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Monday, January 13, 2014

Multi-effects Explained, Part 2


Technique : Effects / Processing


PART 2: Modern multi-effects units provide all sorts of useful processors in addition to the more usual reverb and delay-based effects. PAUL WHITE discusses the extras you might find inside your effects box and how to best use them. This is the second article in a five-part series. Read Part 1, Part 3, Part and Part 5.

Last month, I covered the basics of the main effects that make up a typical multi-effects unit. However, most units these days also contain building blocks based on signal processors, such as mixers, equalisers, gates, compressors, exciters, speaker simulators, overdrive effects, and even swept resonant synth-style filters. As before, I'm going to quickly run through the key aspects of these basic processes, before moving onto the ways in which they can be combined.




Mixer modules are included because a signal can generally be routed in many different ways within a multi-effects unit, through several combinations of effects blocks. Every time two effects block outputs have to be mixed together, or when a dry signal has to be mixed with the output of an effects block, a mixing element is needed. As far as the user is concerned, it's normally only necessary to adjust levels -- the routing system automatically puts mixers wherever they are needed. When the output of one effects block is fed into the input of another, there will be no need for a mixer (series connection), but when the outputs from two parallel effects blocks need to be combined, a mixer will be required. Blocks may also incorporate mixing elements -- for example, a delay block requires a mixer to balance the dry and delayed sound.

Simpler multi-effects units may be limited to connecting the blocks in a series chain, and the simplest of these place the blocks in a preset order, leaving the user with choice of which blocks to use and which to turn off. More sophisticated systems allow the user to rearrange the blocks into a different order, and it's quite common for both series and parallel connection to be permitted.




There's no big mystery about equalisation, but it does come in several guises, specifically shelving, parametric and graphic. The familiar treble and bass controls provide both cut or boost at the extremes of the audio spectrum, and these tend to be based on shelving filters. The various EQ types were covered in more detail back in the February 1997 issue of SOS, so if you're not familiar with these terms, you might want to refer back to it. While a shelving equaliser is a 'broad strokes' kind of tool, parametric equalisers are far more precise devices that can be tuned to specific areas of the audio spectrum as narrow as a single semitone. A single parametric equaliser has three controls for cut/boost, frequency and bandwidth, where bandwidth determines how wide a band of frequencies is affected. A wide bandwidth (also known as a low Q) affects a wide range of frequencies, whereas a narrow bandwidth (high Q) affects a relatively narrow part of the spectrum. Setting up a parametric EQ properly takes a little experience, especially as most contain two or more separate equaliser sections, allowing you to EQ different parts of the audio spectrum at the same time.

Graphic equalisers are so called because the row of cut/boost sliders on the front panel gives an indication as to the shape of the unit's frequency response. The centre position is known as 'flat', because it produces a flat frequency response -- that is, the sound passing through is not modified in any way. The more EQ sliders, the finer the control you have. Graphic equalisers have the advantage of simplicity, though you obviously lose the physical fader control if you're using the EQ within a multi-effects box. Even so, some units provide a graphic-style readout on the display so that you can still see the shape of the curve you've set up.




The filter used in a typical analogue synth is closely related to the parametric equaliser, the main difference being that the frequency of the filter can be controlled electronically, rather than being left to the user. For example, an LFO could be used to sweep the frequency up and down, or an envelope could be generated to provide a filter sweep. You'll probably also find that the filter can be set to a higher Q. Some of the more sophisticated multi-effects processors include not only resonant synth-type filters, but also a variety of possible control sources, including envelopes derived from the input signal level, MIDI-triggered envelopes, LFOs and so on.




Noise gates were covered in depth in the September 1996 issue of SOS, but, to cut a long story short, they are little more than electronic switches that mute the audio path when the input signal falls below a threshold set by the user. If you set the threshold at just about the background noise level, any pauses or gaps in the programme material can be silenced. While the gate is open, both the wanted signal and the noise pass through, but unless something is seriously wrong with your source material, the wanted sound will be loud enough to hide the noise.

