Wireless World, August 1973

First of three articles describing the operation and construction of a modular system with
manual or electronic voltage control of synthesized waveforms.

by T.Orr B.Sc. and D. W. Thomas Ph.D., M.I.E.R.B

Please note: all copyrights by the authors and Wireless world.

Pictures and schematics can be found halfway this article.

The electronic sound synthesizer is an instrument that can generate a variety of complex outputs the parameters of which are variable and are controlled by the device itself. In its most common form, the synthesizer is used as an electronic musical instrument usually being a monophonic keyboard device. It is also to be found in more fixed purpose applications, such as animal “alarm call” generators.
Basically, the synthesizer is capable of generating and processing signals, and by employing such techniques as frequency and amplitude modulation, filtering and mixing, it is usually possible to produce a desirable output. The feature that makes the synthesizer unique from other instruments, such as organs or electric pianos,
is its voltage control capability. This enables parameters such as frequency, amplitude modulation, attack and reverberation, to be not only manually controlled, but also electronically controlled. Couple this voltage control capability to a flexible programming unit and the result is an instrument with an enormous range of
possible tone colours. The versablity of the synthesizer can be further extended by the inclusion of more and more functional units, but this approach is over-sophisticated. It is better to try to analyse just what is required and how best to achieve it. For instance, what particular types of sounds should the synthesizer generate; is it for instance, going to be used as a piece of educational equipment or for quantitatively synthesizing known waveforms, for example bird calls, engine noises, spoken words etc? This is the “deep end” of synthesizer technology where a great deal of effort has been expended for few retums. Where reasonable returns have been achieved it has been, generally, with computer backup.

Sound synthesis

As a musical instrument the synthesizer is well cast. Tbe world of qualitative descriptions is an ideal environment for a machine that continually defies a quantitative approach. The synthesizer is often used to generate special effects and can also be used to produce pseudo-instrumental sounds via keyboard control, or by
modifying real instrument sounds. To synthesize implies the process of generating a result by the summation of many parts, and a musical synthesizer should produce a musical output by the summing of a group of semi-musical elements. Musical instruments produce sounds that have a discernible harmonic structure, the perceived sounds being the result of exciting a resonant structure by percussion, bowing, plucking or blowing. The envelope of the signal is modified by various sorts of damping and excitation, and the pitch of the fundamental is either pre-selectable or in some cases continuously variable. To make an electronic synthesis of a “pseudo-instrument”, a selection of resonators (oscillators) is required. These resonators should have a variable multi-pitch control (voltage controllable) with a large dynamic range and possibly a selection of different harmonic structures (sinewave, square, ramp, etc which have different harmonics; pure tones only have a limited use). Three or four of these resonators can be considered as a basic minimum for any sort of modest synthesizer arrangement. The signal amplitude from the resonators must be controllable and so a means of control (a voltage controlled amplifier, the gain varying with respect to a control voltage) and a source of control (voltage control sources such as other oscillators, joystick, keyboards, potentiometers, waveform generators etc) must be provided. Also, a means is necessary of bringing these units together so that they interact (the patch panel and the voltage summing networks).
When a rapid series of randomly distributed percussions is initiated (for instance, brush drums) the pitch information is low. This group of “pitchless” sounds is characterized by the lack of a significant harmonic structure and can be synthesized by modifying the amplitude and spectrum of a noise source. When a musical instrument is played an amount of reverberation is always introduced, thus a means of adding a controlled amount of reverberation is provided.
The synthesizer is operated to its best advantage using a set of keyboards. However, no dynamic function – i.e. a means of generating a louder note the harder the key is pressed – has been provided as in some other synthesizers. To simulate a percussion envelope, waveform generator having a variable exponential attack and decay has been included. Other circuit functions are included (described later) and these combine
with those units already mentioned to produce a system that is capable of generating a very large range of special effects.
The total collection of units was chosen after monitoring the format of commercially available synthesizers. Such items as oscillators, voltage controlled amplifiers, noise sources, mixer reverberation, patch panel, keyboard, voltage controlled filter, and waveform generator are common to most devices but unusual items included are a joystick, summer/inverter, exponential transfer function, and a very low frequency noise source. These units extend the range of special effects that can be generated. Items that appear in other synthesizers, but which had to be left out due to time, space and money limitations are: the internal amplifer, loudspeaker, an input preamplifer for microphone and pickups (these provide some excellent electronic effects), envelope followers that try to mimic instruments and voices), electronic two-way switches and a programmable memory.
Faced with all the possible combinations of units, the newcomer to sound synthesis will probably be somewhat at a loss to make any decisions as to what units are needed to meet his requirements. Firstly, the system is going to need a power supply. If the synthesizer is likely to be built in modules, which are added when time and money permit it is advisable to allow a more than suffcient power supply capability to enable an unhindered growth.
A current-limited supply would be an improvement over the one given later in this series. The amplifier loudspeaker combination and the patch panel are also essential. The heart of the synthesizer is its oscillators; they generate nearly all of the sound that is produced.
The next most important are the voltage-controlled amplifiers. These are reasonable quality devices, but a cheap f.e.t. modulator could be used if money is tight. Such parameters as linearity and harmonic distorrion will suffer from this particuIar economy. It now becomes more difficult to decide which particular units are most important, so they have been grouped together; the audio mixer, noise sources (coloured), voltage controlled filter, reverberation, waveform generator and keyboards. Lastly, probably the low priority units are the joystick, sample and hold, exponential transfer function, summer/inverter, white and very low frequency noise sources. Even though these last units have the lowest priority, they add considerably to the synthesizer’s versatility. As a guide to cost, the synthesizer described in this article was produced for approximately £100. The performance of the machine, as with other synthesizers, is not sufficient for it to be a main instrument for live performances, due mainly to speed consideratons in setting up patches and pots. The only way to obtain a versatile performance entirely from the synthesizer is to use multi-track recording techniques.

