A/V 107: The Basics of Audio Interconnections - TvTechnology

A/V 107: The Basics of Audio Interconnections

This is for those among us who don't know a lot about audio and are not quite sure we should ask.
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I thought I was done with this series about audio basics. However, reader Eric Smith wrote in and asked if I would write about matching signal levels between equipment and sources, and not just mic/line levels, but something more in-depth.

So, once again, here we go. This is for those among us who don't know a lot about audio and are not quite sure we should ask. In any case, in audio, we have all these different devices that we need to hook together to form a working audio system. Hooking them together (patching) is one of those basic mundane tasks that we take for granted.

However, about 95 percent of the problems we have in audio comes from faulty patches, meaning either a) we've hooked it up wrong, b) one or more of our patchcords is faulty, or c) there is some basic signal incompatibility that we don't understand between devices.

For technical simplicity, devices that serve as sources or outputs of signals are called "gzoutas" (as in "It gzouta the green box") and devices that receive signals are called "gzintas" (as in "It gzinta the purple box").

You got yer analog signal and you got yer digital signal. We'll talk about digital signals first, to get them out of the way.

DIGITAL SIGNALS

A digital signal is, of course, a high-frequency datastream of ones and zeroes (usually masquerading as positive and negative voltage pulses). It is generated in accordance with some formally established protocol in the gzouta. For the gzinta to recognize the signal, it must know and function in accord with the same protocol. As a general rule, digital signals either pass perfectly or they don't pass at all, except for one booby trap we'll talk about in a minute.

AES/EBU is the so-called "professional" digital audio standard. It is transmitted on a balanced line (see below) with XLR connectors. It carries a stereophonic (two-audio channel) signal. There are other details, such as making sure you're using a digital cable.

Sony/Philips Digital Interface or S/PDIF is the so-called "consumer" digital audio standard and is transmitted on an unbalanced coaxial line with RCA connectors (for instance, the yellow RCA-to-RCA video cables). S/PDIF is also stereo. Interestingly, the digital languages of AES/EBU and S/PDIF are the same, and it is generally possible to convert one to the other with an impedance matching transformer (Canare makes one--the BCJ-XJ-TRB).

Sometimes the S/PDIF signal is converted to an optical code for transmission via fiber-optic connector.

There are a slew of other digital formats you probably don't need to know about. However, you do need to know that all the gzoutas and gzintas have to be precisely synchronized for the digital system to work. This is usually no problem, but occasionally, synchronization won't be maintained (usually due to an operator error and related to one of many quirks about digital audio) and the audio will be contaminated by a string of annoying tics, clicks and pops. Watch out for this!

The analog signal is a voltage trace over time that mimics the pressure change of the acoustic sound that generated it.

There are three common analog signal levels in use today:

  • "professional line-level," aka +4 dBu;
  • "consumer line-level," aka -8 dBu or -10 dBV;
  • "mic level," typically -40 to -60 dBu (i.e. much softer than either line- level)



For the purposes of this article, you don't need to know what dBu or dBV means, except that it is an expression of amplitude and that positive numbers have greater amplitude. For example, +4 dBu is 12 dB greater than -8 dBu. It will sound significantly louder.

Professional interconnect cables are typically balanced lines with XLR connectors or tip/ring/sleeve phone plugs. Consumer interconnects are typically unbalanced with RCA plugs (occasionally tip/sleeve phone plugs). Mic cables are almost always balanced lines with XLR connectors.

If you send a consumer-level signal into a professional-level gzinta, the signal will be inexplicably soft but audible. If you send a professional-level signal into a consumer-gzinta, you will probably encounter distortion as the signal overloads the circuitry. A mic-level signal sent into either line-level gzinta will generally not be audible.

IMPEDANCE MATCHING

Impedance matching used to be a big deal, but it's no longer something to be concerned about. Most gzoutas have very low impedance, while most gzintas have very high impedance. It means that one gzouta can be split and sent to many gzintas without trouble.

The only places where this isn't generally true is with microphone signals and with some legacy gear. For microphones, the low-source impedance of the gzouta microphone is usually approximately matched by a somewhat greater impedance at the mic-input gzinta.

Regarding balanced and unbalanced lines, this is where it gets to be a little more fun, as well as confusing. For reasons we do not need to go into here, all devices in any given system will not necessarily have exactly the same definition of "zero volts" or "ground."

When two such devices are connected, it can present problems called "ground loops," which manifest themselves as hum or buzz and with amplitude related to the difference in "ground" or "zero volts" between the devices.

Balanced lines carry the audio on two wires independently of the ground (I'm skipping a lot here!) . Usually, at both the gzouta and the gzinta, the audio isolation from ground is maintained. This means that hum and buzz cannot contaminate the audio signal.

Unbalanced lines carry the audio on the same lines as ground (again, I'm skipping a lot). As a result, hum and buzz can be easily generated where unbalanced lines are used. This is particularly true in large complex systems, or where gzoutas and gzintas are in different rooms or buildings.

Why do we use unbalanced lines at all? Because they are simple and cheap, and because in small systems, where ground or zero volts is the same everywhere, they work really well.

WHAT DOES IT MEAN?

If you have an audio problem, the first thing to check is the interconnections. Further, when you are patching together a system for the first time, it is important (some say absolutely essential!) to carefully review the specs provided for all gzoutas and gzintas and to spend the time, money and resources it takes to make sure you've connected them properly to begin with.

In general, avoid adapters for permanent installations. Instead, have custom cables cut to length and have the proper connectors properly terminated at each end. This may be beyond you. If it is, find or hire a tech person who knows how to do it. It will be far, far cheaper in the long run to do it well from the start. As I said at the top, 95 percent of audio troubles have to do with interconnections.

Thanks for listening.