Tag Archives: Cable

Just What Signal Is It, Anyway?

This business is all about electricity, but the electricity can mean lots of different things.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

signals-smallWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

A fader, an XLR cable, and an Ethernet cable walk into a bar.

None of them could have ducked, because cables and faders can’t walk into a bar anyway. Besides, they don’t play nice with liquids, if we were talking about the other kind of bar.

Look, some jokes just don’t work out, okay?

Every object I mentioned above deals with electricity. In the world of audio it’s pretty much all about electricity, or the sound pressure waves that become (or are generated by) electricity. What trips people up, though, is exactly what all those signals actually are. An assumption that’s very, very easy to make is that all electrical connections in the world of audio are carrying audio.

They aren’t.

The Three Categories

In my experience, you can sort electrical signals in the world of audio into three “species:”

  • Audio signals.
  • Data signals that represent audio.
  • Signals that represent control for an audio-processing device.

Knowing which one you actually have, and where you have it, is critical for understanding how any audio system or subsystem functions. (And you have to have an idea of how they function if you’re going to troubleshoot anything. And you’re going to have to troubleshoot something, sometime.)

In a plain-vanilla audio signal, the electrical voltage corresponds directly to a sonic event’s pressure amplitude. Connect that signal – at an appropriate drive level – to a loudspeaker, and you’ll get an approximation of the original noise. Even if the signal is synthesized, and the voltage was generated without an original, acoustical event, it’s still meant to represent a sound.

Data signals that represent audio are a different creature. The voltage on the connection is meant to be interpreted as some form of abstract data stream. That is to say, numbers. The data stream can NOT be directly converted to audio by running it through an electrical-to-sound-pressure transducer. Instead, the data has to reach an endpoint which converts that “abstract” information into an analog signal. At that point, you have electricity which corresponds to pressure amplitude, but not before.

Signals for control are even further removed. The information in such a signal is used to modify the operating parameters of a sound system, and that’s all it’s good for. It is impossible, at any point, for that control signal to be turned into meaningful audio. The control signal might be analog, or it might be digital, but it never was audio, and never will be.

The Console Problem

Lots of us who louderize various noises started on simple, analog consoles. Those mixers are easy to understand in terms of signal species, because everything the controls work on is audio. Every linear or rotary fader is passing electricity that “is” sound.

Then you move to a digital console.

Are those faders passing audio?


Ah! They’re passing data that represents audio!


I have never met a digital mixing desk that does either of those things. With a digital console, the faders and knobs are used for passing control data to the software. With an analog console, the complete death of a fader means the channel dies, because audio signal stops flowing. With a digital console, a truly dead fader doesn’t necessarily stop audio from flowing through the console; It does prevent you from controlling that channel’s level…until you can find an alternate control method. There often is one, by the way.

And then there’s the murky middle ground. More full-featured analog consoles can have things like VCAs. Voltage controlled amplifiers make gain changes to an analog audio signal based upon an analog control signal. A dedicated fader for VCA control doesn’t have audio running through it, whereas a VCA controlled signal path certainly does.

And then, there are digital consoles with DCAs (digitally controlled amplifiers), which are sometimes labeled as VCAs to keep the terminology the same, but no audio-path amplifiers are involved at all. Do your homework, folks.

Something’s Coming In On The Wire

I’ve written before about how you can’t be sure about what signal a cable is carrying just by looking at the cable ends. The quick recap is that a given cable might be carrying all manner of audio signals, and you don’t necessarily know anything about the signal until you actually measure it in some way.

There’s also the whole issue of cables that you think are meant for analog, but are carrying digital signals instead. While it’s not “within spec,” you can use regular microphone cable for AES/ EBU digital audio. A half-decent RCA-to-RCA cable will handle S/PDIF just fine.

Let me further add the wrinkle that “data” cables don’t all carry the same data.

For instance, audio humans are interacting more and more with Ethernet connections. It’s truly brilliant to be able to string a single, affordable, lightweight cable where once you needed a big, heavy, expensive, multicore. So, here’s a question: What’s on that Ethernet cable?

It might be digital audio.

It might be control data.

It might even be both.

For instance, I have a digital console that can be run remotely. A great trick is to put the console on stage, and use the physical device as its own stagebox. Then, off a router, I run a network cable out to FOH. There’s no audio data on that network cable at all. Everything to do with actually performing audio-related operations occurs at the console. All that I’m doing with my laptop and trackball is issuing commands over a network.

It is also possible, however, to buy a digital stagebox for the console. With that configuration, the console goes to FOH while attached to a network cable. Because the console has to do the real heavy-lifting in regards to the sound processing, digital audio has to be flying back and forth on that network connection. At the same time, however, the console has to be able to fire control messages to the stagebox, which has digitally remote-managed preamp gain.

You have to know what you’ve got. If you’re going to successfully deploy and debug an audio system, you have to know what kind of signal you have, and where you have it. It might seem a little convoluted at first, but it all starts to make logical sense if you stop to think about it. The key is to stop and think about it.

The Cable Termination Isn’t The Signal

The connector on the end of a cable doesn’t necessarily indicate what kind of signal is present.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

xlrWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Just recently, I ran into a musician who decided to solve a problem with a cable.

