Flanging
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Flanging is a time-domain based audio effect that occurs when two identical signals are mixed together, but with one signal time-delayed by a small and gradually changing amount, usually smaller than 20 ms (milliseconds). This produces a swept 'comb filter' effect: peaks and notches are produced in the resultant frequency spectrum, related to each other in a linear harmonic series. Varying the time delay causes these to 'sweep' up and down the frequency spectrum. Part of the output signal is usually fed back to the input (a 're-circulating delay line'), producing a resonance effect which further enhances the intensity of the peaks and troughs. The phase of the fed-back signal is sometimes inverted, producing another variation on the flanging sound.
A flanger is a device dedicated to creating this sound effect.
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Comparison with Phasing
Flanging is actually one specific type of phasing. In phasing, a signal is passed through one or more allpass filters which have non-linear frequency phase response. This results in phase differences in the output signal that depend on the input signal frequency. When used with multi-frequency signals like music, various frequencies in the original signal are delayed by different amounts, causing peaks and troughs in the output signal which are not in a linear harmonic series.
By contrast, flanging relies on an overall uniform phase delay to the entire signal, which is equivalent to phasing as described above but with a filter that has a linear phase response across the frequency spectrum. The result is an output signal with peaks and troughs which are in a linear harmonic series. To put it in simpler terms, flanging results in selective but uniformly-spaced filtering of frequencies across the spectrum, whereas phasing results in selective but non-uniformly-spaced filtering of frequencies across the spectrum. Extending the comb analogy, flanging uses a comb filter with regularly-spaced teeth (like a brand new hair comb), whereas phasing uses a comb filter with irregularly-spaced teeth (like a worn hair comb with teeth that have been bent or broken).
To the ear, flanging and phasing sound similar, yet they are recognizable as distinct colorations. Although both flanging and phasing change the delay used to create the effect over time, the effect is significantly more pronounced when using a phaser, resulting in more 'sweep' across the spectrum and more 'sparkle' or liveliness to the sound.
Origin
The effect was given its name by John Lennon in 1966. However the term used by The Beatles is now known to have specifically referred to flanging's 'parent' process, automatic double tracking (ADT), which was invented in April 1966 by Abbey Road engineer Ken Townshend. ADT used linked tape recorders to automatically create a synchronised duplicate of a lead vocal or an instrument. It was created largely at the instigation of Lennon, who hated the tedium of having to 'manually' record duplicate vocals for Beatles recordings -- the only way this effect could be achieved before ADT. The Beatles were delighted with Townshend's invention and used it routinely thereafter; it was the famously non-technical Lennon who is said to have nick-named the ADT process Ken's flanger or flanging (for reasons outlined below).
The name flanging comes from the original method of creation, which involved playing the same recording on two synchronized tape recorders with the output of one machine out of phase as above, and then mixing the two output signals together. As long as the machines are perfectly synchronized, the result will be silence, as the signals will be cancelling each other out. Given the near-impossibility of keeping two analog tape decks perfectly in sync, however, this silence is only achieveable for a split second before the signals slip slowly out of sync again. If the operator places his/her finger on the flange (that is to say, the rim) of one of the tape reels, that machine can be made to slow down, slipping out of sync by tiny degrees. A listener will hear the familiar "drainpipe" swooping effect as shifting sum-and-difference harmonics are created.
The classic flanging effect is believed to have been first perfected during 1966 by George Chkiantz, an engineer employed at Olympic Studios in Barnes, London. One of the first instances of the sound being used on a commercial pop recording was the Small Faces' 1967 single Itchycoo Park, recorded at Olympic and engineered by Chkiantz's colleague Glyn Johns.
If the frequency response of this effect is plotted on a graph, the trace resembles a comb, and so is called a comb filter. Once the operator takes his/her finger off, that player will speed up until its tachometer is back in phase with the master, and as this happens, the flanging effect will be repeated, with the harmonics swooping gradually higher until both signals pass momentarily through the silent perfect sync point again. It is often aesthetically better not to let the two tapes reach this point, but to start the reel-slowing again just before they get back into sync.
In the 1970s, advances in solid state electronics made the flanging effect possible using integrated circuit technology. Solid state flanging devices fall into two categories: analog and digital. The flanging effect in most newer digital flangers relies on DSP technology. Flanging can also be accomplished using computer software. Even today, though, many studio practitioners prefer the sound of analog tape flanging, finding the serendipitous nature of human intervention more interesting than the clinical perfection created by purely electronic means. Tape flanging requires bulky hardware and takes quite a knack to get right, but some consider the results to be well worth the time and effort.
Note that the original tape-flanging effect sounds a little different (generally much richer!) from the later electronic and software re-creations. This is because, not only is the signal time-delayed, but the response characteristics at different frequencies of the magnetic tape and tape heads inevitably introduced some phase shifts into the signals as well. Thus, whilst the peaks and troughs of the comb filter are more-or-less in a linear harmonic series, there is a significant amount of non-linear behaviour too, causing the timbre of tape-flanging to sound more like a combination of what came to be known as flanging and phasing.