The system

The synthesizer may be considered as a series of separate units, each with their own respective sub-groupings (see Fig. 1).

Voltage controlled units

This is probably the most important set of units, for it is these devices that have their parameters controlled by external electrical signals. Voltage controlled oscillators. Each oscillator’s fundamental frequency is controlled by the sum of the input control voltages and a bias voltage, there being a fixed relationship between the voltage and frequency. From three oscillators, several waveforms are simultaneously available, these being sinusoidal, square, triangular, sawtooth, variable mark/space ratio, pulse and a sequential signal. The operating ranges extend down to frequencies of a fraction of 1Hz and to requencies above the audo range. These oscillators perform all the frequency modulation functions of the synthesizer.
Voltage controlled amplifiers. The gain of the unit is linearly controlled by the sum of the input control voltages and a bias voltage. There are two v.c.a:s and these provide all of the amplitude modulation capacity. Voltage controlled filter. This unit is a bandpass filter, the value of the resonant frequency being linearly proportional to the sum of the input control voltages and a bias voltage. The Q factor is manually adjustable and increases linearly with frequency.

Signal processors

The voltage controlled units require input control signals and produce either control or audio signals at their outputs. Note that the distinction between control and audio signals is not absolute, but as a lineralization, control signals exist from d.c. up to the low frequency end of the audio spectrum. There is no physical reason against control signals extending to high frequencies, except that the effect is rarely a pleasant one! By processing audio and control signals, the range of outputs is considerably enlarged.
Audio mixer and reverberation unit. These two processors are only compatible with audio signals as they are both a.c. coupled. The mixer has three channels, each channel having its own attenuator, and there is also a master gain control. The reverberation unit also has a gain control and provides a source of reverberation up to approximately 4kHz.
Summer/inverter and exponential transfer function. These devices were designed essentially for control signals, but audio signals may also be used. Two of each are used in the synthesizer.
The summer/inverter has three inputs, two with a gain of – 1, one with a gain of – 10.
Sample and hold. This is the only form of analogue memory provided. Sampling is initiated by a positive input pulse that causes the unit to sample the analogue signal for a preset time. This signal is then held for an unspecified period.

Noise sources

  • Three different outputs are simultaneously available. The noise may be used as a control signal or as an audio signal.
  • White noise. The noise source provides on average a continuous flat spectrum (within certain limits and tolerances).
  • Coloured noise source. The output noise spectrum is arbitrarily variable and is controlled by a conventional tone control network.
  • Very low frequency noise source. One of two v.l.f outputs may be selected, the signal’s function being a random control voltage.

Control voltage sources

The units of this group generate control voltages, and provide the main active link between the operator and the synthesizer.
Joy stick control. Two bias voltages are produced, one associated with each degree of freedom of the device. By physically moving the joystick, the bias voltages change, the modified signals being linearly proportional to the stick’s position.

