The G word: How to get your audio off the ground
Making a difference
Of course, such a way of working already exists. XLR connectors have an extra pin compared to an RCA connector. Pin 1 connects the chassis while pins 2 and 3 are the high and low terminals between which the signal is measured.
Sadly enough, this too is riddled with confusing semantics, some of which have turned out to be deadly. Balanced, differential, symmetrical, what shall it be? Before I pitch my tent at any one of those three, let me quickly revisit what school books and audio magazines usually make of it.
The source, they say, produces two signals which are each other's mirror image (figure 5). Any source of interference will affect both wires equally and the error is eliminated when the receiver subtracts the two signals.
Note how the authors of this type of explanation have difficulty shedding GND-think. If those two signals are neatly symmetrical, about what potential exactly are they symmetrical? The source's return node? The chassis? Any of those on the receiving side? And does it even matter? The input should only care about the difference between the two. The whole reason why the input measures the voltage between the two wires is precisely because it's trying to ignore those irrelevant potentials.
You can cut the amount of circuitry on the transmitting end by half simply by arbitrarily choosing some potential that it has handy anyway and connect one wire there. All it has to do is offset the potential on the second wire to make the difference between the two the wanted output voltage, see figure 6.
This is just as good as the previous one. There's no pressing need to drive both wires actively. One will do. On the receiving end it's only the potential difference that matters. If one wire is connected to whatever node the source calls "my zero volts" the receiver duly subtracts the potentials of the two wires, regardless of where its own personal zero volts might be with respect to the source's.
I'm 1.8 m tall when I measure myself standing on the office floor. But this is equally true when I'm standing on a landfill. If you want to know my height, simply subtract the altitude of the refuse horizon from the altitude of my bald patch. There's no need for me to be dug in halfway.
This is seriously good news. To change an output from single-ended to differential all you need to add is an extra wire to carry the reference potential to the receiver. The burden then falls on the receiver to make the subtraction.
Differential transmission of audio doesn't mean you need to make a symmetrical voltage.
An input that expects a symmetrical signal is not differential because it's trying to involve a third node into the equation whereas voltages are only measured between two nodes.
A word of terminology. The signal that we want to transmit is that measured between the two wires. This is also called the differential-mode signal.
The error signal we want to ignore is the one that gets superimposed on both wires (as measured with respect to the receiver's chassis potential). This error signal may be due to interference en route, but in practice it's mainly the difference between the chassis potentials of the transmitter and the receiver. That error signal is called the common-mode signal.
Coming up in Part 2: The ideal differential input.