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What is a DI?
 
What's a DI?  Depending on where you are in the world, "DI" is short for either Direct Injection or Direct Input.  DI's are used both live and in the studio.  The uses are limited only by the design of individual DI's; which can range in price from $20 for a simple passive DI, to well over $500 for a single-channel box with lots of bells and whistles.
 


DI's perform three main functions: Impedance conversion, balance conversion, and ground isolation.  And although it is also used in the reverse; most often what happens is that an unbalanced, high-impedance (hi-Z) signal is converted via the DI to a balanced, low-impedance (lo-Z) signal.
Of course i'm looking at this from the perspective of a guitar player *only*, so...

In addition to those three main functions, many DI's also include among other things: Phantom power (power from a mixer); ground lift; pad/buffer/attenuation; parallel output; phase inversion, and equalization.

Impedance Conversion
Whether you're playing electric or acoustic guitar, your (passive) magnetic pickups are generating a hi-Z signal and this can be a problem in two ways:
1. When plugging directly into a mixer channel that is wired for a lo-Z signal, the mixer will "load" your pickup(s) leaving you with a sound that is thin and unpleasing (understatement).
2. When trying to send that signal any substantial distance live or in the studio, it will be subject to degradation due to signal loss.


mixer channel strip

Typical uses for hi-Z to lo-Z conversion with guitar are:
1. Where an acoustic guitar amplifier is unavailable, DI conversion allows you to plug an acoustic guitar directly into the mixer without the high or low frequency loss.  This works with magnetic soundhole pickups, undersaddle transducers, and soundboard transducers.
2. Where even a processed (POD, V-Amp, etc.) electric guitar signal is to be sent a great distance to the mixer (or a short distance in the bar with neon lights and all of that), DI conversion to a lo-Z signal can mean almost no signal loss.
3. In the studio electric (and acoustic) guitar & bass performances are often recorded dry via the DI in order to be re-amped later.  When re-amping outside the computer, conversion is often 'reversed' (lo-Z to hi-Z).

Balance Conversion
Balanced signals have quite the advantage over unbalanced signals when it comes to noise rejection in less-than-perfect environments such as the bar or for long cable runs.  This is due to the design/shielding of the balanced cables themselves, which have three conductors arranged so that two carry the signal and one surrounds them as a shield.


three-conductor microphone cable

Not only does the braided conductor do an extremely good job of shielding versus a standard unbalanced (two-conductor) cable, it isn't dependent upon being connected at both ends, which means the ground can be "lifted", helping to eliminate ground-loop problems live and in the studio.

Typical uses for balance conversion with guitar are the same as for hi to lo-Z conversion.  Balanced lines help combat signal degradation that would otherwise have been caused by poor EMI & RFI rejection.

Ground Isolation
People have been discussing this since long before my first involvement with music/audio more than 30 years ago: The only way to completely avoid having the audio signal affected by ground loops is to have an entirely balanced system connected to a properly wired AC service.  That's right; no unbalanced (guitar) cables or wall wart power supplies.  If only that were possible.
Ground-loop problems in the form of an audible hum can be complicated to reign in because you have all of this equipment that is tied together via audio signal *and* connection to AC; and it is all wired differently.

Most balanced gear (like a mixer) has two grounds: a signal ground, and a chassis ground.  Even though the chassis might be connected via the metal racks in a case (for example), in theory this would still leave the audio signal unaffected.  In the real world though, many pieces of balanced gear have their signal and their chassis ground connected somewhere in the unit, and most unbalanced gear will have the same (if it has chassis ground at all).

Sometimes the use of a DI might itself be enough to get rid of a ground loop problem, but usually it's the Ground Lift that ends up doing the trick.

Phantom Power & Ground Lift
An active DI needs power.  It can come in the form of a 9 volt battery, or it can take phantom power from the mixer.  It seems like this would be a no-brainer because batteries are so expensive, but the problem though is that some mixers aren't capable of delivering phantom power or are limited in which channels can send it (smaller mixers).
 

Another issue to keep in mind is that when employing the Ground Lift on most (inexpensive) DI's, the phantom power option is removed along with your ground

loop!  I've had gigs where i forgot to check my battery and then didn't immediately realize i was no longer coming through the FOH speakers when my battery died. (Some of the more expensive DI's use an isolated power source for their input amplifier and so the Ground Lift has no affect on their ability to utilize phantom power.)

Pad / Buffer / Attenuation
Most DI's come with an option for Attenuation that includes (typically) 0 dB, -20 dB and -40 dB.  Many passive DI's have a fixed-ratio attenuation of usually -20 dB; this is a compromise setting.  Some expensive DI's have a buffered input that can handle extremely high impedance (30 dB) and so they omit the implementation of a Pad selector.
 

In my own experience with both active and passive DI's i have witnessed many times where one of the three attenuation options (0, -20, -40 dB) was needed.
These involved gear other than electric guitar and bass, plus a few very high-priced DI's.
 
Because of this i'm inclined to only trust having a DI with attenuation options in my gig bag!

Parallel Output
When your hi-Z unbalanced signal is input into the DI, it is split.  You have a signal that goes through the DI and comes out through the balanced lo-Z end; and you have an unaffected signal that comes out the same way it went in: unbalanced and high impedance.

This parallel out signal would typically go to an onstage amplifier or to a modeler or some other processor that needs that unbalanced hi-Z signal.

Most DI parallel outs are true bypass, meaning they do not run through any circuit in the DI.  However, some DI's will buffer that input and send it out affected (sometimes through a 2nd unbalanced 1/4" output).  Always read the spec sheets!


Equalization
Some DI's are marketed specifically for use with acoustic guitar.  By far the most popular are those with EQ; including a variable midrange, or notch filtering.
 

The Low and High frequency controls are often exaggerated/hyped in ways that aren't always pleasing, but necessary because of all the different soundboard transducers, undersaddle transducers, and magnetic soundhole pickups; let alone those acoustic guitar systems which also include a microphone!


< L.R. Baggs Para DI

 

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