Moog Monday- On Synthesizers: Accessories, Part II—Flangers

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Flangers are among the newer stars in the sound-modifier accessory constellation. Made possible by the availability of a new type of integrated circuit called a bucket-brigade delay line, these new units offer the musician convincing vibrato and chorus effects as well as the basic "flanging sound" of a swept, harmonically related multiple resonance frequency.

Actually, flanging has been known to studio musicians for many years. The term refers to the practice of recording a signal on two tape recorders while simultaneously mixing the outputs of their playback heads. As long as the recorded signal arrives at both playback heads at the same time, the output mixture is a delayed replica of the input. But if one tape is slightly delayed with respect to the other, then the mix of the playback head signals is in phase at some frequencies but out of phase at others. For instance, suppose one playback head receives its signal one millisecond later than the other. If the signal were a 500Hz wave (one having a two-millisecond period) or any of its odd harmonics, then the two playback head signals would be exactly 180° out of phase, and would therefore cancel. Put another way, mixing a signal delayed by one millisecond with an equal amount of straight-through signal results in a multiple resonance "comb filter" response, where the dips between the resonances are located at the fundamental and odd harmonics of 500Hz. This type of signal processing is shown schematically in Fig. 1, while the resultant frequency response is shown in Fig. 2. What studio musicians discovered was that they could briefly change the time delay between playback heads by bearing down with a finger on one of the tape reel flanges to slow the tape. When they did this the position of the "comb" frequency response would move through the frequency spectrum. Every time the lowest frequency dip of the comb coincided with a pitched tone, it would null out the fundamental and all odd harmonics, and emphasize the even harmonics, resulting in a pitch one octave above the original tone!

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Of course, life is much simpler with today's electronic flanging. But it's far from perfect! The bucket brigade delay line that all reasonably priced flangers currently employ consists of several hundred electronic "buckets" in a line, with a way of filling up the first bucket with the juice of a narrow time slice of an audio waveform, and a way of transferring the buckets of juice ("charges," to be more technical) down the brigade and out the other end, where the waveform is reconstructed. The faster the bucket brigade transfers the juice the shorter the delay. The bucket-passing speed, and hence the amount of delay, is paced by a supersonic "clock" oscillator that is variable from 10kHz to as fast as 2MHz (megaHertz—millions of cycles per second).

The block diagram of a typical present day flanger is shown in Fig. 3. The input signal goes through the bucket brigade and is mixed with an equal amount of undelayed input signal in the B mixer block. This gives the basic flanging-type comb filter response. The clock oscillator that paces the bucket brigade is voltage-controlled. Its frequency is determined by the combined effect of a front panel Range or Comb position control, and the waveform produced by a low-frequency modulating oscillator. The Range control determines where in the frequency spectrum the overall comb response is centered, while the Rate control determines how fast the comb sweeps through the spectrum. The Emphasis or Color control provides a variable amount of feedback around the delay line, which determines the sharpness of the peaks in the comb response. The lowpass filters help to prevent the high harmonics of the audio signal from interacting with the clock frequency to produce undesirable modulation products.

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Fig. 3 appears complicated for a single-function sound modifier. In fact, flangers are probably the most technically sophisticated small modifiers on today's market. However, the way a flanger is used in performance, and the sort of controls it has, are quite similar to that of a phaser. A flanger is likely to have more limitations than a phaser, and these limitations can be best understood by referring to Fig. 3.

First, the maximum amplitude that a bucket brigade circuit can handle is limited; any attempt to apply a larger signal will result in clipping, a particularly nasty sort of distortion in this application. The other end of the dynamic range is limited by the inherent noise of the bucket brigade process. Some flangers simply don't have the dynamic range necessary to handle keyboard instruments without introducing distortion, audible noise, or both.

Next, a flanger's frequency response is also inherently limited. The lowpass filters must cut off the audio signal's spectrum well below the minimum clock frequency. Since the bucket brigade's time delay increases as the clock frequency is lowered, there is a tradeoff between good frequency response and long delay time. Some flangers have delay times long enough to give "slapback" echo, but their high-frequency limit is as low as 3kHz! This limitation may be okay for guitar application, but is generally unsatisfactory for keyboard use. Keyboard-quality flangers should have a high-frequency cutoff of no less than 12kHz; 15-20kHz is preferable.

Related to limited frequency response is a potential fault that lurks in every sampled-data (clocked) system: "foldback." Foldback is the interaction between the clock signal and the high components of the audio signal, and creates a sound characterized by audible fuzz and dirt. The lowpass filters in Fig. 3 are supposed to prevent foldback, but some may still be audible. Keyboard signals, especially raw square and sawtooth synthesizer waveforms, cause much greater problems with foldback than, say, a typical guitar signal. Foldback manifests itself as a high whistling, swishing, or hissing that comes and goes at the modulation rate.

So if you're seriously thinking of adding flanging to your bag of signal-processing tricks, be sure to listen for adequate dynamic range, good frequency response, absence of foldback, and smooth modulation. These are the basic parameters of quality that any keyboard flanger should have.

We'll talk more about signal-processing accessories in my next column.

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