The previous installment of this series dealt with how sound must be represented as electricity for us to work with it in the modern, pro-audio context. Mostly, we talked about the transducers you’re likely to encounter – microphones, that is.
The next step is to get that electricity passed along to the input stage of a console or other device.
“Wire” You Looking At Me Like That?
The simplest and most robust connection possible is a single cable carrying analog electrical signals. Analog cabling is subject to many problems, of course, including noise induced by electromagnetic interference. However, its simplicity reduces the number of ways that an outright failure can occur, and the connection tends to degrade “gracefully.” In other words, the cable will continue to pass some kind of signal unless it’s completely unable to function.
A cable is a bundle of conductors that have different roles in a successful connection. Audio connections require a minimum of two conductors: Signal (Hot, +) and Ground. It is, of course, entirely possible to bundle more conductors into a single cable, with the extra conductors having different roles. A three-conductor system might be used for a balanced connection, with Signal (Hot, +), Return (Neutral, -) and Ground…but this isn’t the only possible configuration! A three-conductor cable might also be unbalanced stereo, with two “hots” and a shared ground.
Cables do not have brains, and don’t “know” which application they are meant for – although certain cable constructions are better for different situations. The application, or interpretation of the carried signals is up to the manufacturers of input and output devices.
In pro-audio, there is a definite preference for “balanced” connections. As stated above, a balanced connection requires Signal and Return conductors for each audio signal being carried (along with a proper Ground conductor scheme as needed).
Balanced connections are helpful because of their ability to suppress induced noise. Interference gets “into” a balanced cable just like any other cable. All cables function as antennae, and the longer the cable the more functional it is. With a balanced connection, though, the induced noise is the same on both conductors, whereas the actual signal is inverted on one conductor. Balanced input stages are meant to accept only what is DIFFERENT between the two conductors, while – to some degree – canceling anything that is common. If the signal on one conductor is inverted, but the noise is the same on both, the signal should be preserved while the noise largely disappears.
Because of what I said above (that cables are unaware of how they’re supposed to be used), you can theoretically put connectors on any cable with the appropriate number of conductors, and use that cable in a balanced situation. However, cable made specifically for balanced connections will use “twisted pairs,” so that the noise exposure for any given conductor is the same along the length of the cable. Cables built for unbalanced use may not have any twist, which can cause one conductor to be more susceptible to noise. If the noise on one conductor is greater than on the other, the differential input stage will happily amplify the difference and pass the noise along.
I’m A Terminator
In a pro-audio context, we have a strong tendency to name cables by referring to the connectors attached to the ends. (We could connect everything via bare wire, but that wouldn’t be quick or convenient.) It’s important to note that cable termination does not necessarily guarantee that the cable will function a certain way.
For example, I can terminate an unbalanced cable with a connector generally meant for balanced operation. The cable will never actually be capable of carrying a properly balanced signal, even though the connector is capable of doing so.
Some common termination types – and thus, cable names – are:
— 3-Pin XLR, commonly referred to as XLR, or “mic-cable.” XLR connectors offer robust construction, with relatively large electrical contact points arranged in a triangle, and the ability to latch built in at both ends. Please note that XLR is actually a reference to an overall connector form-factor. An “XLR cable” might have connectors with 5 pins instead of three. 3-Pin XLR is most often used for mono, balanced audio, but there’s nothing to prevent the connector from being utilized in other applications. (Some older equipment even used 3-Pin XLR for loudspeaker connections.) XLR cables are most often wired with a male end and female end.
— TS (Tip-sleeve), often called a phone cable or guitar cable. TS connectors and their variants are available in various sizes, with 1/4″ and 1/8″ diameters being the most common. TS and other similar connectors are not quite as heavy-duty as XLR, because their “all in a line” construction can be more easily damaged by sideways force, and also because TS connectors are not often built with latching capabilities. The tip being larger than the sleeve offers some protection against accidental disconnect, though, and some manufacturers have also created latching jacks. TS cables are often seen with two male ends. TS cable finds applications in mono, unbalanced audio applications, along with certain switching operations.
— TRS (Tip-ring-sleeve), also called a phone cable (confusingly with TS), or stereo phone cable. TRS can be used for stereo audio, balanced, mono audio, and switching applications where two switches are to be addressed with one connection.
— TRRS (Tip-ring-ring-sleeve), which ALSO may be called a phone cable (causing even more confusion), or headset cable. TRRS, due to having four connection points, commonly finds use where stereo, unbalanced audio travels to a receiving end, and a mono, unbalanced signal travels from a sending end located on the same device – such as a headset with a built-in microphone.
Multiple TS variants can be physically inserted into a mismatching jack, with varying results. Cables with “too few” connection points will often seem to work normally when plugged into a jack with more connections. The opposite, however, is much less certain. TRRS-equipped headphones, for instance, can’t be counted on to pass audio as expected if mated to a TRS headphone output.
— RCA, also called a pin-jack. Any single RCA connector only has the electrical capabilities of a TS connector, limiting it to unbalanced, mono audio or single-point switching. Two or more cables with RCA termination are often combined by the manufacturer for convenience. RCA connectivity is even less robust than other types, especially as simple friction is the only barrier to a cable being inserted or removed. Many RCA cables are wired with male connectors at both ends.
A cabling “super-type” is that of the snake, or multicore. These are effectively a cable of cables, meant to help collect and run a number of connections in a single, physical bundle. Snakes come in many varieties, with many combining cables with different termination types. Some include a stagebox for one end, which may be removable if the snake is a high-end model.
At some point, you will very likely encounter a situation where a device producing an audio signal is incompatible with the receiving device, such as a mixing console. In certain cases, this is a purely physical problem, such as a balanced output on TRS faced with a balanced input on XLR.
In such a situation, the only conversion necessary is a simple adapter. There’s no problem to be solved with the signal itself. Rather, the connection for the signal needs to be reconfigured. A simple adapter has no electronics, but only the necessary wiring required to pass the audio straight through to a different connector. You must be careful that the wiring is as you expect, however. Some TRS to XLR adapters don’t actually include all three conductors!
Simple adaption is also all that’s necessary in many situations where you are reducing electrical complexity, such as mating a balanced output to an unbalanced input.
The problem gets a little more complex when you need to convert an unbalanced signal to a balanced one, and/ or where the signal producing device can’t effectively drive the console input. (The latter case is an impedance problem. A real discussion of impedance is beyond the scope of this series, but a tell-tale sign of an impedance issue is a connection where the signal seems surprisingly weak and sounds VERY poor.)
When you need to improve/ upgrade the ELECTRICAL part of the equation, a simple adapter is insufficient. You will need intervening electronics in the form of a DI box or similar device.
Some of these converters are very bare-bones, stuffing an electrical transformer into the body of an adapter and doing nothing else. Others may be a bit more full-featured, operating as an actual DI box with, perhaps, a pad for high-level signals, a pass-through, and a ground-lift on the output. In both cases, though, the internal transformer does all the real work. The signal is converted to a balanced output, and a reasonably broad range of impedances can be successfully handled.
These simple devices are especially easy to use because they are passive, requiring only the input signal to be present in order to function. They can’t handle every possible input situation, though, because their transformers have inherent limitations regarding input impedance.
Active direct boxes are more flexible at the cost of complexity. The core of an active DI is a circuit involving an op-amp which requires some sort of steady power supply to operate. The op-amp can be effectively driven by almost any input, though, making active DIs compatible with pretty much anything you’re likely to encounter. A reasonable rule of thumb, then, is to use an active DI box whenever you’re in doubt about what kind of DI is appropriate.