Last time in “Mixing 101”, we zoomed through a lot of information concerning the basics of the ever-mysterious MIXING CONSOLE *cue ominous music!* Granted, on subjects like this one, there's a lot to cover and it isn't always easy to keep things simple AND accurate. Lucky for you, I'm told I have a knack for being complicated and irrational, so here comes PART 2!
“That Weird In-Between Part”
Sound, whether acoustic energy or a bunch of zippy little electrons, still exists and interacts with things in the material world. Between the microphone and the mixer input, there is still physical distance for that audio signal to travel. This distance is usually overcome via a cable connection (although yes, wireless is a thing, but not today, suckers!) which serves as the bridge between the signal source's output to the input of the mixer. The official technical term for this part of the audio signal's journey is That Weird In-Between Part, and it will be the focus of this week's blog.
First thing to know before launching your hopeful little audio signal across a desert of RF and capacitance (more on those in a minute)...
NOT ALL SIGNALS ARE CREATED EQUAL
When describing an audio signal in the magical PA realm, you'll hear all kinds of different categories for different types of signals. As previously mentioned, there's everything from “microphone”, “instrument”, and “line-level”, to terms like “high-z” and “low-z”, to still even more things like “balanced” and “unbalanced”. To sum up, it's fair to say that signals vary from each other in many different ways. In order to keep things simple, there are two main ways in which one audio signal can differ from another in this Weird In-Between Part: signals can vary in strength and in efficiency.
When I talk about signal strength, I'm basically eluding to the output voltage of the signal source itself. It's a pretty straightforward approach and makes sense. Obviously, the volts produced from your run-o-the-mill SM58 will be significantly less than the voltage coming out of the back of your Marshall Half Stack. Even the volts produced from un-amplified signals can differ considerably from each other: for example, many microphones output somewhere around 1.5 millivolts. That's teeny-tiny. Whereas, it's fair to say that the line-out on your keyboard is pumping out around 750 millivolts of output. So yeah, they're different. A guitar pickup will produce a certain amount of voltage, whereas a condenser microphone likely produces a very different amount, etc. Actual numbers aside though, the big idea here is that signals vary in output strength...ah, but there's even a more interesting and, frankly, weirder way in which they differ.
Ever heard of impedance?
2. Hi-Z and Lo-Z Hubbub
Let me be up front here: Impedance is funky. It's a murky concept and a real easy one for me to get sucked into and lost amid all the squabble and numbers and theory. We won't be doing that today. Instead, I encourage people to think about impedance simply as a way of measuring how efficiently a signal travels from the output of one thing to the input of another. The terms used for these audio starts and stops are often referred to as “Source” and “Load”. In case of the Weird In-Between, your source is...well, your source and your load is the mixer input.
Audio signals vary in impedance as well as output voltage. That's why you'll come across terms like “High-Z” and “Low-Z”; “Z” just simply meaning “impedance”. In most PA applications, you'll find the following things almost always to be true:
Microphone Signals: almost always Lo-Z (in other words, not very much voltage but excellent current and the most efficient traveler).
Instrument Signals (guitars, basses, piezos): almost always Hi-Z (quite a bit of voltage, but not very much current. Doesn't last over long distances, not super efficient).
Line-Level Signals: almost always Hi-Z.
What does this actually mean? In the real world (all theory aside), you will find that Lo-Z signals travel best over long distances, whereas Hi-Z signals peter out after about twenty-five feet of cable. Ever notice how you almost never come across guitar cables longer than 30 feet? This is because the high-impedance signal coming out of your guitar pickup can only get so far before the cable starts acting like a big antenna, picking up interference and eventually withering up a perfectly good electrical signal. So, unless your mixing desk is about six feet away from your lead guitarist, you'll find that plugging a guitar straight into a mixing desk is often bad ju-ju (aside from the obvious windmilling and spin kicks that will yank your mixer right off the table and come crashing down at the guitarist's feet).
3. A Crucial Balancing Act
If you've ever done any reading up on the Lo-Z/Hi-Z thing, you've probably also seen terms right alongside it: “balanced” and “unbalanced” signals. In the world of electrical theory, a signals' impedance and whether it's balanced or not are separate concepts...but in the practical applications of pro audio, you'll see a lot of people using Lo-Z and balanced synonymously (same with Hi-Z/unbalanced). This is because, honestly, 99.9999% of Lo-Z applications use a balanced connection from source to load, and just as many percents of Hi-Z will be unbalanced.
