5 Jun
2010
5 Jun
'10
11:53 p.m.
On Sat, Jun 05, 2010 at 01:49:02PM -0700, Niels Mayer wrote:
For reference:
http://en.wikipedia.org/wiki/Balanced_audio#Differential_signalling
..............
Signals are often transmitted over balanced connections using the
differential mode, meaning the wires carry signals of opposite
polarity to each other (for instance, in an XLR connector, pin 2
carries the signal with normal polarity, and pin 3 carries an inverted
version of the same signal).
Despite popular belief, this is not necessary for noise rejection. As
long as the impedances are balanced, noise will couple equally into
the two wires (and be rejected by a differential amplifier),
regardless of the signal that is present on them.[1][2] A simple
method of driving a balanced line is to inject the signal into the
"hot" wire through a known source impedance, and connect the "cold"
wire to ground through an identical impedance. Due to common
misconceptions about differential signalling, this is often referred
to as a quasi-balanced or impedance-balanced output, though it is, in
fact, fully balanced and will reject common-mode interference.
Common mode interference will be rejected provided the input at
the other end is truly differential. Many inputs on consumer or
pro-sumer type of equipment are not, even if they are presented
as such.
But calling such an output a 'fully balanced output' is
stretching the truth - describing it as such in a spec
or sales literature would be a plain lie.
There are many variations for both 'balanced' inputs and
outputs. For outputs there is at least:
1) Impedance matched, as described by the OP.
All the following must also be impedance matched, exept
the last for which the concept doesn't make sense.
2) Providing signal on pin 2 and inverted signal on pin 3,
both signals are independent. Connecting such an ouput
to a single-ended input will short-circuit one output
(which may or may not survive, and may or may not
produce nasty distortion), and reduce the signal by 6 dB.
3) An output that basically drives pins 2 and 3 with opposite
signals (as above), but controls the difference voltage
between pins 2 and 3 rather than each of them separately.
This means for example that if either pin is shorted to
ground the difference signal remains the same.
4) An output that uses pin 3 as an _input_ with the same
impedance as pin 2. The signal on pin 3 is added to
the ouput appearing on pin 2. This provides ground
loop compensation even if the receiver is unbalanced
and connects pin 3 to ground.
5) Fully floating output - the signal is between pins 2
and 3 and isolated from the ground potential.
Ciao,
--
FA
O tu, che porte, correndo si ?
E guerra e morte !