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 ! _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
