Sawtooth Processor, Graphic Oscillator
The basic oscillator waveform common to even the simplest synthesisers is the sawtooth, as it contains a full complement of even and odd harmonics, suitable for subtractive synthesis. It can, however, be irritatingly harsh and even when processed by a lowpass filter, is often rather characterless.
The circuit was developed to derive, from any sawtooth, an alternative basic waveform, which is audibly interesting over all parts of the audio range. The circuit is inexpensive to build and can be constructed on a small piece of Veroboard or suitable PCB.
Since the circuit can only use a 'clean' sawtooth signal it must be placed directly after a VCO before being processed further.
The complete circuit diagram is shown in Figure 1. The circuit consists of an input amplifier IC1b driving two cascaded wave-shapers and an output amplifier IC1a. Both active stages are inverting so that overall they cause no inversion. The first wave-shaper creates a DC offset, so C4 is included to provide suitable decoupling.
This waveshaper consists of C2, D1, R3 and R4 and generates a waveform whose actual shape varies with the input frequency. The waveform typically produced at C4 is shown in Figure 2a. It can be seen that the linear ramp of the sawtooth is passed unchanged by D1, but the fast reset part of the cycle (which causes the undesirable high harmonics of the sawtooth) is replaced by the capacitor discharge curve of C2 back through R3, D1 remaining non-conductive until the point P where the ramp meets the curve.
Note that the amplitude as well as the shape of the resulting waveform will change with input frequency, increasing with decreasing frequency. This would normally cause lower notes to be louder than higher ones, so some form of amplitude correction circuitry is required. The second waveshaper provides this, consisting of D2-3, C5 and R5-8, where excess amplitude of lower frequency notes is limited and converted into extra harmonics. Progressively less of this action occurs towards middle notes and eventually none at all on the highest notes. Overall this gives a 'self-tracking' capability where the lower the note, the more its excess energy is fed back into higher, more useful areas of the audio spectrum. A typical output is shown in Figure 2b.
Two components D1 and C4 are orientated as shown to process a rising sawtooth. They should be reversed, however, if a falling sawtooth input is to be used. The supply voltage is not critical and can be in the range of +9V to +15V which can be tapped off most synthesisers.
The amplitude at pin 7 of IC1b should be roughly 6V P-P. This signal can be obtained for most sawtooth signals by adjusting R2 according to the simple relationship R2= 1200/A Kilohms where A is the amplitude of the available sawtooth in volts P-P.
The value of R9 can also be calculated to give the desired output level by R9 = A x 180 Kilohms, where A is the required amplitude in volts P-P.
By changing the value of R3, the discharging rate of C2 is altered, which can yield some interesting tonal changes. Therefore a 100K lin pot plus a 15K series resistor could be used to replace R3 and provide variable control.
Another interesting chorus-like effect can be obtained by adding a low frequency sine or triangle signal (1 to 6Hz) via a 1M resistor to pin 6 of IC1b. There is no problem with breakthrough of the LFO signal into the final output as the signal is 'absorbed' by the action of the waveshapers.
The two frequency-dependent processes just described produce an audibly varied and unusual 'mobile' sound, which, since its harmonic structure alters with changes in frequency, is particularly expressive for arpeggios or with portamento.
D. G. Walton
This circuit is a refinement of the idea of a Graphic Oscillator. That is an oscillator whose output waveform is variable and made up of a series of points set on slider pots. The slider panel therefore looks like that of a Graphic Equaliser, hence the name.
Eight slider pots are provided allowing eight points in time to be set for the positive going half cycle of a wave. The negative going half is then produced by running through the settings in reverse order and with inverted voltages, as shown in Figure 1. This allows a large number of complex waveforms to be produced with a graphic representation of the shape on the controls.
The circuit diagram of the unit is shown in Figure 2. Voltages tapped off each pot are connected to eight inputs of an analogue multiplexer, the 4051. The selected output, from pin 3, is then connected to a 741 which acts as a buffer amplifier. An offset voltage is applied to pin 3 to bias the amplifier at half rail. Output level can be adjusted using the 10K pot.
Channels are selected on the multiplexer by the 3 bit address provided by the 4024 binary counter, via 3 exclusive-OR gates.
When pin 6 of the counter is low the counter levels are passed directly to the 4051. When high the 3 levels are inverted and the channels selected in reverse order. The fourth Exclusive-OR gate is used to change over the voltage levels which supply the sliders. When pin 6 is low the top of the slider is at 0.53V and the bottom at 0.47. High and the top becomes 0.47V and the bottom 0.53V.
To allow the circuit to be integrated into an existing synthesiser the counter is included in a Phase Locked Loop. The 4046 is the PLL which, with the counter, provides the frequency multiplication necessary to make the counter run up to eight times as fast as the input signal.
The input signal from the synthesiser should be a clean square wave set to 4' range which allows the PLL to track accurately. Using the octave switch, output waveforms generated by the circuit can be -2, -4 and -8 or 8', 16' and 32' respectively.
Output signals from the graphic oscillator can be mixed directly into the synth's VCF for further modification. Supply voltage should be in the range of 9 to 15V which could be tapped off the synth's supply.
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