The Harmony Generator published in E&MM October 1981, although very useful, is only capable of generating square waves. Also, only one octave output is available at a time. The circuit shown in Figure 1 not only provides the Harmony Generator with triangular and sine waveforms on a selected octave output, but also provides facilities for mixing these and all seven octave outputs simultaneously.
The additional waveforms are produced by a voltage controlled function generator, IC 11, which is used in a phase locked loop (PLL) in conjunction with IC10. The PLL IC10 is used only for its phase comparison and error voltage generation functions, its internal VCO not being used. Instead, IC10 controls the frequency of oscillation of IC11 by a control voltage applied to pin 8 via an inverting and level shifting stage, TR2. The CV is adjusted such that the loop frequency is the same as the input frequency presented to pin 14 of IC10. IC10 and IC11 thus constitute a square wave to sine wave (and triangular wave) converter.
Since waveforms other than square waves are involved in the mixing, the envelope shaping must be linear rather than by means of the very elegant chopper system used in the original Harmony Generator design. An operational transconductance amplifier, IC12, is used both as a virtual earth mixer and a current controlled amplifier to perform the envelope shaping function. The mix is set up on RV4-12 and summed to the virtual earth node at IC12 pin 3 by R31-39. The rectified input voltage from IC2 in the Harmony Generator is converted into a control current for IC12 by TR3 operating in the common base mode. The final output mix between direct and harmony signals is affected by RV13 which is in a rather odd configuration best suited to the current output of IC12.
This circuit is not suitable for operation on the original +9V battery derived supply, so a +12V mains operated supply is shown. The duty cycle of the square wave, which should be monitored at IC11 pin 9 is adjusted by RV3. The 50% duty cycle setting can be clearly detected by ear since its harmonic structure is quite markedly different from any other duty cycle setting. After the duty cycle has been trimmed, the sine wave output will be of reasonably low distortion. However, if very low distortion is required then R29 can be replaced by a 100k preset which is adjusted for minimum sine wave THD.
Although it would add considerable complexity, there is no reason why the circuitry of IC10 and IC11 should not be duplicated for all octave outputs, allowing sine and triangular waveforms to be produced on all seven octave outputs simultaneously.
The component at the heart of the circuit above is a recently introduced elapsed time indicator (ETI), a device that was previously electromechanical, and therefore too large and costly to be justifiable in everyday applications. The new format here is a ¼" fuse-like barrel containing a blob of electrolyte, this moving along the barrel in response to the passage of current. The FSD (full scale deflection), hence maximum duration in hours (T) that can be measured is governed by the magnitude of current (I) viz:
T = 0.00677 / I
NB: Imax=85uA Imin=0.6uA
In the application circuit above, the device is intended to act as a guide to timing stylus replacement, thus it's connected across the turntable's mains supply. The bridge rectifier (BR) provides the requisite DC voltage, and R1 sets the current for a 500hr FSD. Once this period has elapsed, the fuse-like ETI is simply removed from its enclosure (a standard ¼" fuseholder) and turned around, the inversion of polarity causing the electrolyte blob to set course on another 500hr trek.
Obvious applications are as an objective, if arbitrary guide to stylus and tape head maintenance, as a means of qualifying reliability or simply for the measurement of hours of use for warranty or determining hire charges.
Electronic keyboard instruments often have some form of dynamic or touch sensitivity added in order to allow the player more expressiveness. The circuit shown here enables touch sensitivity to be added to most keyboards without the need to modify the keyboard contacts or the keyboard itself. The circuit makes use of a pressure sensitive pad placed underneath the keyboard rest. Under normal playing conditions, the circuit is adjusted so that the output voltage is zero. However, if during 'normal' fingering additional pressure is applied to the keys, then a variable output voltage is generated by the circuit, the value of which is dependent upon the pressure applied.
This type of sensitivity is particularly useful with a synthesiser, when this 'pressure' dependent voltage can be applied, for instance, to the VCO frequency input so producing pitch bend effects. Alternatively, the voltage may be applied to any control voltage input to produce various effects, i.e. VCF cut-off frequency, VCA and soon, all controlled by the pressure applied to the keyboard.
The touch sensitive pad (made from conductive foam as supplied with CMOS ICs) undergoes a reduction in resistivity with increasing compression. One side of the pad is fed with a small voltage via R1, and the other side is connected to the inverting input of IC1. Since without compression the resistance of this pad is over 20M, the output of IC1 is close to zero. Once the pad begins to compress, the resistance drops rapidly until the ratio of the 'input resistor' (the pad) and R2 is such that IC1 amplifies the few millivolts up to a maximum of about 5 volts (negative). This voltage is applied to the inverting input of IC2 where the output voltage is adjusted by RV1 to suit a particular application. This positive going voltage is applied to a low pass filter (IC3) acting as a slew limiter necessary to smooth out the voltage which would otherwise suffer small but rapid fluctuations due to unwanted variations in the pressure being applied by the player. The remainder of the circuit is very straightforward. RV2, R5, 6 & 7 enable the output voltage to be nulled when no pressure is applied and allow any offsets in the ICs to be cancelled out.
The touch sensitive pad is constructed from a piece or pieces of conductive foam (as supplied with CMOS ICs) and with the circuit values shown should be approximately 20mm square by 10mm thick and as 'springy' as possible. The two springs at either end of the keyboard ensure that the output returns to zero volts when no pressure is applied, but relatively inelastic foam will result in loss of control during playing. These springs should hold the keyboard approximately 10mm higher than its normal resting position, allowing, the pressure pad to sit underneath the centre of the keyboard uncompressed. Two metal plates (or aluminium foil) are used to act as the contact plates to the conductive foam pad and these are wired into the circuit as shown in the circuit diagram.
Of course RV1 could be replaced by a potentiometer if various levels are required for different uses, and if your synthesiser has no accessible inverter then it might be wise to add a further inverting op-amp so as to be able to apply negative going voltages as well as positive going voltages, for downward pitch bend, for instance.
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