The problem was that he couldn’t get his instrument pickup to work with direct boxes. He had heard that the signal from the pickup was “mic level,” and so he did a bit of thinking. Pro-audio microphones that connect directly to general-purpose preamps (whether the preamps are outboard or contained within consoles) have XLR connectors. His instrument pickup has a 1/4″ phone jack. It seemed reasonable, then, that a TRS phone plug wired to a male XLR would help.

One the one hand, this is rational. Although his pickup is almost certainly an unbalanced output on a 1/4″ TS connector, the TRS cable has a good probability of working. The likelihood is that the tip and sleeve portions will mate with the jack, while the ring simply floats. At the other end, the XLR connector can’t be mistakenly mated with the input side of a direct box, which would increase the likelihood of the instrument being connected to a mic pre. Purely as a question of physical connectivity, the cable solution is okay.

However, the basic, physical connectivity probably isn’t his issue. My guess (which ended up appearing to be correct) was that what he really had was an impedance problem. He has probably been running into audio humans who assume that his pickup will play nicely with basic DI boxes. Basic, passive DI boxes usually have input impedances that are too low to get proper voltage transfer from pickups with high-impedance outputs. (For more, you can read this article I wrote for Schwilly Family Musicians. You’ll have to scroll down a bit.) When we connected his instrument pickup to an active DI via a bog-standard TS cable, everything worked beautifully.

I should also mention that, if his custom cable had been mated to a jack with phantom power applied, he might have ended up with a very dead pickup. Some things these days are built to tolerate having 48 volts DC applied. Some things simply “release their magic smoke,” and that’s that.

Now, I can’t say that I know everything that was going on the player’s head. It’s entirely possible that his solution was just a “shorthand,” and that he’s entirely aware of the separation between cable connectors and the signals on the cable.

Some people aren’t aware of that, though, and that’s why this is worth talking about. If you’re new to audio, here’s what you need to remember:

The termination used on a cable does not guarantee any aspect of the signal flowing on that cable. The termination only represents an upper-limit to the functionality of signals flowing on the cable.

Let’s flesh that out a bit.

Voltage Level Uncertainty

Let’s say I hand you one end of a cable. The end is terminated with a male XLR connector. You don’t know anything about the other end. If you complete a circuit by mating that male XLR with another device, what will the RMS voltage across the connection be?

Millivolts? (Common microphones subjected to SPL levels in the 90 dB range – “mic” level.)

Volts? (“Line” level devices, like mixers and pro-audio signal processors.)

Tens of volts? (“Speaker” level. Twenty volts RMS across an 8-ohm load is 50 watts continuous power.)

Well? Which one is it?

You don’t know. That XLR connector doesn’t guarantee that some particular, overall voltage level can be expected. The other end of the cable might be joined up to a microphone. Or a signal processor. Or even a power amplifier. Yes, it’s not likely that the output of a power amp would be on a cable terminated with XLR, but it’s entirely possible. It has been done.

All you can really guess at is the upper-limit of the XLR connector’s functionality, and that’s not even all that useful in this context. Assuming that anything larger than 16 AWG would be too hard to stuff into the connector, the upper amperage limit of what’s practical on a common XLR connector is something like 3.7 amps. In theory, you could use a specially-built cable to successfully supply power to some models of 120 V lightbulb via an XLR connector. (DO NOT ATTEMPT THIS. You may electrocute yourself, burn yourself, or end up setting fire to something.)

The point is that the presence of XLR connectors does not mean mic-level audio. Not necessarily. You can have a similar range of voltages on TS and TRS-terminated cables. To make an educated guess, you need to know what’s connected to the send-end of the cable…and that’s at a bare minimum. To be 100% sure, you need a reliable meter.

The Unknown Balance

Let’s continue the thought experiment above. Is the signal on the cable balanced?

Again, you don’t know. Cables terminated with XLR and TRS connectors can support balanced signals, but they don’t guarantee balanced signals. It’s quite common to use TRS for unbalanced stereo. It’s also possible (although I’ve never run into it) to use XLR for unbalanced stereo. From an electrical connectivity standpoint, TRS and 3-pin XLR connectors are the same thing – three terminals. What’s done with those terminals is up to equipment manufacturers, not the connectors.

It’s entirely possible to connect an unbalanced output to a connector that supports balanced signals. The reason is because of what I said above. The connector only indicates the upper functionality limit. If one of the signal terminals is left unconnected, or just isn’t supplied with any voltage, the signal on the cable is unbalanced. The connector doesn’t care.

Because of the connector imposing an upper functionality limit, you CAN sometimes determine if the signal on a cable is unbalanced. If you’re handed a cable end that’s terminated with a connector that has only two “poles,” like a TS cable, then you can’t have balanced audio on that line. Balanced audio requires three poles: Two for actual signal, and one for ground. If a connector doesn’t have the required number of terminals, it can’t handle balanced signals.

But a connector can certainly be capable of handling a balanced line, and yet not be handling balanced audio at that particular moment.

Looking at the ends of a cable isn’t enough to know what’s going on. You have to dig a little deeper, because the cable termination isn’t the signal.

Over-Under (Why, Instead of How)

Over-under wrapping helps maintain the original twist in the cable, and this is a good thing.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.