Wireless World, August 1973

Last 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.

Final circuit details, interconnection of functions by patch-panel , inteconnection of functions by patch-panel, keyboard and joystick control The final part of this series describing the construcrion and operation of a sound synthesizer completes the circuit functions provided with sample and hold, noise sources and the waveform generator ciruitry.

Sample and hold

It is very useful to have an analogue memory function, for use in such cases as a long fadeout where a constant control signal may be required throughout. One method of implementing this requirement is to use a sample and hold device with the following characteristics. The output should have a very small offset voltage coupled with a low output imp˛ance; also a long storage time, so that the output voltage will only drift by a few per cent per minute ; and a high accuracy over the specified input range. The sampling period is relatively short, being initiated by a positive-going pulse. Also, then is no input buffer because the output impedance of all the units of the synthesizer is low. The input voltagr range is approximately -0,5V to + 6,5V, being deliberately limited by D1 (Fig. 28). The signal is stored on C3, a low Ieakage capacitor, which is connected to the input voltage by an f.e.t. (Trl). This transistor is used as an analogue gate and is controlled by a monostable (Tr5,6,7). During the monostable period, the gate is opened and the signal is sampled: The voltage stored on
C3 is monitored by Tr3 , a current-driven source follower which can be preset to give a zero input/output offset voltage. Using a 500 Ohm source resistor, the spread in Vgs may range from about -0,5V to -5,0V, for drain current drives from about 0,5mA to 10mA, respectively. The constant current source may be preset to lie anywhere in this range. Thus by keeping Tr3 operating in its saturation region, and maintaining Id virtually constant, variations in Vgs can be kept very low for considerable changes in Vds.
Setting up procedure: set R4 to about 500 Ohm (this is the “fine adjust” and it is preferable that R4 is a trimmer) and, with the input short-circuited, initiate the sampling with a positive pulse (this clears any charge on C,). Adjust R8 until the output voltage is as near to zero as possible and then use R4 to finely “zero” the output.
Storage time with input short-circuited is 30 minutes for 5% droop and sampling time l4ms.

Noise sources

The noise sources fill two functions, firstly, a source of noise that can be filtered and modulated, and secondly, a low frequency source that can be used as a randomly fluctuating control voltage. This was achieved by constructing a white noise source and injecting the output into a spectrum shaping network and a low pass filter.

White noise source

The major difficulty in producing a simple reliable white noise source is the very nature of noise itself; it is non-deterministic. Several methods were available but the simplest and cheapest seemed to be the use of the leakage current Icbo of a faulty (high leakage) germanium transistor. However, this approach requires that the leaky transistor is specially selected, or even manufactured by gentle frying! A suitable device (Trl) Fig. 29, should produce an average noise level of approximately 40mV pk-pk, when used in the configuration shown. The white noise generator consists of three parts; the noise source Trl, an equalized high gain amplifier, and an output buffer. A high gain amplifier is used because the signal level from Tr1 is relatively low, thus particular care must be taken to isolate Tr1 and the input of the amplifier from any power supply fluctuations. Preset R3 is adjusted to give a suitable output level of between 2 to 3V pk-pk average.

Colored noise source

Coloured noise is produced by driving a spectrum shaping network with white noise, this network being a Baxandall tone control. Preset R17 is adjusted so that with both tone control pots at maximum the output
shows no signs of clipping.

V.L.F. noise

Very low frequency noise is extracted from the white noise source by two low pass filters, only one of which is available at one time, the selection being made by operating switch S1, Fig. 29. One of the drawbacks of
this method of producing v.l.f. noise, is that very little signal remains after filtering, the amplitude rapidly diminishing with decreasing cut-off frequencv.
Preset R42 is adjusted so that the two v.l.f. outputs have the same amptitude, of approximately 3V pk-pk average.

Waveform generator

The waveform generator produces a control voltage that may be used to either frequency or amplitude modulate other units. The start of the waveform is initiated by a pulse input, the output rises “exponentially” and, after a predetermined period, falls “exponentially” (Fig. 30). Three controls are provided, attack, duration and decay and tbe pulse may be introduced electronically or from a manual pulse source.
The circuit operation is as follows (see Fig. 31 ). The first section is a current driven monostable, the monostable period or duration being controlled by the current drive which is proportional to the wiper
setting of R6. The monostable is triggered by either a positive going input pulse or from a manual pulse upon release. The square wave produced is then fed into the attack/decay section where a capacitor is charged
via the attack con˛rol R14 and diode D3. When the monostable period is over, the capacitor discharges via R12, the decay control, and D2. The potential across the capacitor is monitored, and an attenuated and buffered output signal is produced. A choice of duration times is available (C3 or C3 + C4 with S1 closed) and also a choice of time constants (C6 or C6 + C7 with S2 closed).

