DI (Direct Injection) boxes

Like an Amp Simulator these are not considered effects however they have significance to performing musicians (connecting to an in-house PA system or studio recording rig) so I'm giving them a mention.

A DI box converts the guitar/effects output to a mic-lead connection. Besides the obvious difference in the shape of the connector there is also a difference in the signal transmitted:

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• The guitar output (jack-lead) is a hi-Z (or high-impedance) signal
• The DI outputs a lo-Z (or low-impedance) signal

Firstly these are both 'mic-level' or 'instrument-level' inputs (they carry very quiet signals) but hi-Z signals are more prone to interference. The lo-Z signal consists of the instrument's mono signal (hot) and it's inverted waveform (cold), the cables are twisted around one another such that any interfering signal generated in one is negated by the other (much the same as the way a humbucking pickup works).
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The result of this noise-cancellation is that the signal deteriorates less over distance.

There are two types of DI, active and passive, which have their own advantages and disadvantages a comparison can be seen here.

The next stage of signal processing is the pre-amp.