Waveform generator. A “rectangular” waveform with an exponential attack and decay is generated, the process being initiated by a manual or electronic signal. The attack and decay time constant, and the duration are all arbitrarily variable.
Key boards. A standard four-octave key-board is used to generate a d.c. control voltage, which is linearly proportional to the key position. As the synthesizer is essentially a monophonic instrument, then only one key may be pressed at a time. If two or more are pressed simultaneously, the highest note is automatically selected. Also a pulse is produced at the start of each new note.
Three other units must be introduced to complete the total system. The first is the patch panel which enables the rapid interconnection of units into any desired configuration.
Secondly, an external amplifier and loudspeaker is required. The third requirement is an external feedback system with pattern recognition facilities and a versatile complement of servo systems – an operator. The selection of units may be varied to suit one’s particular requirements.

Design in general

There are certain rules that have to be enforced if the synthesizer is to work satisfactorily.
Firstly, it is essential to generate and measure all signals relative to OV, and this requires a reliable grounding system. A stack of star terminals was employed for this, to which were connected the ground wires from the control pots and all the OV supply lines from the edge connectors.
A signal level of 3V was selected, this giving ample room for larger signal excursions. Also as there is a considerable amount of wiring between the pots circuits and patch panel, the input and output impedance of the units was kept low so that unscreened wiring could be used without any serious interference or crosstalk problems occurring. The input impedances are typically lkOhm and the output impedances must be correspondingly lower to avoid loading. Some control signals are low frequency or even direct voltages and so a.c. coupling between units is not a practical proposition (with the exception of the audio mixer and the reverberation unit). The most significant problem with direct coupling is the fact that control signals are never what they ought to be, but always have an offset voltage added to them. Most of these offset voltages are
only a few hundred millivolts (positive), but this is enough to cause disturbing effects. However, the variable bias on the voltage controlled units should be capable of overcoming most offsets.
The general layout of the synthesizer can be seen in the photograph. Most of the circuitry was constructed on plugin boards and although the connectors increase the cost, they do provide the advantage of making the boards removeable for servicing. Also a spacious layout has been used, enabling clear access to the control pots. Even with a stablized supply and a reasonable ground system it may prove necessary to decouple the power supply on each board. Minor transients of the supply levels can be disturbing as they can build up into a noticeable background noise, and may even cause the v.c.os to lock on to each other’s harmonics. The synthesizer bears a strong resemblance to an analogue computer, with an array of control pots to vary parameters, a patching system and a selection of functional electronic units. However, whereas the analogue computer makes an attempt at being quantitative and accurate, this synthesizer does not, relying strongly on the qualitative perception of the operator.

First voltage controlled oscillator

This oscillator has a linear frequency/voltage characteristic and produces four outputs as shown in Fig. 2. These are square, triangular, sinusoidal and a variable mark/space ratio rectangular waveform. The oscillator has three frequency ranges, the top range covering the audio spectrum, the bottom two extending to subsonic frequencies. The quiescent operating point may be shifted by altering the bias level, and the input control voltages (VC1, VC2) may be attenuated by control pots. The final operating frequency is linearly proportional to the sum of the bias voltage and the attenuated control voltages, and should have a dynamic range of at least three decades.
The heart of the oscillator is a triangle-squarewave generator (Fig. 3) where a Schmittrigger provides positive feedback around an integrator; the integrator’s output thus ramps up and down inside the hysteresis window of the Schmitt trigger. The oscillator is both self starting and stable, having a large dynamic operating range and a defined amplitude. Two outputs are produced, a triangle at the integrator’s output and a square wave from the Schmitt trigger. The ramp rate, and hence the operating frequencv, may be varied by altering either the ntegrator’s gain and/or the drive voltage.
The two voltages V and V (Fig. 3) are alternately switched into the integrator by the electronic switch (a diode ring switch D7, 8, 9,10, Fig 4), which is controlled by the Schmitt trigger. The voltage V is produced at the output of IC3, where the output is depressed by the forward drop across diode D6. Ideally D6-10 should all be matched and so should resistors R21, 24, 36 and R22, 23, thus preserving as far as possible the linear voltage/frequency characteristic and signal symmetry. However, as matched diodes are relatively expensive, it was decided to use unmatched unselected diodes. This had the effect of causing some nonlinearities which were only noticeable at low frequencies where the diodes were conducting very low currents. To obtain the required gain from IC3, resistor R36 had to be much larger than R21,24, and this resulted in a loss of voltage/frequency linearity at low freqnencies. This effect is not very noticeable, but imbalance in the ring switch may cause a disturbing loss of symmetry (Fig. 7). This can be nulled by preset R2 (Fig. 4) which is set to cancel the offset caused by the ring switch’s imbalance at its minimum operating point. To preserve as much symmetry as possible, R21-24 are all 2% tolerance resistors.
Diode D3 (Fig. 4) is included to protect Tr1, TrZ, against emitter-base breakdown; if for any reason the feedback loop is broken, the output of IC1, may ramp down unhindered, with irreversible results. The Schmitt trigger used is the SN7413N, a ttl integrated circuit.
The whole of the circuit operation relies upon the stability of the hysteresis levels; if they alter, then the amplitude and frequency of the output will change. Thus it is particularly essential to have a stabilized and decoupled 5V supply for IC2 as well as for Vcc. If this is not achieved then spikes on the power supplies will cause oscillators VCO1 and VCO2 to have a tendency to lock onto one another’s harmonics. To reduce the generation of spikes, the output of the Schmitt trigger is capacitively loaded; this however, has little effect on the square wave production at audio frequencies.