Joystick control

The joystick is a mechanically controlled voltage source having two degrees of freedom, and thus generating two independent control voltages, which are proportional to the stick’s position. The device is essentially
a position transducer (Fig. 32) with two sense pots (R4 and R8, Fig. 33) mounted orthogonally. The range of the joystick is limited by the rectangular opening in the front panel giving approximately 90degrees of freedom in both the x and y directions. An extra pot can also be seen (Fig. 32) but this is used only as a spindle. The connecting cable should be thin and flexible so as to present as little restriction as possible to the stick’s movement. Also, this cable should be firmly held by two P-clips, one on the joystick assembly and one on the front panel so as to stop continual wear on the soldered connections.
The circuit function is illustrated in Fig.34. A constant potential is maintained across the control pots R4,8, in Fig. 33 and by the zener diodes D2,3. Also, the potential of these pots relative to 0V may be shifted by presets R2,6. Wiper crackle is attenuated by capacitors C3,5 and the wiper is buffered to the output by Tr2,3 and Tr5,6. With the joystick in the bottom left hand corner of its range, the two outputs x and y are zeroed by adjusting R2, 6 ; movement of the joystick in the x and y directions will then produce corresponding positive increases in the potential of the respective outputs.

Keyboard

The keyboard generates a control voltage that is linearly proportional to the status of the key that is pressed. This voltage is produced for the duration of the key’s depression, retuming to 0V when the key is released. If two or more keys are pressed, the highest frequency key is selected automatically. Also, when a key is pressed, a pulse is generated, this being intended to trigger the waveform generator or the sample and hold unit. However, if the production of this trigger pulse is required, then care must be taken when playing the keyboard to ensure that each key is released before the next key is pressed. If this procedure is not observed, then, even though
the control voltage does change correctly, no pulse will be generated. The result is the production of a signal somewhat different to that intended.
The keyboard control circuit is shown in Fig. 35. A constant potential is maintained across resistors R1 to R48 and as all these resistors are the same, they forrn a potential divider composed of equally spaced steps. The switches S1 to 49 are operated by the keyboard and form, with diodes D1 to D49 and resistor R51 a “Minof” analogue gate. Thus, whatever combination of switches are pressed the most negative voltage is selected, this voltage appearing at the emitter of Tr3. Note that when no switches are pressed, the emitter of Tr3 rises to nearly + Vcc. This voltage must be modified so that it is in a suitable form to act as a control signal. It is attenuated (R55), inverted and its d.c. level is shifted (R59) so that the range of outputs is from 0V to + 3V. Also the feedback around IC1 is such that when no keys are pressed, and the emitter of Tr3 rises to nearly + Vcc, the output (Vc) is prevented from going negative, and stays at 0V.
It is required that a pulse is generated a the moment when a key is pressed, but not when it is released. This would be a simple response to achieve (by detecting the transition direction of the “Minof” voltage) if it were not for the phenomenon of contact bounce. The spikes produced by the bounce can be largely suppressed (C3) but there is still a possibility of generating a pulse by mistake. One method of overcoming this dilemma is to use a Schmitt trigger with a sizeable hysteresis loop, so that, as the “Minof” signal plus spikes rises or falls, it causes the Schrnitt to change state only once. The direction of this change is determined by whether the input is rising or falling (i.e. whether the key is being released or pressed) and can thus be made to produce a pulse only on the falling transient.
Some applications of the keyboard are given in Fig. 36. Fig. 36(a) shows a patch diagram of simulated piano sound. A sinusoidal signal is given a fast attack and a slow decay. Note that the control output (Vc) from the keyboard is modified by the exponential converter, so that an equally tempered scale is produced. However, if the key is prematurely released, the output promptly changes frequency. Fig. 36(b) overcomes this difficulty, by using the sample and hold circuit to store the control signal. Also, reverberation with a slow sinusoidal modulation has been added producing a pleasant effect similar to a xylophone. Fig. 36(c) shows a network for producing bell-like “clanging” noises.

