ICs for Electro-Music (Part 1)
Overheard at a recent meeting was a comment to the effect that if cars had made the same technological progress as electronics then a certain well known make would now cost 1.75p and do three million miles to the gallon. We will not go into the logic of this comparison but at least it puts the progress of our industry in perspective. Just one of the spin-offs is that it is now practical to produce specialised integrated circuits in relatively small quantities and end up with a product which is much cheaper than the equivalent circuit based on discrete components and having the same performance. In the last few years there has been a steady growth of IC's specifically designed for electronic music applications which in turn have made a large impact on the development of voltage controlled synthesisers. The designer can now readily apply his skills to producing compact units essential for both lead synthesisers and for polyphonic systems; to offering equal performance at a lower cost; or greater performance capabilities at an acceptable cost.
Other common features of the CEM devices include housing in standard plastic 16 or 18 pin DIL packages and their ability to operate from a wide range of power supplies. The latter includes the widely adopted ±15V supplies as well as the increasingly popular +15V, -5V supplies used in synthesisers incorporating CMOS logic controllers.
We will plunge straight in at the heart of the synthesiser, the VCO, since by dealing with the most critical part first we can avoid spending time on some simpler aspects later. We have extolled the virtues of exponential control but how is it achieved and how does one get the scale accurate over a wide range coupled with good temperature stability? Such generators make use of the fact that the current, Ic, flowing into the collector of a transistor is an exponential function of the base-emitter voltage, Vbe, namely the difference between the voltage at the base and at the emitter of the transistor. Hence Ic = Fe(VBE/VT) and a curve of Vbe versus Ic is shown in Figure 1. Unfortunately one of the components of F, namely the emitter saturation current, doubles for every 10°C rise in temperature and the first step is to cancel out this term by using a dual transistor, preferably contained on a single chip to ensure temperature equalisation. The front end of a typical VCO then becomes like Figure2. IC1 is a standard inverter and its inverting input provides a summing node for the input voltages via resistors R1 and R2 (any number of resistors may be added). The gain of the opamp is set by R3 plus R4. Most synthesisers adopt a scale of 1 volt/octave, that is, one volt applied to R1, assuming that this is the resistor connected to the keyboard voltage, will double the frequency of the VCO. To achieve this with the arrangement of Figure 2 the component values are chosen to obtain an 18mV change at the base of Tr1 for each increment of one volt applied to R1. So that we are operating in the most accurate range of TR1 we need to apply a reference current to its collector and this is obtained from R7 connected to a positive reference voltage. The function of IC2 is to sink excess current from the emitters of TR1 and TR2. The output current, Ic, taken from the collector of TR2 then usually goes to some switching device, such as a FET to charge and discharge a capacitor and generate a waveform.
The circuit will now give Ic = Iref e- (Vb/VT), where VT is equal to about 26mV at 20°C with the emphasis being on the temperature. If the temperature of the transistor changes from 20°C, as it surely will, then the 1/T component will alter by 0.33% for every one degree Centigrade change and this would certainly not be acceptable for a synthesiser oscillator. To counteract this we have to alter Vb by an equivalent amount and the usual technique is to employ a special temperature compensating resistor either in the feedback loop of IC 1 or in place of R6.
Now turn to the functional block diagram of the CEM 3340 shown above and also refer back to Figure 2 where the numbers in brackets relate to the pin numbers on the CEM 3340. The similarity is apparent. The first step is to set the scale of the VCO and a 1k8 resistor to ground from pin 14 provides greatest multiplier accuracy. To obtain the widely used 1V/octave scale a 100k resistor is connected to the frequency control pin 15.
As already mentioned this latter pin is a summing node and so any number of resistors may be connected to this point. For example, suppose we wish to add a fine frequency control of ±0.5 octaves whose voltage is obtained from a potentiometer connected between +15V then a resistor of 3M0 (3M3 would be near enough) connected to pin 15 would achieve this. Likewise a resistor or a variable resistor, or both, may be connected between the positive supply and pin 15 to set the oscillator at the desired initial frequency, i.e., the frequency with no keyboard or other external voltages applied. The reference current is taken to pin 13 and for the CEM 3340 the current should be in the range of 3uA to 15uA, so let us use 10uA which requires a 1M5 resistor connected to a +15V supply. Next connect the timing capacitor to pin 11 and this should be chosen to keep the current within the range of 50nA to 100uA and the frequency is given by f = 31eg(Vcc.CF) where Ieg is the output current from the exponential generator, Vcc is, as usual, the positive supply voltage and CF the value of the capacitor at pin 11. Thus if the most accurate range for the VCO is to be 10Hz to 10kHz then the capacitor should be 1nF.
Where the CEM 3340 differs substantially from Figure 2 is in its unique method for compensating for the temperature dependence of the VT term discussed earlier. Within the IC is a Tempco Generator which produces a temperature compensating offset and is generated by the same mechanism which causes the temperature dependence within the exponential generator. The cancellation of the latter is therefore nearly perfect. It means however that precise adjustment of the volts/octave scale is different to that expected from Figure 2 and so the arrangement of Figure 4 should be used, in which the 10k preset potentiometer provides for accurate adjustment of the scale.
One further problem with these exponential generators is the bulk resistance of the base-emitter junction which becomes significant at higher currents and so causes the oscillator to go flat at higher frequencies. In the CEM 3340 pin 7 outputs a current which is a quarter of the exponential generator current. This current is converted to a voltage by connecting it to a grounded trimmer (10k) and taking part of the voltage from the wiper back to the input summing node, pin 15, via a resistor (say, 1M0) and then using the trimmer to cancel out the bulk resistance effect by an additional calibration at high frequency, i.e., about 10kHz.