What the heck is up with balanced and unbalanced anyway? Put simply, their physical guts are different. Slice open your guitar cable right now. DO IT. Nah, I'm kidding...but seriously...the inside of your guitar cable is a pretty simple construction: two conductors, one “ground” or “cold”, the other is “hot” or “actual signal thingy”. Just two conductors. But if you decide you're going to sadistically rip open a microphone cable, for example, you'll find three conductors: two “hots” and a “shield”, which is grounded. Wha- huh??
Why the extra “hot” in the microphone cable? Seems redundant, right? But check this out...now we're getting into some sweet, scienc-y magic. In a balanced cable, such as one coming from the output of a microphone, one of the “hot” conductors carries the audio signal while the second “hot” carries an exact copy of that signal, just flipped 180 degrees out of phase with the first one. If you don't know what the hullabaloo about phase is, don't worry: all you need to know is that if an audio signal is flipped out of phase with an exact copy of itself, the two signals cancel each other out, yin-yang style. You get one good signal and one evil signal and, just like a perfectly matched fight between a Jedi and a Sith Lord, bam! No signal at all. Mmm...nerd references...
Obviously, you may be wondering why we would want to cancel an audio signal completely? Well, remember how I mentioned before that an unbalanced, Hi-Z signal will eventually degrade so much over distances longer than about twenty feet that if you were to plug it into the mixer, you'd get a lot of noise and hardly any signal? This is because a cable essentially acts as an antenna and eventually picks up a bunch of unwanted noise and crap in the signal itself. But say you were to make two copies of the same original signal and flip one out of phase before it begins to travel down the line to the mixer? The audio signal has disappeared, but the noise is still being collected along the signal path as usual. Well, clearly that's not a super useful thing until we come to the coolest part! By the time a balanced signal reaches the input of a mixer, the evil Darth-signal is flipped back into phase with the good signal, and somehow...magically...you get perfectly clear, noiseless audio at the end of the mixer. How is this possible? All that noise that was polluting the cable run between the microphone and the mixer was collected on the signals when they were out of phase. By flipping the signals back in phase with themselves at the input end of the mixer, the noise is subsequently flipped out of phase with itself, canceling itself out. BOOM. Balanced.
All this to say, when you're looking for the most efficient way for an audio signal to travel the necessary distances to get to the mixer intact and clean, you want to be able to make a signal travel in a balanced, Lo-Z format. How do we do this with all the different types of output signals out there? Oh, there are ways...
4. Let's Be Direct
The most common and useful way is through the use of one of my favoritest things in the whole world: a little thing I like to call the D.I. Box! Yay!
You may have seen these little guys around stages and studios alike and for as insignificant as they may seem, they serve a very important purpose and perhaps the most important function in the Weird In-Between Part.
Put simply, a D.I. (direct or direct injection) box is used to convert Hi-Z and/or unbalanced sources into efficient and noiseless Lo-Z, balanced signals in order to best get to and be used by the mixer. The guts of a DI are pretty straightforward: unbalanced signals are fed into the quarter inch input, they are sent through the magical magnetic transformer and viola! They come out the other side through the XLR output as a Lo-Z, balanced signal. Your instrument or keyboard signal can now travel more efficiently over longer distances with less noise and better signal quality.
Assuming you're setting up a live PA with a fairly typical cable-to-snake-to-mixer arrangement, here are some general guidelines:
Acoustic guitars, electric guitars, basses* and keyboards should all be run through a DI box and then out from there to the XLR inputs on the stage snake. This converts their Hi-Z signals into Lo-Z signals for the sake of efficiency and organization as well as getting them balanced in order to travel the distance to the mixer.
Microphones are usually already Lo-Z and balanced. In theory, you don't even really need to run microphone outputs into the stage snake assuming you have long enough mic cables. However, I do recommend using the snake anyway seeing as how it keeps everything better consolidated.
Once you've got your inputs plugged into the mixer and everything's clean and clear and ready to go, we're ready to start with the mixy business. Way to navigate that Weird In-Between Part!
Stay excellent and have a good week!
*Quick note: most direct boxes come equipped with a parallel output on the unbalanced end in order to run your guitar or bass through your amplifier or stage monitor. That way you have a balanced signal being sent to the mixer while simultaneously having your unbalanced signal fed back into the amp as if you were just running through your amp anyhow. Often times, however, electric guitarists prefer to have the tone of their amp present in the signal being fed to the mixer, so an instrument mic in front of your amp is often considered the best approach.