This is the most popular modulation effect. The original meaning of the word refers to a number of singers performing in unison this sounds big not just because of the total volume produced, but because every voice is different. Many instrumental sounds that we take for granted have this type of natural chorus effect built in, from an orchestral string section, to most of the notes on a piano (because each hammer strikes three strings tuned in unison), and of course the twelve-string guitar.
When two notes of different frequencies are heard together, all sorts of effects are produced by the mathematical relationships between them - in certain conditions both the sum and difference frequencies can be heard. If the two notes are close together, the difference frequency is too low to be heard as a pitched note, but can still be perceived as a wobble or beating sound. Traditional piano tuners are trained to hear these beats very clearly and count them precisely enough to be able to achieve the crucial difference between pure and tempered tuning - the smaller the difference, the lower the difference frequency and the slower the beating sound. Many traditional instruments such as the organ and accordion have rich chorus stops which exploit this phenomenon by using two or three reeds for each note. The amount of chorus can only be varied when tuning the instrument by trimming the reeds.
A chorus pedal produces a sound which is similar to the effect weve just discussed, but a little more sophisticated in conception. The goal is to reproduce the effect of several instrumentalists playing together - no matter how good the players, there will always be small differences in timing and tuning, and this produces the chorus effect. A conventional chorus pedal uses a continuously variable delay line - an LFO (low frequency oscillator) is used to vary the delay time, and the delayed signal is then mixed back in with the original dry signal. While its easy to see how this replicates the variable delay between several performers playing together, the less obvious, but satisfyingly clever part of the trick, is that pitch variation is introduced simultaneously. If youve ever experimented with the controls of a delay pedal you will have noticed that playing with the delay time has a temporary effect on pitch. The workings of vintage tape-based delay units show why this happens - delay time is determined by the speed at which the tape passes from the record head to the replay head (the two are placed a fixed distance apart). At any given constant speed this will have no effect on pitch, but varying the speed during recording or playback introduces a difference between the two speeds - a sound played faster than it was recorded will be sound higher, and one played slower will sound lower. This is essentially an instance of the Doppler effect experienced when a high speed train hurtles through a railway station - rising pitch on approach, falling pitch on departure.
There are many parameters which at least in theory, could be user-controlled - wet/dry balance (effect level), LFO depth (height of the wave, determining maximum and minimum delay times), LFO rate, and LFO waveform (sine, sawtooth etc). In practice, many of these parameters are preset, partly because the sound we have come to recognise as chorus is actually defined by a fairly narrow range of values. For example, chorus pedals tend to use a delay range of around 20-30ms whereas flangers (see next page) have a sound characterised in part by much shorter delay times.
Of course, the delay between players in a section varies randomly, but most chorus units use a periodic waveform, and this gives the chorus effect its characteristic slight rhythmic whooshing sound.
The basic recipe has changed little since the legendary BOSS CE-1 chorus pedal appeared in the 70s. To achieve a really big sound, many players started to experiment with stereo setups - two amps fed with independent chorus sounds, or chorus and dry signals respectively. Dedicated stereo chorus pedals have become common in response to this trend - these can work in several different ways, although most often the chorus signal is split and one output is delayed very slightly so as to introduce a phase difference.