While the number of external components around the exponential generator have been kept to a minimum consistent with freedom for the designer the following rules should be observed in order to retain the accuracy of the device.
1. The positive supply is used as a reference voltage to generate the reference current and also possibly to set the initial frequency. This requires that the positive supply for the VCO is extremely stable in respect of both time and temperature.
2. All of the resistors should be metal film, or similar, with a temperature coefficient of 100ppm/°C, or better, and the trimmers should be cermet types. Resistor accuracy of 1% is adequate so long as only one of the frequency control input resistors is required for precise frequency control, namely from the keyboard. Do not have any inputs to pin 15 where vibration or a small change in temperature are likely to affect the frequency unless some means of isolating them when not in use is also installed.
3. Pins 13 and 15 require compensation components of a 470R resistor in series with a 10nF capacitor connected to ground.
4. The timing capacitor should be a low loss, low leakage type such as silver mica, polystyrene or, for larger values, polycarbonate. It is also worth noting that at low frequencies the currents are only a few nanoamps and leakage through residual solder flux or general dirt can create problems. It is good practice to clean the foil side of the PCB after soldering with a proprietary solvent and then apply a PCB varnish.
Next we turn to the subject of FM (frequency modulation) of the VCO. Pin 13 is referred to as linear control, and some times as linear FM input. It is a summing node and so an additional resistor may be connected to this point to obtain linear FM using an AC waveform. Take note of what is happening in these circumstances, namely, one is effectively increasing and decreasing the reference current. The first effect is that if the current developed at this input were to exceed the reference current then the oscillator will be gated off (you may wish to use this fact in some application). Suppose, however, that we keep the 1M5 resistor connected to +15V for the reference current and add a 1M0 resistor for a linear FM input. This will give us about a 10% change in frequency for a one volt change in input, i.e., a +50% frequency change for a ±5V waveform. There are two main uses for such an input. First to provide some dynamic timbre modulation using an envelope shaper (ADSR plus VCA) to shape the modulating waveform. Secondly, if a modulating waveform is applied to the linear FM input of two or more oscillators set to different intervals then a type of chorusing effect will be obtained since the tracking of the oscillators has been affected. The more usual FM effect is vibrato and one usually requires a deeper modulation as well as maintaining a constant depth over the frequency range and tracking between oscillators. Vibrato is therefore obtained with a modulating waveform applied to the exponential control input, pin 15, via an appropriate potentiometer and resistor network.
The CEM 3340 provides three simultaneous waveforms: sawtooth, triangle and pulse. They are nominally referenced to 0V and their amplitude is determined by the positive supply voltage, being: two thirds of the supply for the sawtooth; one third for the triangle; and 1V5 below the supply for the pulse. A particular point to note is loading of the triangle output will lower the VCO frequency since the output is also connected to an internal comparator. Even into a 100k load the frequency may drop by 0.15% and in any event the output should not directly drive a load of less than 10k and/or more than 1nF to ground. On the other hand the pulse output is an open NPN emitter and requires a pull down resistor to ground or a negative voltage. This output may be readily clamped to a lower voltage using a zener diode. The amplitude of the signals and whether they are ground referenced or AC is the free choice of the designer. For example, if a rotary switch is used to output one waveform at a time and the desired amplitude is, say ±5V, then this is easily accomplished by switching into an op amp inverter and selecting the appropriate input resistors to obtain the required gain for the different waveforms and then place a capacitor in the output line.
Pulse width may be varied from 0 to 100% duty cycle by 0 to +5V (Vcc = 15V) applied to pin 5. The +5V may be derived from the positive supply using a resistive divider and a trimmer in this network may be desirable if it is essential that the pulse does not completely disappear at the extreme ends.
The remaining facilities on the CEM 3340 VCO are for synchronisation. The hard synchronisation input, pin 6, may be configured in several ways but in its simplest form a 1nF capacitor connected to pin 6 will respond to inputs of both positive and negative pulses. The effect is illustrated in Figure 5 which shows that a positive pulse causes waveform reversal only during its rising portion whereas a negative pulse causes reversal during the falling portion of the waveform. Clearly this technique is only applicable to two or more oscillators. The oscillator of lowest frequency is referred to as the 'master' oscillator and the other(s) as 'slave' oscillator(s) and the pulse output from the master is used for synchronising the other oscillator(s). The complex waveforms which result are capable of yielding some pleasing timbral effects especially when only the slave oscillators are frequency modulated. Soft synchronisation requires negative pulses of about -5V amplitude and coupled via a 1nF capacitor to pin 9. The oscillators in this instance will usually be connected to a soft sync, bus and when one oscillator discharges its sync, pulse will cause premature discharge of other oscillators in such a manner that their oscillation period is an integral multiple of the reset pulse period.
The final but important point relating to the CEM 3340 concerns the negative supply voltage. If this is greater than -7V5 then a current limiting resistor must be placed between the negative supply line and pin 3. The value of the resistor is determined by (Vee -7.2)/0.008 which for -15V gives a nominal 975 ohms and a practical choice of 910 ohms.
Next month I shall continue this look at the Curtis range of IC's, starting with the CEM 3320 Voltage Controlled Filter.
Feature by Charles Blakey
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