1. Field of the Invention
The invention relates generally to electronic variable filtering circuits and more particularly to low-pass variable filter circuits for electronic musical instruments utilizing voltage controlled current sources.
2. Description of the Prior Art
Electronic filters of either the low-pass or high-pass variety are commonly used in the design of electronic musical amplifier systems. Usually these filters employ a variable filter comprised of a resistance and a capacitance, either or both of which may be variable. The cutoff frequency (f.sub.c) is that frequency above or below which the filter will not pass signals, depending upon whether the filter is low-pass or high-pass. The prior art designs made the cutoff frequency (f.sub.c) dependent on an external voltage applied to the filter circuit. In variable filters employed in electronic musical instruments, it is useful to control the cutoff frequency (f.sub.c) of a low pass filter by using such an external voltage. In general, low pass filter design seeks to limit conduction to those frequencies below the cutoff frequency (f.sub.c). Thus, a graph of signal amplitude versus frequency would illustrate all frequencies below the cutoff frequency (f.sub.c) at a fixed amplitude. All frequencies above the cutoff frequency (f.sub.c) would be attenuated at a rate which is frequency dependent. Every doubling of the cutoff frequency (f.sub.c) is an octave and for every octave above the cutoff frequency, the amplitude of the signal will be halved if the filter is designed for a six decibel (dB) per octave roll off. This calculation is based upon the equation that decibel (dB)=20 Log.sub.10 GAIN or 20 Log.sub.10 amplitude out/amplitude into the filter. Thus, a filter stage designed having an amplitude attenuation of 6 dB/octave but with a very low cutoff frequency (f.sub.c) might pass only the fundamental frequency. Likewise, a filter stage designed having an amplitude attenuation of 6 dB/octave but with a high cutoff frequency (f.sub.c) might pass the fundamental frequency and several harmonics. An example of variable filter design is illustrated by U.S. Pat. No. 3,475,623 issued to R. A. Moog entitled "Electronic High-Pass and Low-Pass Filters Employing the Base to Emitter Diode Resistance of Bipolar Transistors". Moog employs an adder, a signal input buffer, a filter input, a filter, a filter output, a signal output buffer, and a feedback loop. The output of the adder is applied to the emitter of an input transistor to the filter. The base of the input transistor is maintained at a constant voltage by being connected to a forward biased diode and a fixed biasing voltage. The volt-ampere characteristic of the input transistor is exponential over a wide current range resulting in high current gain. The collector current which is nearly equal to the emitter current is fed to a pair of filter driver transistors connected in a push-pull common emitter configuration. The driver transistors drive a string of transistors which provide the dynamic resistance elements of the filter. The driver transistors have a very high collector-to-collector impedance so that they serve as a nearly perfect current source for the filter transistors. This defines the voltage controlled current source feeding the filter. The filter portion includes four identical sections each comprising a fixed capacitor and two transistors whose input diodes are effectively in series. Since 6 dB/octave attentuation is inadequate for music applications, four stages are cascaded to provide a cummulative attenuation of 24 dB/octave. The dynamic resistance from emitter-to-emitter of each of the filter transistor pairs is inversely proportional to the standing collector current. The cutoff frequency of each filter section is f.sub.c =1/2rc where "r" is the dynamic resistance of the two base-emitter diodes in series and "c" is the capacity of the emitter-to-emitter connected capacitors. At frequencies low compared to f.sub.c, nearly all the signal current flows into the emitters of the filter transistors and at frequencies high compared to f.sub.c, the signal is by-passed by the emitter-to-emitter capacitors.
A second example of the variable filter is disclosed by U.S. Pat. No. 3,805,091 issued to Dennis P. Colin, entitled "Frequency Sensitive Circuit Employing Variable Transconductance Circuit". Colin employs a variable transconductance means with an output fed to an operational amplifier with a feedback capacitor forming an integrator. A portion of the integrator output is fed back to the transconductance. The circuit includes a plurality of circuit gains the proper choice of which results in either high-pass, low-pass, or phase shift operation. The transconductance means includes a differential amplifier and a current reflector. The differential amplifier includes a pair of matched transistors with intercoupled emitters and connected to an expodential voltage controlled current generator which provides a control current. The current reflector includes a pair of matched transistors with their base and emitters respectively interconnected. The collectors of the current reflector transistors are connected to the cathode of a diode and the emitter of a fifth transistor, respectively. The base of the fifth transistor couples to the anode of the diode and to the collector of a first of the pair of matched transistors of the differential amplifier. The collector of the fifth transistor couples to the collector of the second of the pair of matched transistors of the differential amplifier, and to an output terminal. Thus, the circuit provides a current controlled transconductance means wherein the relationship between the output current and the input voltage is controlled by the control current. When the circuit gains are chosen to provide low-pass operation, the cutoff frequency (f.sub.c) is dependent upon the control current. At higher control currents, more high frequency components of the input signal are passed and at lower control currents fewer high frequency components of the signal are passed. Since the cutoff frequency (f.sub.c) of the filter is dependent upon the control current, the cutoff frequency (f.sub.c) doubles for each one volt increase in the control voltage. Thus, Colin discloses a low-pass filter response whose frequency or other characteristic is controlled by an external voltage or current. The control current controls the amount of transconductance and the mathematics demonstrate a transfer function for a low-pass filter from the combination of the variable transconductance and integrator output. This design has been an industry standard.
