From: "William Sommerwerck"

The "characteristic impedance" of a cable -- balanced or unbalanced -- is
determined by a number of factors that are under control of the cable designer.
One type of cable is not inherently higher- or lower-impedance than the other.
(In fact, I've never seen a balanced cable with a spec'd characteristic
impedance.)

You won't see characteristic impedance specified for cable designed
for microphone or console/rack wiring, but cable that looks like auido
cable and is constructed in much the same way, but designed for
digital signals nearly always has the characteristic impedance
specified. AES/EBU cables look like mic cables and work great as mic
cables, but they are built for a characteristic impedance of 110 ohms,
and are also constructed so that the impedance is uniform over the
length.

The characteristic impedance is not generally seen as a significant
performance factor at audio frequencies.

Which is why they don't bother to specify it or even design the cable
to a characteristic impedance specification.


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In article writes:

I attended Bill Whitlock's tutorial at the AES and this is at odds with his
definition of a balanced system. He says that it is the balance between source

and load impedances that determines whether a system is balanced or not and
that
the differential nature of the signals is irrelevent.

The input must be differential for common mode noise reject to work.
The source need not be differential (you don't need one lead going
positive with respect to ground while the other one goes negative with
respect to ground by the same amount). The only requirement is that
when looking back "upstream" to the source, each conductor of the pair
has the same impedance to ground (or the audio reference point).

This is why companies that hang a resistor between Pin 3 of an XLR
output (or the Ring of a TRS jack) and ground aren't lying when they
say that the output is balanced, even though onlyt Pin 2/Tip are
driven.


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Mike Rivers wrote:

In article writes:

I attended Bill Whitlock's tutorial at the AES and this is at odds with his
definition of a balanced system. He says that it is the balance between source

and load impedances that determines whether a system is balanced or not and
that
the differential nature of the signals is irrelevent.

The input must be differential for common mode noise reject to work.
The source need not be differential (you don't need one lead going
positive with respect to ground while the other one goes negative with
respect to ground by the same amount). The only requirement is that
when looking back "upstream" to the source, each conductor of the pair
has the same impedance to ground (or the audio reference point).

This is why companies that hang a resistor between Pin 3 of an XLR
output (or the Ring of a TRS jack) and ground aren't lying when they
say that the output is balanced, even though onlyt Pin 2/Tip are
driven.

I would only call that configuration impedance balanced or quasi-balanced. But then I don't
like fibbing about specs.

Not to be confused with 'ground sensing' outputs that use pin 3 to detect any ground
differential between equipment A and B and apply a 'correction signal'. Costs a few more
resistors around the op-amp output..


Graham :-)

In article writes:

This is why companies that hang a resistor between Pin 3 of an XLR
output (or the Ring of a TRS jack) and ground aren't lying when they
say that the output is balanced, even though onlyt Pin 2/Tip are
driven.

I would only call that configuration impedance balanced or quasi-balanced. But
then I don't
like fibbing about specs.

No, this is balanced. Period. I started using the term "impedance
balanced" (in fact I may have invented it, though I don't claim to be
the only one to invent the term) to distinguish such an output
configuration for people who expect a balanced output to have both
sides of the line driven with a signal with respect to ground. (But
then, how do you explain a transformer output which has only leakage
to give you any signal referenced to ground to those people?)

Not to be confused with 'ground sensing' outputs that use pin 3 to detect any
ground
differential between equipment A and B and apply a 'correction signal'. Costs a
few more
resistors around the op-amp output..

That's yet another scheme, but it's to give a differential output.
Then there's the single ended output with an inverter going to the
other lead to give a differential output - but those outputs are
rarely balanced because the impedance looking back into the two active
lines isn't the same. But to someone who looks only for voltage, it
looks like a "balanced" output.

The point that Bill Whitlock makes is that balanced and differential
outputs are different and that having one does not mean that you have
the other (or at least an effective version of the other).


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On Sat, 06 Nov 2004 06:55:13 +0000, Pooh Bear
wrote:

Mike Rivers wrote:
This is why companies that hang a resistor between Pin 3 of an XLR
output (or the Ring of a TRS jack) and ground aren't lying when they
say that the output is balanced, even though onlyt Pin 2/Tip are
driven.

