The waveshaper circuit described on this page produces a wide variety of evolving waveforms in response to an external control voltage. Driven by a triangle wave, it can produce an output wave that continuously evolves from a triangle to an antisymmetric pulse waveshape. In addition, the section of the antisymmetric waveform that is at a zero-volt level can be replaced by an evolving series of successively folded triangular sections, somewhat in the spirit of the popular Serge Wave Multiplier.
To understand the circuit's operation (figure immediately below), begin by assuming that its input (In) is driven by a triangle wave, with voltage Vt, and that the controls labeled "In Mix" and "Fold Mix" are set to zero. The input triangle wave is then passed through amplifier A5 to the circuit output unless the FET switch is closed, in which case the output is zero. The state of the switch is set by the output of comparitor A4, whose inputs are the Shape control voltage Vs, and a full-wave rectified version Vr of the input. Thus the output signal is "shorted" to zero whenever the absolute amplitude of the input is less than the threshold set by the Shape control voltage. The resultant output is a pair of positive and negative steeple-shaped pulses and evolves continuously from a triangle wave to an antisymmetric double-pulse wave with increasing control signal. (Refer to the second figure below.)
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When the "Fold Mix" control is turned up, the shorted section of the output wave is replaced by the output Vf of A7. This signal is an evolving series of triangular peaks produced by the four high-gain (saturating) differential amplifiers labeled DA, whose inputs are Vr and a series of voltages below Vs set by the string of diodes. This section of the circuit was inspired by a waveshaper described by Juergen Haible; however, the present circuit uses the diode string to derive voltages related to the control voltage rather than to the input signal, and it generates a separate waveform as opposed to distorting the input signal.
Finally, the control labeled "In Mix" allows some of the input signal to be subtracted from the output, reducing the relative strength of the lower harmonics, as described on the Double-Pulse Waveshaper page.
Below are some drawings of typical Wavolver signals. On the left the relations between the input signal Vt, the rectified input signal Vr, the shape control voltage Vs and the double-pulse output are shown. On the right are two typical output signals with the Fold Mix control set to 50%. The weaker peaks are from the folding circuitry, and the large peaks are the double-pulse signal.
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There is considerable flexibility in how the wave folding circuitry is designed. Increasing the gain of the DA's results in trapezoidal or square folded waveforms. Increasing the number of DA's and reducing the diode drops between them from two to one results in more folded sections per period. I settled on the configuration shown because it gives quite smooth timbral sweeps, whereas sudden jumps in timbre are audible with more and sharper folding. There is much room for individuality and experimenting with this circuit.
Bill Brett has designed a circuit board for the Wavolver including the circuit above plus input summing amplifiers for the In and Shape inputs. A photo of this board, hooked up to a breadboard for supplying power and connecting the potentiometer controls, is shown below.
In the following are five sound clips (mp3 format) demonstrating some of the timbres available from the Wavolver. The clips each have two waveforms played in an a:b:a sequence with a timbral sweep between the sections. I have also included scope photos of the waveforms and results from spectral analysis.
Clip 1 --The "a" section of this clip is the signal with no shaping, i.e., the input triangle wave (220 Hz). For the "b" section, the Shape control has been turned up part way, with the Fold Mix and In Mix controls at zero. The photo (and all the following ones) shows the input wave at the top and the shaped wave at the bottom. Spectra are given for both the input and output waves.
Play clip 1
Clip 2 --The "a" section of this clip is a signal with a rather narrow double pulse. For the "b" section, the In Mix control has been advanced to cancel the fundamental. The Fold Mix control is at zero.
Play clip 2
Clip 3 --The "a" section of this clip is the same as the "b" section of Clip 2 above (photo not repeated). The "b" section (photo shown) has the folded waves added at a 50% level. Spectra are given for both "a" and "b" sections. Note the presence now of strong even-numbered harmonics.
Play clip 3
Clip 4 --The "a" section of this clip is a signal with a somewhat wider double pulse than in the above. The In Mix and Fold Mix controls are at zero. For the "b" section, the Fold Mix control has been advanced to its full value.
Play clip 4
Clip 5 --This clip demonstrates the range of timbres produced by the second wave folding. The Fold Mix control is at full value and the In Mix is at zero. For the "a" section of this clip the Shape control is set to where the first folding is complete and the second folding about to begin. For the "b" section, the Shape control has been advanced to where the second folding is complete (upward pointing triangular sections).
Play clip 5
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