DIs, Speaker Simulators and Amp Modelers
I think a lot of players don't understand the differences among a DI, a speaker emulator and an amp modeler. These are commonly-used tools for guitarists who want to play their instrument "direct to the board", usually in situations where it's necessary or desirable to minimize stage volume.
Briefly, here's what each unit does:
- A DI ideally does nothing to the sound. It converts from a high-impedance unbalanced signal to a low-impedance balanced signal.
- A speaker emulator alters the frequency response of the sound to make the signal "sound like" it would have sounded had it been run through a speaker cab. This doesn't include any of the tone shaping you'd get from an amplifier. A speaker emulator may be built into a DI, or it may be a separate unit.
- An amp modeler emulates the tone shaping of a guitar amp and its speaker(s). The most important features of a guitar amp are its effect on frequency response (the EQ) and its distortion. BTW, most of the highly-regarded "clean" guitar amps are not free from distortion.
So why would you use each of these units, and why use one unit in preference to another?
A DI's sole function is to match an unbalanced signal line to a balanced signal line. Unbalanced lines are common in guitar rigs because the circuitry needed to drive the line is inexpensive. The downside of an unbalanced line is that longer cable runs tend to degrade the signal by attenuating higher frequencies and by picking up electrical noise (hum and buzz).
A balanced line is designed to overcome these limitations at the expense of additional cost and complexity in the circuitry on both ends of the line. If you're running twenty or thirty feet of cable from you guitar to your amp, then an unbalanced line is good enough. But if you want to run a signal through a snake to the house mixing board fifty or more feet from the stage, you're going to be much better off running a balanced signal. Think of the DI as a special adapter to match the unbalanced connection to the balanced line.
A guitar speaker's frequency response is anything but flat and neutral. The reason guitarists have a preference for one brand or model of speaker over another is because of the unique tonal coloration that each speaker provides. Also, the size and shape of the cabinet, whether the cabinet has a back, the number of speakers, and even the type and thickness of the cabinet material can all affect the sound. Most speaker emulators give you one or two preset choices.
If you run your guitar through a distortion stompbox and some other effects and then try to run it through the board, you may find that the sound is excessively harsh or fizzy. This is because you're hearing all the high end that any guitar speaker naturally suppresses. A speaker emulator, either as part of your DI or between your board and the DI, will smooth out the high end and make your sound more listenable.
A speaker emulator alone will not mimic the contribution of the guitar amp (or some facsimile thereof). You'll get a lot more mids than you're used to hearing. (Most guitar amps have a prominent mid-scoop centered somewhere between 200Hz and 800Hz, depending upon the design of the amp.) You'll also miss some of the "warmth" added by the low levels of distortion from a clean guitar amp.
A modeler simulates the EQ and distortion of a guitar amp and the coloration of a guitar speaker cabinet. Some modelers even simulate the micing of a guitar cab with commonly-used microphones in selectable positions. A modeler (plus a DI if your modeler doesn't have a balanced output) is a soup-to-nuts solution for running your guitar directly to the board.
You can approximate a guitar-amp sound without using a modeler, if that's your goal. Emulate the mid-scoop using a graphic or parametric EQ pedal. Add a "transparent" OD (i.e. no mid-scoop or mid-bump -- avoid metal distortion and tubescreamers, respectively) set low to give you some of that amp warmth. Then run the whole thing through a speaker emulator. That adds two pedals and a box (the DI/speaker emulator) to your board.
If I was starting over today I'd take a close look at the Boss "Legend" pedals that are just starting to hit the stores. There's a Deluxe Reverb emulator and a Bassman emulator. From what I've read, these can be plugged into a DI and sound good. I haven't yet had the chance to confirm that for myself.