I would only call that configuration impedance balanced or quasi-balanced. But then I don't
like fibbing about specs.

Mike has been writing long enough to have possibly invented the
term impedance-balanced. But when I've tried to defend it as the
optimum no-transformer solution, he has made the case that larger
headrooms are inherently available driving both lines actively.
And Monte has made the case that driving both lines minimizes
ground contamination. So, ya pays yer nickle...

My gripe is with the limitations of the term "differential input".
Of course we all know what it means, but *all inputs* are
differential. What is a better term for three pin inputs?

Chris Hornbeck

In article writes:

My gripe is with the limitations of the term "differential input".
Of course we all know what it means, but *all inputs* are
differential. What is a better term for three pin inputs?

Oh, man! Now you REALLY want to confuse people. g


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"Mike Rivers" wrote in message
news:znr1099785841k@trad...

In article
writes:

My gripe is with the limitations of the term "differential input".
Of course we all know what it means, but *all inputs* are
differential. What is a better term for three pin inputs?

Oh, man! Now you REALLY want to confuse people. g

The natural result of revisionism. I'd be fascinated to see an
explanation of how "all inputs are differential".

The natural result of revisionism. I'd be fascinated to
see an explanation of how "all inputs are differential".

Simple. An unbalanced input senses the difference between ground and the signal
line. In that sense it is "differential."

This is why unbalanced lines are so susceptible to hum and ground loops -- the
ground is not absolute; it can be altered.

On Sun, 7 Nov 2004 05:55:42 -0800, "Richard Crowley"
wrote:

The natural result of revisionism. I'd be fascinated to see an
explanation of how "all inputs are differential".

Even "single-ended" inputs (hot and ground, yada yada) are
sensitive to the difference between their two input pins.
That's the literal definition of differential.

The unique quality of three-pin "differential" inputs is a
somewhat different dealio.

Chris Hornbeck

"William Sommerwerck" wrote in message
...
The natural result of revisionism. I'd be fascinated to
see an explanation of how "all inputs are differential".

Simple. An unbalanced input senses the difference between ground and the
signal
line. In that sense it is "differential."

This is why unbalanced lines are so susceptible to hum and ground loops --
the
ground is not absolute; it can be altered.

"William Sommerwerck" wrote ...
Simple. An unbalanced input senses the difference between
ground and the signal line. In that sense it is "differential."

Yes, and "that sense" is what practically nullifies the meaning
of differential. Exactly what I meant by revisionisim diluting
the meaning of our language. But perhaps it is so pervasive
that people don't notice it anymore. Too bad.

Simple. An unbalanced input senses the difference between
ground and the signal line. In that sense it is "differential."

Yes, and "that sense" is what practically nullifies the meaning
of differential. Exactly what I meant by revisionisim diluting
the meaning of our language. But perhaps it is so pervasive
that people don't notice it anymore. Too bad.

Right. "Differential" implies that the two inputs are "floating" -- neither is a
reference.

In article writes:

The natural result of revisionism. I'd be fascinated to see an
explanation of how "all inputs are differential".

That's so simple that even ** Phil Allison could explain it without
insulting anyone, but he's pretty clever.

An electrical voltage is a "potential difference" - that is the
difference between the electrical potential at one terminal and
another terminal. If one of those terminals is defined as zero
potential by calling it "ground" (even though it in itself may have a
potential difference relative to absolute zero) then the signal
applied between ground and the input terminal of an amplifier is
expressed as the potential DIFFERENCE between the input terminal and
the reference point, or ground.

When we speak of a differential input, we usually refer to a
differential amplifier where neither of the two inputs is ground, but
we still measure the voltage DIFFERENCE between them.

Where we get a noise advantage with the above type of amplifier is
that the potential difference between either one of the input
terminals and ground results in an output. For a positive voltage
applid to the input, the output is positive when the voltage is
applied between the non-inverting input and ground, and negative when
it's applied between the inverting input and ground. If the same
voltage (at the same time - for example noise picked up by the input
leads) is applied between both inputs and ground, the positive and
negative voltages, being equal, sum to zero and that noise is
cancelled.

On an "unbalanced" input, since the reference for the potential
difference is tied solidly to ground, it's always zero so you can
never have an opposite voltage to cancel out noise.



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