Don't bother to read this article if you never record vocals, don't perform or mix live, or produce electronic music using only software synthesizers and prerecorded loops processed through plug-ins, bounced to disk, and burned to disk internally. You are among the few musicians and engineers who never need to connect a music-making device to another piece of audio gear, other than to headphones or powered speakers.
For the rest of us, it takes mics, amps, instruments, speakers, cue boxes, recorders, computers, mixers, and a bunch of other physical devices to produce and perform music. All those things have to be connected somehow, which requires cables. Unfortunately, cables are susceptible to transmitting various types of induced noise, such as electromagnetic interference (EMI). Balanced lines are often used to combat this problem.
FIG. 1: In a balanced circuit, the matched (balanced) impedance of two conductors causes induced noise to be identical between them. The two noise signals then cancel each other at the differential amplifier.
Photo: Chuck Dahmer
A balanced line uses two primary conductors whose impedance is precisely matched. Their matching impedance renders them equally susceptible to interference, so they end up carrying the same induced noise signal. The receiving device effectively inverts one signal before adding the two together, thereby canceling the noise.
This is known as common-mode rejection. In other words, the balanced circuit rejects the signal that is common to the two conductors (the interference). The effectiveness of a balanced circuit is judged by the relationship between the common-mode input (or induced) signal and the remnant of the common-mode signal at the output. This value is the common-mode rejection ratio, or CMRR.
The desired signal, then, must not be common to the two conductors, or it too will cancel when the signal on one side is inverted and the two signals are combined. Although it is perfectly acceptable for the signal to be present entirely on one conductor, the more common arrangement is for one conductor to carry an equal but opposite copy of the other. In other words, its polarity is inverted so that a positive voltage on the second wire corresponds to a negative voltage on the first wire (see Fig. 1). If the signals carried by the two wires were to be combined directly, they would completely cancel each other, resulting in no output. Instead, the receiving device sees the difference between them, effectively adding them together.
This differential signal is created at the source and resolved at the destination by either a differential amplifier (electronic balancing) or a transformer (transformer balancing). Although the sonic merits of each are the subject of some debate, the performance of the balanced line is not affected by mixing and matching the two. In any case, the critical aspect of a balanced line is not whether the high (normal) and low (inverted) halves of the signal are perfectly equal, but whether the impedance of the two lines matches exactly.
When connecting balanced inputs to unbalanced outputs or vice versa, it may be minimally acceptable to tie the low conductor to the cable shield at the unbalanced end. To retain as much of the benefit of the balanced line as possible, however, it's far better to use a transformer to isolate the two from each other at the unbalanced end of the cable. A device designed for this purpose is known as a balun (for “balanced/unbalanced”). A direct injection (DI) box is fundamentally a balun, although it often incorporates additional features.
Their robust resistance to induced noise makes balanced cables useful in carrying analog audio, digital audio (such as AES3), and data (including Ethernet). There is a less-is-more school of thought that holds that balanced connections introduce unneeded circuitry into an audio chain, and that it's therefore better to prevent interference in the first place and use only unbalanced lines. For most real-world studios and P.A. systems, however, balanced lines offer powerful insurance against induced noise and EMI.
Brian Smithers is department chair of workstations at Full Sail University in Winter Park, Florida, and the author of Mixing in Pro Tools: Skill Pack (Cengage Learning, 2006).