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Many budding recordists have heard the term "direct box," or even used one without really knowing what it does. Maybe you've heard that a direct box (or DI) matches the "impedance" of your devices, but you really have no idea what that means. If all you're looking for is a nice tone from your instrument into your computer, then don't fret--a strong understanding of impedance is not necessary--but, you may want a little back-story to help wrap your mind around the concept (if not, skip to the section about types).
All electronic devices contain some sort of resistance to the flow of electricity. In an alternating current (AC) circuit-as in what comes out of your standard outlet-that resistance is called "impedance." It's easy to remember since it "impedes" the flow of electricity. This may sound like a negative thing, but in reality impedance is always an integral part of any AC circuit's design. Different types of devices have different impedances-which are measured in ohms (Ω). For the purposes of most basic recording issues we can think of almost everything as either high-impedance (Hi-Z) or low-impedance (Low-Z).
Your guitar, synthesizer, keyboard, or almost any electronic instrument will typically have a Hi-Z output-maybe somewhere around 12,000 Ω (or 12 kΩ.) This has to do with the way your instrument is meant to "drive" its amplifier. Microphones, on the other hand, are usually Low-Z-usually around or below 200Ω. That standard came about to deal with the long cables that are usually connected to microphones before they hit their preamplifier.
Standard recording devices are built with this in mind-so their microphone and line-level inputs are "expecting" a Low-Z connection-squeezing the most juice out of Low-Z outputs. That's why you can't typically reach decent volume by plugging an instrument (Hi-Z) directly into a mixer or just any old input on your audio interface. A direct box serves as a meeting place between the two types of devices-it helps them to resolve their differences.
Diving just slightly deeper we find out that this is based on a pretty simple rule-known as the 10:1 rule. That ratio refers to the impedance of your input in relation to the impedance of the output device. In actuality, you may not ever hit a 10:1 ratio exactly, but you will load your input with significantly lower-impedance devices. It might seem odd that you would want a higher-impedance on your input than your output device-why impede the flow into the input? The best analogy I've ever heard for this is the water hose analogy; if you are trying to maximize the pressure out of a water hose, you install a nozzle-which actually impedes the flow of that water-the same is true when maximizing the output of your device. So rather than "matching" the impedance the goal is really to find the proper load for your input (or to squeeze the nozzle just right to maximize the affect of its impedance.)
Now that you understand what a DI does, let's review the two basic types of DI's and their differences. DI's are basically active or passive-terms you may have heard used to refer to many other types of audio equipment--always meaning powered or non-powered.
Passive DI's tend to be cheaper, and also don't require a power source--so no dealing with dying batteries, AC adapters, or phantom power sources. The biggest disadvantage is that the "matching" of impedances is completely dependent on the devices you're connecting--so it would still be possible to encounter impedances that are still too low for your instrument.
An active DI requires a battery, AC adapter, or phantom power supply. Since most microphone preamps have phantom power this is the most common way to power an active DI. One of the biggest advantages of an active DI is that the input and output impedances are usually set so you can be sure that your instrument sees a high enough impedance, and your input sees a low one. You're also likely to find more features like pads, and high/low pass filters on an active direct box--since the electronics can more easily be introduced. Active direct boxes also tend to color the sound of their output a bit--which can be viewed as a positive or a negative. Many engineers prefer to only use passive boxes whenever possible--believing that any coloration is best left up to the mic preamp.
Impedance Matching for Recording
The benefits of direct recording are so vast that they'll be covered in greater detail in subsequent articles. For now, just be aware that the average home studio is not the best environment for live recordings. Most home studios are plagued by poor room acoustics and bad isolation from unwanted noise. In addition, the average home studio is also suffering from a severely limited amplifier and microphone selection. Direct recording can eliminate the problems of a poor setting, and also allow you to try out tons of mic and amp combinations through an affordable amp modeling software. In fact, this newsletter also contains my review* of Amplitube 3-IK Multimedia's newest amp modeling release.
These days, you might find impedance matching built into your devices. Most audio interfaces will have an input labeled "instrument" or "DI" that enables you to plug directly into your interface for recording. Even on a high-quality interface these inputs may not always be your best bet. The tone you get from plugging a high-quality DI into your high-quality microphone preamp can often be much more satisfying. Countryman and Radial Engineering both have solid reputations for building durable, high-quality DI's.
Tone aside, using a separate DI can increase flexibility for musician and engineer alike. Most DI's enable you to split your signal--or more accurately--send an unaffected version which remains Hi-Z. This signal can be sent into your favorite amp or into a hardware amp modeler, such as a Line 6 Pod. This allows the performer to hear their instrument just like they're used to--without any latency, and through an amplifier. This may not seem like a huge deal, but the smallest latency or the simple lack of reverb in a room can throw off even the best performer.
You may be saying to yourself, "Forget that! I don't need a direct signal, because I always prefer the sound of a real amp." In the perfect environment with unlimited time and options there are few who would disagree. But, having the direct signal as a back up might also save a once-in-a-lifetime performance from undetected issues with your mic-like the sound of that airplane flying by.
Some keyboard/guitar amps will include outputs labeled "studio," "direct," or "DI" which would also allow you to record both amp and direct simultaneously--though, you may still be missing out on some pretty vital features. Almost all DI's have a ground lift-which might make or break a direct recording by isolating it from unwanted ground hum. The output on an amp is also less likely to have other key options like a pad or a high/low pass filter.
Up to this point, I have been mainly discussing the traditional concept of a DI box, but today there are more impedance matching options than ever. MOTU recently released their take on impedance matching transformers-called the ZBox. This device is just like any basic DI, but it is optimized for guitar specifically-featuring two guitar inputs made to emulate those of classic guitar amplifiers. This ensures that the tone and dynamics remain as true to your instrument as they would be when plugged into a real amp. Add this benefit to the already impressive amp modeling software of today, and you'll have no trouble feeling like you're playing through the real deal. In fact, B&H has made it easier than ever to turn your computer into the most realistic computer-based guitar rig you've ever played-combining the unique characteristics of the ZBox with Amplitube's most realistic emulations yet. This simple kit bridges the gap between real and unreal--unlocking the full potential of software emulation.
Direct boxes also come in stereo--allowing you to match stereo instruments like keyboards and drum machines with only one device. Radial's ProD2 and Whirlwind's DIRECT2 are two great examples. Today direct boxes even come with as many as six or eight channels. Radial's JD6 offers 6 full-featured DI's in a single rack space. These units are designed to cover the inputs and outputs of numerous pieces of equipment making this the ideal solution for tracking a live band without any bleed through between tracks!
Hopefully, I've opened your eyes to the world of direct recording, or at least provided a few valuable insights. If this is all over too soon--then don't worry--we're just getting started. Thus far, I've really only touched on the basic concepts and uses of a direct recording. In the next part of this series I'll explore all of the fun that can be had with that signal after it's been captured.