Digital Sound & Music: Concepts, Applications, & Science, Chapter 1, last updated 6/25/2013
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achieving. In a recording situation, you can often find great success connecting these instruments
directly to your recording system. Since these instrument audio outputs usually have high output
impedance, you need to run the signal through a transformer in order to convert the audio signal
to a format that works with a professional microphone input. These transformers can be found
inside devices called direct injection (DI) boxes like the one shown in Figure 1.21. A DI box
has a ¼" TS input jack that accepts the signal from an instrument and feeds it into the
transformer. It also has a ¼" TS output that allows you to connect the high impedance instrument
signal to an instrument amplifier if desired. Coming out of the transformer is a low impedance,
balanced microphone-level signal with an XLR connector. This can then be connected to a
microphone input on your recording system. Some audio interfaces for a computer have
instrument level inputs with the transformer included inside the interface. In that case, you can
connect the instrument directly to the audio interface as long as you use a cable shorter than 15
feet. A longer cable results in too much loss in level due to the high output impedance of the
instrument, as well as increase potential noise and interference picked up along the way by the
unbalanced cable.
Figure 1.21 A direct injection box
Using these direct instrument connections often offers complete sonic isolation between
instruments and a fairly high signal-to-noise ratio. The downside is that you lose any sense of the
instrument existing inside an acoustic space. For instruments like electric guitars, you may also
lose some of the effects introduced on the instrument sound by the amplifier. If you have enough
inputs on your recording system, you can always put a real microphone on the instrument or the
amplifier in addition to the direct connection, and mix between the two signals later. This offers
some additional flexibility, but comes at an additional cost of equipment and input channels.
Alternatively, there are many microphone or amplifier simulation plug-ins that, when added to
the direct instrument signal in your digital audio software, may be able to provide a more
authentic live sound without the need for a physical amplifier and microphone.
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