"Pseudo" balanced interfaces.
“Warnings” have been posted to the effect that not all “balanced” equipment interfaces are really balanced. Some are “pseudo” balanced. Sounds bad, doesn’t it?
I recently purchased an electronic crossover with “pseudo” balanced outputs (inputs are true differential balanced) and have therefore given some thought to this “pseudo” thing. I conclude that it’s not so bad after all. By the way, I have never had any problem with single ended interconnects, and this unit can be operated that way, and the differential interface had nothing to do with why I bought the unit.
A pseudo differential output simply means that what would be a ground wire in a single ended circuit is isolated from ground by the same impedance as the active wire, twisted up with the signal wire, and applied as the (-) signal to the differential input of the destination hardware. This configuration will exhibit the same common mode noise rejection as a true differential (balanced) interface. The (-) wire carries just the pickup noise, instead of the pickup noise plus the inverted signal. It is the (true) differential INPUT of the DESTINATION hardware that provides the rejection of common mode noise pickup.
So why bother with a true differential output? It doubles the voltage level of the signal interface, and this improves signal- to-noise ratio with respect to INTERCONNECT NOISE PICKUP. However, it adds whatever the noise of the extra (-) output circuit might be. With decent interconnects of any kind noise pickup in the wires is less than the circuit noise, and except in unusual circumstances an active true differential output doesn’t buy much.
I have suggested that having balanced interfaces is not a good reason to buy a particular piece of electronics. Equipment with balanced interfaces may be very good, but the balanced interface has little to do with it. (Complementary active gain stages are another matter). Now I add to that suggestion that one should not reject a unit out of hand just because the OUTPUT is “pseudo” balanced. What this says is that the designer was concerned more with technical performance than with marketing features.