Guide To Electronic Music Techniques
Last month I explained how phase effects could be produced electronically, and mentioned briefly how this was produced in the early days by using two tape recorders.
In actual fact, the method used by broadcasting and recording studios was referred to as "flanging", because the speed of one of the tape machines was varied by pressing the tape reel "flange". However, modern flange effects are more sophisticated than the early attempts, and some modern electronic phase effects sound subjectively more like the early flanging effects!
If you are now completely confused, do not worry as all will be revealed; but it must be realised that in electronic engineering, many words used to describe effects are often chosen for historic reasons, and flanging is no exception.
There is one other point that should be made here, and that is that there is nothing to stop a particular manufacturer producing an effects unit and calling it a flanger, although it may not comply with the generally accepted meaning of the word. Each unit must therefore be taken on its own merits and its features carefully examined.
The term flanging in general relates to an effect that is produced by having a series of notch filters, with the frequencies of the nulls being harmonically related; for example 100Hz, 200Hz, 400Hz etc. An audio delay line can be configured to produce a comb filter response as described last month and by slowly changing the delay, the basic flanging effect can be achieved.
"What is the difference between flanging and phasing?" you may ask. The simple answer is that from a systems point of view they are very similar; the main differences being in the frequencies of the notches, and in the overall frequency response of the filter. Phasing is often defined as when the rate of the sweep is higher and the frequencies of the notches need not be related harmonically.
Flanging effects are always produced, then, by notch filters whose frequencies are harmonically related. Another characteristic of flanging is that the notches of the comb filter response extend down to lower frequencies than those of the phaser; and as shown in a previous article, this requires a longer delay because the lowest notch occurs at a frequency corresponding to a half wavelength at the maximum delay. For example if it was required to extend the lowest notch frequency to 50Hz, then: 1 Wavelength = 1/50 = 20ms.
i.e. ½ wavelength = 10ms = Maximum delay time.
Flanging is often described as "Tunnelling" because of the characteristic sound produced. It is also sometimes referred to as a "Soft Phase" or L.F. (low frequency) phase.
A block diagram of a flanger system with all the effects that have been described is shown in Figure 1 although the facilities available with any particular unit will obviously have a bearing on its cost.
There are many flanger units on the market today, but they differ mainly in the range of variations of the basic flanger effect that can be produced, and in the technical quality of their output.
However, facilities possible with this effect are as follows:
In this mode of operation, the delay is varied by a low frequency oscillator which has the effect of moving the peaks and notches in the filter response up and down the frequency range. The rate at which this is carried out is usually slower than that used to produce phasing effects and is typically 3 to 5 seconds per cycle, i.e. a frequency of about 0.2 to 0.3 Hz. Most units allow the sweep frequency to be varied manually to give some variation of this effect.
This control varies the amplitude of the delayed signal that is mixed with the original; hence the depth of the notches in the comb filter response can be greater. More depth will increase the attenuation at the frequencies of the notches, and will increase the height of the peaks, therefore causing the tunnelling effect to be more prominent; whereas the converse will occur when the depth is reduced. Zero depth will result in the original signal appearing at the output.
Rather than have the comb filter sweeping automatically, as in the slow sweep mode, the operator can control the sweep manually in this mode. By connecting the sweep control in the flanger either to a manual control knob, or as is more usual, to a foot pedal, the performer can control the sweep whilst keeping both hands free for his or her musical activity.
This again exercises control over the sweep of the flanger but in this case uses the envelope amplitude to control the sweep. This means that the position of the peaks and notches in the filter response are proportional to the instantaneous amplitude (i.e. volume) of the instrument that is driving it.
There are various methods of producing stereo effects from a mono signal, and in fact some of the up-market professional flangers produce some rather interesting pseudo stereo effects. One method of achieving this is to take the output of the delay line and add it to the original signal for the left hand channel, whilst the delayed signal is subtracted from the original signal for the right hand channel.
The positions of the filter notches will now be different in each channel, but still harmonically related. For example, a signal that has been delayed by a half wavelength at a particular frequency or multiple thereof, when added to the original will cause cancellation and therefore a notch; and when subtracted it will produce a peak. So, if the filter is now subjected to a sweep, frequencies that are cancelled in one channel will be enhanced in the other channel, the sweep effect causing cancelled frequencies in one channel to appear to "tunnel" in the other channel.
This produces an effect which subjectively can be described as producing a feeling of space, and one which some would describe as "rotational".
Before we leave the subject of flanging, it should be mentioned that not all flanger effects use a delay line to produce the comb filter response; although this is the most effective method, it is also the most expensive. Another method which produces a softer flange is to have three voltage controlled filters whose notch frequencies are harmonically related. This produces a three notch comb filter which can be controlled using the same sweep features previously described. The flange produced using this method is not as prominent as the delay line approach, but may be preferred by some.
Feature by Paul Conway
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