Click here to purchase the entire book in PDF format. Balanced transmission lines It is commonly believed even at the highest levels of the audio world that a balanced signal and a differential or symmetrical signal are the same thing. This is not the case. A differential (or symmetrical) signal is one where one channel of audio is sent as two voltages on two wires, which are usually twisted together. These two signals are identical in ever respect with the exception that they are opposite in polarity. These signals are known by such names as ``inverting and non-inverting'' or ``Live and Return'' - the ``L'' and ``R'' in XLR (the X is for eXternal - the ground). They are received by a differential amplifier which subtracts the return from the live and produces a single signal with a gain of 6 dB (since a signal minus its negative self is the same as 2 times the signal and therefore 6 dB louder). The theoretical benefit of using this system is that any noise that is received on the transmission cables between the source and the receiver is (theoretically) identical on both wires. When these two versions of the noise arrive at the receiver's differential amplifier, they are theoretically eliminated since we are subtracting the signal from a copy of itself. This is what is known as the Common Mode Rejection done by the differential input. The ability of the amplifier to reject the common signals (or mode) is measured as a ratio between the output and one input leg of the differential amplifier and is therefore called the Common Mode Rejection Ratio (CMRR). Having said all that, I want to come back to the fact that I used the word ``theoretical'' a little too often in the last paragraph. The amount and quality of the noise on those two transmission lines (the live and the return) in the so-called balanced wire is dependent on a number of things. 1. The proximity to the noise source. This is what is causing the noise to wind up on the two wires in the first place. If the source of the noise is quite near to the receiving wire (for example, in the case of a high-voltage/current AC cable sitting next to a microphone cable) then the closer wire within our ``balanced'' pair will receive a higher level of noise than the more distant wire. Remember that this is inversely proportional to the square of the distance, so it can cause a major problem if the AC and mic cables are sitting side by side. The simplest way to avoid this difference in the noise on the two wires is to wrap them together. This ensures that, over the length of the cable, the two internal wires average out to being equally close to the AC cable and therefore we pick up the same amount of noise - therefore the differential amplifier will cancel it. 2. The termination impedance of the two wires. In fact, technically speaking, a balanced transmission line is one where the impedance between each of the two wires and ground is identical for each end of the transmission. Therefore the impedance between live and ground is identical to the impedance between return and ground at the output of the sending device and at the input of the receiving device. This is not to say that the input and output impedances are matched. They are not. If the termination impedances are mismatched, then the noise on each of the wires will be different and the differential amplifier will not be subtracting a signal from a copy of itself - therefore the noise will get through. Some manufacturers are aware of this and save themselves some money while still providing you with a balanced output. Mackie consoles, for example, drive the signal on the tip of their 1/4'' balanced outputs, but only put a resistor between the ring and ground (the sleeve) on the same output connector. This is still a balanced output despite the fact that there is no signal on the ring because the impedance between the tip and ground matches the impedance between the ring and ground (they're careful about what resistor they put in there...)