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Multiple-Effects in Live Sound
An effects unit is used to change what something sounds like. As some multi-effects units include dynamic processing (principally EQ and Compression), the distinction between ‘effects’ and ‘processors’ can get a little hazy. Generally, however, and for the purposes of discussion here, the following differences apply:
Effects are used to enhance or add to a sound, while processors are used to correct or control it.
Effects are mixed with and added to the original sound, while processors are used in line to treat the whole sound.
Examples of effects include Delay, Echo, Reverb, Chorus, Flange, and Pitch Shift (AKA Detune).
Examples of processors include Equalisers (including basic tone controls), Compressors, Limiters, Expanders and Noise Gates.
Basically, effects work by adding something to the sound that wasn't already there, whereas processors work by amplifying or reducing some or all of what was already there.
An effects unit is a device that changes the original sound and mixes the changed version with the original sound to create various effects. Some effects (e.g. reverb and delay) mimic natural sound effects. Others (e.g. flange and reverse reverbs) are used to alter the sound for artistic purposes.
Physically, an effects unit will probably be a 19″ 1U rack-mounted box with knobs or buttons and some sort of menu display on the front. However, there are units controlled by pedals (primarily designed for guitarists) that are intended to live on the floor, as well as individual effects pedals (also primarily designed for guitarists) that produce only one effect.
The most common effect used in live sound production is Reverb (Echo - or Delay - is also common, however). This is usually produced by an effects unit that can also produce other effects if required. A multi-effects unit might also include any or all of Delay (single or multiple), Chorus, Flange, Pitch-Shift (also known as Detune), Artificial Double Tracking (ADT, also known as Automated Double Tracking), and Gated and Reverse Reverbs.
It makes things sound different.
Most modern effects units use digital processing to alter the sound. The type of processing depends on the effect required. Most effects use delay in combination with other algorithms to create the required sound.
If all else fails, read the manual. In the menu-driven digital age you will not get the most from it otherwise. However, not all manuals (or menus or devices) are equally easy to understand. If you want to use it yourself, make sure you can understand it before you hire or buy one.
To connect it, use a postfade auxiliary send from your mixer. It isn't necessary to connect to the inputs in stereo (most mixer channels are handling what is effectively a mono source, and most effects units allow mono in/stereo out). Connect the outputs to dedicated effects returns, a stereo channel, or two mono channels*.
*If your system is mono, there is no need for stereo returns. However, most effects - particularly reverb - have a spatial quality, and will sound less natural (and will generally require higher effect levels to be noticeable) in mono.
Generally, things sound more pleasant if there is a spatial aspect to the sound, and a small amount of Reverb can often be a Good Thing. For that reason, any fully equipped PA system will include an effects unit that can produce at least a few different types of Reverb. However, many small-format mixers have built-in effects that are adequate for most standard requirements, and digital mixers generally have a more substantial effects section, so if your mixer already has at least a few basic reverbs and delays you probably don't need to go any further unless you need something specific. Also:
If other effects (e.g. timed Echo on part of a song) are part of the production and your mixer doesn't include them, then you definitely need an effects unit. More importantly, however, you need an effects unit that can produce the particular effect required, so any old effects unit will not always do.
Generally you will want something that produces one or more of the following:
Discrete delays of the original sound. Delays with a Tap function are best, as these enable you to synchronise the delay with changes in tempo.
Reverberation: an effect mimicking the sound dying away in a reverberant space.
Other common effects, including Phase, Flange, Chorus and ADT.
You need one that can produce all the effects you want. Most multi-effects units will produce most of the most common effects reasonably well, but check before you hire or buy.