Back in the days when people walked 10 miles uphill in the snow to get to school, producers had far fewer jacks and cable types to fuss with. Now, a seemingly endless (and expanding) variety of analog and digital cables and connections exists. Yet for many, just getting a firm grip on the analog letter jumbles such as RCA and TRS is a challenge. Round-peg-in-round-hole is not the whole picture with analog connections — a brief exploration into physics and electrical engineering is in order.
KILL THE NOISE
To begin, it's important to understand the difference between balanced and unbalanced signals and connections. An unbalanced cable essentially contains two wires: One is hot, carrying the audio signal, and the other is ground, or “earth,” transmitting no audio. An unbalanced connection is prone to noise from electromagnetic and radio-wave interference. Because there is no inherent way to eliminate this noise once it corrupts the signal, the shorter the cable runs used, the better. A balanced cable uses three wires for transmission: hot (positive), cold (negative) and neutral (ground). Balanced connections are preferable because they eliminate unwanted noise to a large degree. Briefly, it works like this: The hot and cold wires carry duplicates of the same audio signal, but at the source, the cold signal is flipped 180 degrees out of phase with the other. They equally pick up and transmit noise as they travel down the cable, and at the arrival source, the cold signal is again swapped 180 degrees and the resulting audio signals combined. The result is an in-phase audio signal that becomes naturally stronger while the noise from both lines is now 180 degrees out of phase. Due to laws of energy, this noise cancels itself out, producing silence.
With analog audio patching, little has changed. Most equipment still makes use of a few major cable and connection formats: phono (both ¼-inch and ⅛-inch), XLR (most commonly used for microphones) and RCA (typical for consumer-grade equipment). Phono cables can be balanced or unbalanced; balanced types are called TRS (tip-ring-sleeve), and unbalanced types are TS (tip-sleeve). TRS cables are characterized by two indented rings on the plug whereas TS cables have only one ring. The top portion is the tip; the area between the two indents on a TRS cable is the ring; and the space below the rings is the sleeve. You can use TRS or TS cables on balanced or unbalanced connectors, but using a balanced cable does not automatically balance a signal; the sources on both ends must be balanced jacks. Conversely, unbalanced cables used with balanced source equipment indeed unbalance the signal. Most professional-grade audio equipment uses balanced connections, and the most notable examples of unbalanced gear are the vast majority of guitars, basses and amplifiers. But TRS cables can sometimes serve a different purpose: to transmit a stereo signal down a single cable, like with headphones. This is not a balanced connection, however. When conducting a stereo signal, a TRS cable carries it unbalanced.
Yet another unique type of analog connector is the insert, which typically employs a ¼-inch jack (sometimes, it's XLR). This connector doesn't carry a balanced mono or a stereo signal in one direction; rather, a signal is sent from and returned to the same jack. These are often found on console channels and are designed to interrupt the signal chain to “insert” an external signal processor. A special insert cable, which employs (typically) a single TRS jack on one end and a pair of either TS or XLR jacks on the other, is used to send the signal down one wire and return it via the other.
The last notable ¼-inch — type cable is a speaker cable. These are TS yet are designed for much higher-powered signals and shouldn't be interchanged with instrument cables.
CHECK THE SPECS
Three-pin XLR cables also transmit balanced signals and can be used for microphone or line-level sources. If you plug in a 3-pin microphone directly to an XLR jack on a console or preamp, the signal will be balanced. The same is generally true if you connect two line-level devices that both sport XLR jacks, but in some rare circumstances, a jack may be internally wired unbalanced. The same XLR cable can likely be used, but, again, it will not automatically balance the signal. Also, 3-pin connectors on gear can be internally wired in different configurations, so it is essential to ensure that two jacks are wired the same before interconnection — read your manuals.
RCA connections, which are always unbalanced, are the small type typically used with home stereos and turntables. It is always best to keep RCA cables as short as possible and to invest in good cables. Any quality cable will be shielded, which helps with interference rejection. Note that analog RCA cables are almost always found in stereo pairs and are often colored red and white. One cable carries the left signal while the other (usually red) carries the right. When the right analog RCA jacks are not found in pairs on equipment, a mono signal is present.
Although this column merely scratches the analog I/O surface, plenty of good resources for proper cabling are available. One of the best references is Yamaha's Sound Reinforcement Handbook by Gary Davis and Ralph Jones. This book dives deep into the basics of sound theory and sound equipment and its application. It's written with live P.A. in mind, but the book covers everything from the basics to detailed engineering.