It should be pointed out that using the SN7413N for the Schmitt trigger has its drawbacks. The separation between its hysteresis levels is small, making it vulnerable to interference by other v.c.os. Its fast rise and fall times can generate significant interference and also it does not like driving long lengths of cable. These difficulties have been largely overcome, but a Schmitt trigger of discrete components would still be an improve-
ment. Also, delays in the loop cause some unwanted amplitude modulation. This becomes apparent at frequencies above 10kHz, but the change in amplitude and harmonic content (in the case of the piecewise generated sinewave) is not obvious to the observer. The sinewave output is generated by feeding the triangular wave at the output of IC1, (Fig. 4) into a diode function generator (Fig. 5). Thus, by adjusting the bias, R2, and the gain, R3 a sinewave can be produced as shown in Fig. 8.
The mark/space signal is produced by driving the circuit shown in Fig. 6 with the “triangle” waveform. Transistor Tr1 forms a level sensitive switch, and R4 effectively shifts the d.c. level of the input sigal. The resultant mark/space output is buffered by Tr2. Preset R3, is adjusted so that Tr1, comes on just at the peaks of the input drive with the wiper of R4, set at – Vcc. This should provide a mark/space range from about 15 to 85%.
To set up VCO1 select the highest frequency range, disconnect any inputs, set the bias to mid position and set R2 and R32 (both as in Fig. 4) to mid position. Monitor the triangle output and switch on. Tum the bias level down to zero and if the oscillations stop increase R32 until they start again. If the oscillations become badly asymmetric just before stopping, compensate by adjusting the offset control R2. Thus by adjusting R2 and R32, optimize the balance between minimum operating frequency and symmetry. Having done this, increase the bias pot setting to give an output frequency of about 1kHz. The triangular wave should now be symmetrical and the diode function generator and mark/space generator presets can now be aligned.

Second voltage controlled oscillator

This oscillator is similar to VCO1. It produces sine, square and triangular waveforms as before and also pulse and ramp waveforms (Fig. 9). The heart of the oscillator is basically the same as shown in Fig. 4, except that four frequency ranges are employed (see Fig. 10), thus giving an extended low frequency range. The sinewave generator is the same as before (Fig. 5), but two new generators, a pulse and a ramp generator are provided (Fig. 11).
The pulse generator is a monostable; it is triggered on the positive edge of thesquare-wave output and produces a pulse of approximately 20us duration (Fig. 12).
The ramp generator is a differential amplifer with a switched gain (Fig. 13). The square-wave is used to control switching transistor Tr1 , so that the differential amplifier has an alternately positive and then negative gain. As the triangle and square-wave are always phase locked, output of the differential amplifier is a ramp. As the triangular wave will have a d.c. offset voltage associated with it, a step will be produced in the middle of the ramp, but this can be zeroed by cancelling out the offset. For this purpose, preset R11 in Fig.13 has been provided. There will, however, be some distortion generated at the crossover point which cannot be removed, but this is relatively small.
In the article by R. A. Moog’, the VCO described takes a different approach to the waveform synthesis. It first generates a ramp using a current-driven unijunction relaxation oscillator, and then converts this ramp into a triangle. This type of VCO has a smaller dynamic range than VCO1,2, but has a much higher immunity to locking onto harmonics of other oscillators.
The series will be continued with details of a sweep frequency oscillator, VCO3, voltage controlled amplifiers and filters, mixer and summer/inverter, sample and hold and noise sources. The final part describes the joystick control, waveform generator, keyboards, patch panel and power supply.

References

  1. Moog R. A., “Voltage Controlled Electronic Music Modules”, Journal of the Audio Engineering Society , July 1965.
  2. Kindlmann and Fuge. “Sound Synthesis”, IEEE Transactions on Audio and Electroacoustics, Dec. 1968.

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