A further example of a variable filter is illustrated by U.S. Pat. No. 3,924,199 issued to Alan R. Pearlman, entitled "N-Pole Filter Circuit Having Cascaded Filter Sections". The N-pole filter generally comprises four filter circuits cascaded in series, each filter circuit including a transconductance amplifier, an integrating amplifier, and a feedback means. A current source having four output lines coupling respectively to the transductance amplifier of each filter circuit for controlling in concert the transconductance of each of the four transconductance amplifiers. A variable and controllable signal is provided for controlling the common current source. The transductance amplifier includes a pair of input transistors with their emitters tied to a current control input terminal. A third transistor and a diode comprise a current reflector. The cutoff frequency (f.sub.c) for each filter section is dependent upon a capacitor and a control current provided from a current source to the transconductance amplifier of each stage. A variable input voltage signal is coupled to a voltage-to-current converter which couples in parallel to the bases of each of four transistors providing the current source for each stage. The gain of each current source transistor is 1.0 and mirrors the control current produced by the voltage-to-current converter. Each current source produces an equal current and since the cutoff frequency (f.sub.c) is dependent on the current source, each stage has an identical cutoff frequency (f.sub.c). Thus, a 24 dB/octave attenuation is achieved.
A further example of an active filter is illustrated in U.S. Pat. No. 3,792,367, issued to Fleischer, et al, entitled "Active Controllable Filter Circuit Using Variable Transconductance Amplifier". Fleischer, et al teaches a filter circuit which incorporates a plurality of variable transconductance amplifiers. Two or more transconductance amplifiers are cascaded and interconnected by a feedback loop. Resistors and capacitors are selectively coupled to the input and output of the various amplifiers to provide the desired overall transfer characteristics. By selectively altering control signals supplied to the transconductance amplifiers, the overall transfer characteristic of the amplifier may be changed. In another embodiment, the transfer characteristic is altered by applying a plurality of diverse input signals to various components of the filter circuit. Thus, a variable filter is realized which is capable of being used as a sweeping bandpass filter, as an adjustable allpass (delay) section, or as a stable filter whose characteristics may be changed to compensate for temperature or time induced variations in performance.
It is desirable to integrate whole systems on a single integrated circuit chip. The variable filter designs of the prior art including the variable transconductance followed by the operational amplifier integrator or the filter employing the base to emitter diode resistance of bipolar transistors require at least four stages. Although whole systems have been integrated on a single chip a problem of the prior art is that the plurality of elements associated with the multiple stage filter cause the integrated circuit to become too large resulting in a loss of cost effectiveness. The variable filter is just one component of the whole system that must fit on the chip. Thus, space limitation on the integrated circuit chip is a major problem. The integrated circuit is placed within an integrated circuit package with a plurality of pin connectors extending therefrom. The pin connectors are used to electrically connect the functions on the chip to external circuitry. The number of pins extending from the package is always a limiting factor. The present state of the art permits only transistors, resistors, and very small capacitors to be integrated onto the chip. Thus, a large capacitor must be external to the chip and must connect to at least two pin connectors. The Moog '623 patent has four external capacitors and would require eight external pins. Thus, the number of pins available for connection to external circuitry is a limiting factor. As the cutoff frequency (f.sub.c) is increased, higher frequency harmonics are permitted to pass the filter. Thus, more electrical noise passes the variable filter. Each additional transconductance stage generates additional electrical noise and the cummulative effect is to reduce the overall signal-to-noise ratio of the variable filter.
Thus, the problems of integrated circuit space limitations, the number of pin connectors as a limiting factor, and the generation of electrical noise remain in the prior art.