Re: turning a consumer soundcard into "prosumer" w/ quasi-balanced outs

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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 !
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