What is a Flanger?
The term Flanger was coined sometime in the mid 20th century, It was discovered when two tape machines played the same signal at the same time. The story is that a recording engineer placed a finger on a tape reel’s flange, slowing one tape and throwing the two out of sync. When the engineer released the reel, its speed gradually returned to normal, giving a psychedelic whoosh. The origins of the term “Flanger” has various explanations…
How a Flanger works.
Flanging is an audio effect produced by mixing two identical signals together, one of the signals is delayed by a small and gradually changing period, usually between 5, and 20 milliseconds. This produces a swept comb filter effect: peaks and notches are produced in the resulting 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. A Flanger is an effects unit that creates this effect. 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 Flanger sound, the phase inversion causes peaks in the frequency response instead of notches (see the red line in the chart below).
As you can see Flanging differs from phasing in a notable way: A phaser generally has one notch frequency, whereas a Flanger has a number of harmonically related notch frequencies giving a comb like spectrum.
Creating a basic Flanger in SynthEdit.
The easiest way is to use a Delay2 module. Below is the structure of a simple Flanger. I find it’s best to keep the Mod depth’s maximum value to about 9 Volts as the full 10 Volts can sometimes produce some odd artefacts over 9.5 volts, we do this by setting the maximum value of the Mod Depth Slider control to 9 Volts.. The offset 5V is to keep the delay time in the centre of the useful delay time “window”.
Note: When constructing a Flanger in SE you should check the Interpolate box in the properties panel for the Delay2 module, and make sure it’s ticked. This greatly reduces any “stepping” or “zipper” effects in the audio when we are modulating the Delay2 module.
Adding an external Feedback loop with switchable phase.
With a few extra modules, we can create an external feedback circuit to give you a choice of negative or positive feedback loop for the Flanger.
Normally feedback loops are not allowed in SynthEdit, but if we add a special Feedback module we can have feedback. It does however introduce a delay of approximately 2mS into the loop however, which will affect the response of the filter above 2 kHz. See the structure below for the layout (the Feedback – Audio Module is actually Feedback – Volts Module, but in this context I think saying audio makes much more sense). The Phase Inverter is the standard Inverter2 Module, which just inverts the polarity of the audio, which is in effect a 180 degree phase change. The Feedback amp is a standard Level Adj Module and we use a Switch (1->Many) with a List Entry Module to switch from 0 degrees feedback to 180 degree phase shifted feedback. The feedback plug in this structure is not connected, and can be set at 0 Volts in the properties panel, or if you wish you can use a Fixed Value (Volts) module to hold the feedback level at 0 Volts.
Improving the SynthEdit Flanger.
Flangers may often produce a loud low frequency hum at high feedback levels. By adding a high-pass filter after the delay line we can fix this
problem.
Adding a High Pass filter in the output.
Here I have used two single pole filters to give a reasonable 12dB/Octave cut-off slope. You could have a fixed filter frequency, but I have used a slider control to allow a little manual control between 50 and 800Hz. To do this set both the 1 Pole HP filters Frequency Scales to 1V/kHz in the properties panel, and then set the Slider control’s Minimum to 0.05 Volts and the Maximum to 0.8 Volts.
There is a reason for this variable high pass filter, often we can improve the clarity of the Flanged signal by cutting out low frequencies so it’s nice to be able to control the cut-off frequency, and it can also remove some low frequency aliasing by-products.
Making a stereo Flanger in SynthEdit.
Its quite a simple job to convert the simple Flanger to stereo.
We just need to add an extra input and output channel, so we have left and right sides, then copy parts of the basic Flanger setup: see below.
A nice touch is to change the LFO to the TD_LFO_B_ST module.
This is a “Stereo” version of the LFO we used previously, with two outputs instead of one, these outputs are also adjustable in terms of their phase relative to each other, so we could set our Left hand Flanger channel sweep voltage to be 180 degrees out of phase with the Right hand channel. Having the two channels sweeping differently will give us not only a more complex flanging effect, but also an interesting psych-acoustic panning effect too due to the phase and time differences between the two channels.