Gates have additional parameters that set how fast they open and close. Simpler versions may dispense with the attack time, or make it automatic, but the release time needs to be adjustable to ensure that slowly decaying sounds, such as reverb tails, are not cut off early.
Though not a complete solution to noise, gates can be useful when working with electric guitars, which often generate a lot of hiss and hum, especially when overdrive is being used. They are also used to keep pauses in vocal tracks clean. In the context of a multi-effects unit, you could even gate the input signal to reduce the effects of mix buss noise, gate reverb to create a gated reverb effect, or simply to ensure that the song starts cleanly.




Essentially, a compressor monitors the level of the incoming signal and turns it down when it looks like getting too loud. Signals below a user-adjustable threshold pass through unaffected, but signals exceeding the threshold are reduced in level by an amount determined by the Ratio control. The Ratio control sets the number of dBs of change in output level that will occur for a given change in input level, so a ratio of 2:1 means that an input-level increase of 2dB will only cause a 1dB increase in the output level. At very high ratios, the compressor behaves as a limiter -- once the signal level reaches the threshold, it is 'limited' from going beyond it.
An attack control is used to govern the rate at which the compressor responds once a signal has exceeded the threshold, while the release control sets how long the gain takes to come back to normal after the signal has fallen back below the threshold.

The most popular use of dedicated compressors is to control vocal, drum or bass levels, though in the context of a multi-effects unit they may also be used to add sustain to guitars, to re-shape the decay characteristics of a reverb decay, or to compress the input of, or output to, one of the other effects blocks. In other words, the compressor is more likely to be used in a creative rather than corrective mode in a multi-effects environment. Because compressors increase the level of programme noise during quiet passages, they are often used in conjunction with a gate. As a rule, the gate goes before the compressor.




Ducking is a technique for using a compressor to control the level of one signal with the level of another signal -- for example, the radio DJ's voice that causes the background music to 'duck' in level every time he speaks. Some multi-effects boxes now include ducking delay and ducking reverb algorithms, the idea being that when the playing is busy, the effect remains at a low level so as not to get in the way. However, when there's a pause the effect swells up to its full level. The user generally has control over the degree of ducking that takes place. In some multi-effects units, ducking is provided within some delay and reverb algorithms so that the effect is more predominant during pauses and is reduced in level when the material is busy. This can help keep a mix uncluttered, while still creating the illusion that heavy effects processing is being used.




There are several types of enhancer on the market today, but most tend to create a sense of brightness and transparency by manipulating the high-frequency end of the spectrum. The Aphex principle synthesizes new upper harmonics, while other manufacturers use dynamic EQ to add top boost during transient sounds. The results can be remarkable, but it is easy to over-use any type of enhancer, in which case an aggressively bright sound may result.
Within a multi-effects unit, enhancers may be used to brighten individual instruments or, if the routing will permit it, they may be placed after another effects block to brighten only one component of the sound -- for example, the output from a reverb or delay.




While keyboards tend to work best through a hi-fi type of system with a flat frequency response and minimal distortion, guitar and bass amplifiers are 'voiced', which means that their frequency response is shaped to suit the instrument rather than being left flat. Furthermore, the loudspeakers and enclosures used in guitar and bass amplification tend to have a very limited frequency response, which enables them to filter out the rougher-sounding components of amplifier distortion. If you were to DI (Direct Inject) a distorted guitar without EQ, the result would be very thin and raspy compared to what you would hear from an amplifier. To make DI'ing the guitar a more practical proposition, the amp/speaker simulator was devised. This is a filter circuit that mimics the amplifier and loudspeaker voicing of a typical guitar amplifier. Many multi-effects units now include amp/speaker simulators as well as overdrive, which enables the user to create a fully produced guitar sound within one unit. The output may then be recorded without further processing. In addition to creating authentic miked guitar amp sounds, amp/speaker simulators are also useful for warming up digital synths where a fatter, more 'analogue' sound is desired.




The overdrive sound of electric guitar amplifiers is so complicated that some designers are now resorting to physical modelling as a means of replicating it digitally. However, even without physical modelling, it is possible to produce a digital equivalent of an overdrive pedal, and that's what many multi-effects boxes give you. Serious guitarists may still want to use their own analogue distortion pedals before the multi-effects box, and that's fine, but digital overdrive can also used to great effect on organ and pad sounds if it is used in moderation. Drum sounds and loops may also be 'crunched' up in interesting ways. 

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