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There are many occasions when it is useful to be able to control the cut-off point of a filter by electronic means, rather than by adjusting one or more variable resistors. The most direct way of doing this is to employ the equivalent of electronically variable resistors in the filter circuit. A control voltage alters the effective value of the 'resistor' and so changes the response of the filter.
A device that has the properties of an electronically controlled resistor is the transconductance amplifier. We have already used this in the envelope generator circuits of Project 9 (Fig. 73). The amplifier produces a current, the size of which is proportional to the difference in voltage between its input terminals. This is analogous to a resistor, since the size of the current passing through a resistor is likewise proportional to the difference in voltage between its two ends.
The ratio voltage/current for a resistor is its resistance, while the inverse ratio, current/voltage, is its conductance. Similarly the ratio current/voltage for the amplifier is its transconductance, gm. The value of gm is determined by a voltage applied to the control input of the amplifier.
Fig. 83
Transconductance amplifiers can be used in most types of filter to replace the frequency-determining resistor or resistors.
Several amplifiers, all controlled by the same voltage level, can be used to replace ganged variable resistors in multi-stage filters. The project at the end of this section shows how a pair of transconductance amplifiers are used in a state-variable filter (Fig. 83). The circuit is essentially the same as that of Figure 48, except that the resistors linking the summer to the first integrator and the first integrator to the second integrator are each replaced by a transconductance amplifier. These are the equivalent of the ganged variable resistors VR2 and VR3 of the practical filter of Figure 51. The filter of Figure 83 also differs in that the resistors between the OV rail and the (+) inputs of the op amps are omitted, but this is only because they are an unnecessary refinement for this particular project. One popular field for the application of voltage-controlled filters is in electronic organs and synthesizers. They can be used to enhance the realism of certain sounds. For example, the sound of a cymbal contains more of the higher frequencies when it is first struck. By lowering the cut-off point of a voltage-controlled low-pass filter while the cymbal is sounding, an overall fall in the average pitch of the white noise can be obtained. A similar technique is used when simulating brass instruments, to filter out the higher frequencies after the attack phase is over.
In electronic synthesizers, voltage-controlled filters are used to produce sounds unlike those produced by normal musical instruments. In most instruments the spectrum is strongly influenced by resonances at a number of fixed frequencies. This is the basis of formant filtering described in the previous section. If, instead of a fixed filter, we use one in which the cut-off point varies with the pitch of the note being played, the apparent resonances shift and we obtain a sound which has a distinctly different quality.
Level 1--PROJECT 11--Electronic Guitar
This is an inexpensive and easily constructed fun project, to amuse the younger members of the family. It produces a sound very similar to a plucked string, which is why it is called a 'guitar'. The nature of the sound varies slightly depending on exactly how the board is laid out and upon the pitch of the note. In its higher registers its sound is more reminiscent of the Indian instrument, the sitar.
Without the keyboard section (VR2, etc.), with the addition of a 10 k-O input resistor connected to pin 2 of IC1a, and with a 10 k-O variable resistor for VR1, this circuit functions as a straightforward voltage-controlled filter. The values of C1 and C2 may be altered to make it operate over a different range of frequencies.
How It Works
The circuit (Fig. 83) is essentially a state-variable filter based on four operational amplifiers all contained in a single IC. It also has a pair of transconductance amplifiers to act as voltage-controlled resistors, as explained on page 146. The inverter amplifier ( IC1d, compare with Fig. 48) has a low-value feedback resistor, VR1, so that the damping signal it sends to the summer amplifier ( IC1a) is much attenuated.
The result is that the filter has low damping or, conversely, high Q. Any electrical stimulation of the circuit sets the filter oscillating for a length of time depending on the setting of VR1. In use, VR1 is set so that the filter oscillates for a second or so, the oscillations gradually dying away in the same way as those on a plucked string.
The damping signal is taken from the band-pass output so that the frequency of oscillation depends on the center-frequency of the filter. This is controlled by the voltage applied to the transconductance amplifiers. The higher the voltage, the higher the pitch of the note. The voltage for each note is obtained from a series of variable resistors, of which only one (VR2) is shown in Figure 83. From the wiper of each resistor a wire leads to a keypad. This can be one of a number of copper pads etched on a PCB to resemble the keyboard of a piano. Or it could just be a row of drawing-pins (thumb-tacks) pressed into a strip of wood or strip of plastic. The stylus is touched briefly against one of the keypads. This sets the control voltage and at the same time stimulates the circuit into oscillation. The signal from the band-pass output goes to an audio amplifier (see Fig. 61 or Fig. 67).
Construction
The project is suitable for battery-power, two 6V batteries or battery holders each with 4 AA cells being required. It may be built on a single piece of stripboard, but leave enough room around IC1 to allow for the wiring, resistors and capacitors associated with that IC. As explained earlier, the 'keyboard' can take a number of forms. If stripboard is being used, the ends of the strips can be used as keypads. The stylus is a 4mm wander plug on the end of a flexible lead. The number of notes provided can range between 8 (an octave with no sharps or flats) and 24 (two octaves including sharps and flats). If a high-gain audio amplifier is used (e.g. Fig. 61) it may be necessary to wire a resistor of up to 3.3Mf2 between the BP output and the amplifier input.
Setting up the circuit requires careful adjustment of VR1.
If VR1 is set to too high a value, the sound is damped out quickly, giving a pizzicato effect. If VR1 is set too low the circuit goes into continuous oscillation. Set VR1 so that the circuit oscillates for a second or two when the stylus is touched to a keypad.
Special Components
Capacitors:
C1, C2 330pF polystyrene
Integrated Circuits:
LF347 quadruple JFET op amp
IC2, IC3
CA3080E transconductance amplifiers
IC1