conductor hollow and put the other one inside it.

The second thing to cling to is this: AC doesn't require any more wires than
DC. Imagine connecting a DC battery (such as an AA cell) to 2 wires which
will deliver power to its circuit (a light bulb). Now swap the battery back
and forth in its holder (put it in forward, then backward). Either way, it
only needs 2 wires. Imagine somebody who could swap the battery back and
forth hundreds of times per second. That would be AC. Still only 2 wires.

Notice that this light bulb circuit has no "ground" until YOU CHOOSE some
point in it to designate as "ground." That is your choice and is simply a
name you give to one point in the circuit. The laws of physics do not make
reference to "ground."

Now remember again what I said about the 4-terminal amplifier. Imagine a
little amplifier to which you connect a microphone (which has 2 wires coming
out of it) and a speaker (which also has 2 wires). The mic goes to
terminals A and B and the speaker goes to terminals C and D. On opening up
the amplifier, you find that B and D are tied together inside. That is, B
and D are a single point in the circuit which is "common" (shared) between
both input and output.

Well... because that point in the circuit (B and D) is common to input and
output, it is convenient to connect the cable shields to it (so that the
inner conductors of the cables go to A and C); then all the cable shields
are connected together, which is a Good Thing for minimizing hum pickup.

And we call that point in the circuit "ground."

In audio, "ground" means "common to input and output signals," which is what
B (alias D) is in this circuit.

That's all there is to it. Rods in the earth have essentially nothing to do
with audio.

Enjoy!


.

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