Patch panel

To provide a flexible means of programming the synthesizer, a patch panel similar to the type used in analogue computers has been included. As the input and output impedance of all the units is low, it was possible to use an unscreened system. In fact, ordinary 4mm banana plugs and sockets were eventually chosen, this deci- sion being greatly influenced by cost factors. This choice, however, presents a danger of damage due to misuse. If two outputs are connected together, then it is possible that some damage will eventually occur, although how long it takes is difficult to predict. Certainly, from previous experience of a similar synthesizer, no lasting damage was seen to occur when an error of this sort was made. To minimize this danger the sockets are coloured, all the inputs being yellow, the outputs being any other colour.
The synthesizers on the market appear to have overcome this difficulty, but at some cost. One method is to employ a series of horizontal and parallel conductors, one set being the inputs, the other set the outputs. Pins are then plugged in to make a connection between an input and an output, thus the danger of an “output to output” never arises. Other methods are to use switches or jack plug instead of pins. These systems an all pre-wired and so another problem, that of the “birds’ nest” of patchcords (an all too familiar sight to those who have ever used an analogue computer) has also been eliminated. However, this advantage has been gained at some expense.
The layout of the patch panel was determined on a logical basis ; that is, all the oscillators on one section, the v.c.as and v.c.f. in another, the noise sources in one block etc. Also, to make connections with an external amplifier, a coax. socket was included as well as two sockets which were connected to “ground” potential, these being used as a 0V reference point for external equipment such as voltmeters or oscilloscopes.

Power supply

Many units of the synthesizer are sensitive to power supply fluctuations and so a stabilized supply is desirable. The circuit diagram of the supply used is given in Fig. 37. Without this suppression it is possible to trigger a response by switching on and off unconnected (except via the mains) equipment. Care should be taken in
constructing the power supply to avoid introducing any high rurrent paths that might adversely affect the circuit operation.

Appendix

Voltage controlled filter

Consider a bandpass filter consisting of a series LCR network. The behaviour of this system is characterized by a linear second order ditferential equation with constant coeffcients. Using anatogue techniques, it is possible to model this system, but more important it is possible to make the coeffcients variable, in fact, voltage controlled.


The general equation of a linear second order system is
F(t) = xi + 2kwnx + wnE2x
Where wn is the undamped natural frequency, k is the damping factor (note, the quality factor Q = 1/2k); and F(t) is a generalized forcing function. The solution of this equation consists of two parts ; the particular integral that depends on F(t), and the complementary function that depends on the solution of the right hand side only. Using the network shown in Fig. 38(a) it is possible to implement the complete solution. Different forms of F(t)
can be inserted, and by varying pots 4 and 5, the values of wnE and 2kw can be modified. By monitoring the voltage at the output of integrator l ( -x), the response of a bandpass filter, with the same coefficients, under
the influence of the same forcing function F(t), is observed. (The coefficients for a series LCR cinuit would be wn = 1/LC and k = R/2 C/L). By monitoring x, a low pass response would be seen, and xi a high pass response. If pot 4 were an electronic multiplier, then wn (and hence k) could be voltage controlled. Now 1/2k = Q, so it is thus possible to control both the resonant frequency and thus the quality factor. Two points are immediately noticeable; one, the Q factor increases with frequency. This is because if pot 5 remains constant, we have
2kwn = constant

but k = 1 / 2Q .:. wn/Q = constant.

Fractional changes in pot 4 i.e. wnE2, result in the square root of that change in wn.
One mefhod of curing both of these effects is to use two multipliers Fig. 38(b). It is easily shown that there is a linear relationship between the control voltage Vc and wn”. Also the Q factor is invariant with resonant frequency changes (assuming multipliers 7 and 8 are matched), and the dynamic range of the filter is equal to that of one of the multipliers. It would also be possible to control tbe Q factor with yet another multiplier, but the use of multipliers is both expensive and introduces complications. It was for these reasons that the configuration shown in Fig. 38(c) was finally choosen. Hence, the relationship between Vc and wn is “linear”, the dynamic range is nearly 10 to 1 and the Q factor increases with frequency. The variation of the Q factor is not as disturbing an effect as it may appear to be, especially when it is considered qualatively.

Acknowledgement

We wish to acknowledge the help received from Henry’s Radio in the supply of certain parts, especially for the donation of the keyboard.

Capacitor ratings

Voltage ratings of electrolytic capacitors shown in Figs. 28-38 an as follows :

  • Fig. 28-C2/35V.
  • Fig. 29-C1/25V, C3/l0V, C4/10V, C7/l0V, C8/25V, C9/25V, C10/16V, C11/40V, C14/40V, C17/16V, C19/16V, C20/16V, C27/l0V.
  • Fig. 31-C3/40V, C4/16V, C5/40V, C6/40V, C7/25V, C8/25V.
  • Fig. 33-C1/25V, C2/25V, C3/l0V, C4/25V, C5/l0V.
  • Fig. 35-C1/25V, C2/25V, C6/16V.
  • Fig. 37-C2/40V, C3/16V, C4/25V, C6/25V, C7/ 16V, C8/40V.

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