US 3544694 A
Description (OCR text may contain errors)
United States Patent Inventor William A. Freeman 372 Memorial Drive, Cambridge, Massachusetts 02139 Appl. No. 807,655
Filed March 17, 1969 Patented Dec. 1, 1970 PICKUP AND WAVEFORMING CIRCUITRY FOR ELECTRONIC MUSICAL INSTRUMENT 8 Claims, 1 Drawing Fig.
Primary Examiner-W. E. Ray AItrney-A1fred B. Freeman US. Cl 84/1.l1, ABSTRACT: Circuitry for changing the waveform and other 84/1.14, 84/1.19 characteristics of musical instrument tone signals in electrical Int. Cl. Gl0h 1/02, form which consists of a pickup, a special amplifying and GlOh 3/04 limiting stage driving tone filters, frequency dividers, and Field of Search 84/1.01, other waveforming circuits including LC networks switchable 1.04. 1.06, 1.11, 1.13, 1.19, 1.24, 1.26(B, F) to either high pass or low pass response.
11 fi/Pw- '5 Sol/RC Il '6 is Z5 .4; l 4% t I I j I I7 '9 5 0/8/2197 FREQ. I4 22. 24 OACU/T o/r/az/e 27 I AMPl/F/E/P 007, 07 C/RCU/T PICKUP AND WAVEFORMING CIRCUITRY FOR ELECTRONIC MUSICAL INSTRUMENT BACKGROUND OF THE INVENTION 1. Field of the Invention v This invention is directed to apparatus for changing the spectrum, or timbre, of musical tones, for increasing the sustain time of percussion-type tones, for dividing the frequency to lower the pitch of musical tones by octave intervals.
2. Description of the Prior Art The prior art includes apparatus for changing the timbre and increasing the sustain time of percussive tones by using several successive amplifying, limiting and differentiating stages. this apparatus frequently produces abrupt and random changes in the pattern of distortion with changes of input waveform which are undesirable sand its performance is very sensitive to component variations.
The prior art also includes apparatus to divide the frequencies of musical tones to change pitch by octave intervals. This apparatus uses complex filters tailored to particular input signals and frequency ranges and associated amplifying and limiting stages as well as frequency divider stages.
SUMMARY OF THE INVENTION The present invention provides a single stage circuit which is driven to saturation by very small signal swings in either direction and which limits its response to a wide range of larger signals to a small region about their zero, or center, value. The circuit avoids the abrupt and undesireable changes in output waveform of prior art devices which result from unsymmetrical limiting and transient shifts through interstage coupling networks. The output waveform is further shaped by selecting different linear and nonlinear circuits to connect it to a sound transducer including series inductor-capacitor circuits switchable to high pass or low pass response. For many input signals, the output is a substantial square wave of the same period as the pitch an so satisfactorily drives a divider to produce another signal an octave lower in pitch. The circuitry is not only simpler than prior art apparatus but can operate v over the full musical pitch range. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partial schematic and partial block diagram of an embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Input source 11 may' consist of a guitar with an electrical pickup, or any other musical instrument with a microphone or electrical pickup capable of producing musical tone signals in electrical form. The output of source 11 is applied to one arm of switch 12 which connects it to one side of capacitors l3 and 14 in one position and to the input of amplifier stage 15 in the other position. The other sides of capacitors 13 and 14 connect to the bases of transistors 16 and 17 respectively. Transistor 16 is a PNP type and has its emitter connected to the positive side of the supply. Transistor 17 is an NPN type with its emitter connected to the negative side of the supply and its collector to the collector of the PNP transistor 16 and to a contact on switch 12. Resistor 18 is connected between the bases of transistors 16 and 17.
Each of transistors 16 and 17 is forward biased by the current flowing through their bases and resistor 18. If each had the same current gain, the voltage on their collectors would be midway between the positive and negative supply terminals. A slightly greater current gain in one will swing the collector voltage to its side. One polarity of an input signal from source 11 will reduce base current in one transistor 16 and 17 and increase it in the other while the other polarity does the opposite. The common collector voltage will swing from one side to the other with changes in the input polarity so long as the signal amplitude is enough to drive transistors 16 and 17 to alternately have the largest collector current.
The size of the forward biasing current is determined by the size of resistor 18 and the supply voltage. It must be kept small enough to avoid producing collector currents which could exceed the transistor power dissipation ratings. The two bases receive extra current drive on large signals during the first half cycles and then less as capacitors l3 and 14 retain charges which tend to add reverse biasing to their respective transistors 16 and 17. The clamping applied by the bases for both directions of signal swing maintain a substantially symmetrical signal swing about the zero point for all signal inputs. The circuit will perform adequately with a l.5 -volt battery supply which is very convenient for small portable applications. The use of larger voltage supplies increases the effective voltage gain and the voltage output which may be desireable for some applications.
In the position shown, switch 12 connects the collectors of transistors 16 and 17 to the input of the emitter follower consisting of capacitor 19, transistor 20, base bias resistor 21, and emitter load resistor 22. The emitter follower offers a light load on the collectors while providing a suitable current drive for the following circuitry. Capacitor 23 connects the output of the emitter follower to resistor 24, contacts of switches 25 and 26, and the inputs of distort circuit 27 and frequency divider circuit 28.
Switches 25 and 26 have three positions with the center position being open and the two end positions connecting their respective networks in opposite directions across the output. The respective networks consist of capacitor 30 and inductor 31 and of capacitor 32 and inductor 33 which forrri series resonant circuits and have their respective center junctions connected through resistors 34 and 35 to output circuit 36. Connected in one direction, the networks provide a low pass response, and connected in the other direction, a high pass response. Two different tonal effects are thus obtained from each network.
Distort circuit 27 changes the input waveform, which is substantially a square wave, to another shape, such as a pulse or a sawtooth wave, to provide additional variety in the timbre available. Frequency divider circuit 28 may consist of a pulse amplifier and flip-flop or other circuit for dividing the input frequency by two or more. It thus provides a signal which is an octave or more lower than the input. Output circuit 36 may include signal mixers, additional tone forming networks, and switches for controlling the passage and routing of signals. The combined signal output from output circuit 36 goes to sound transducer 37 which converts it to audible sound.
When switch 12 is in its other position, the output of source 11 is applied to amplifier l5 and the emitter follower associated with transistor 20 is driven from the output of amplifier 15. This allows the following tone processing effects to be applied to the source signal directly. It is not expected that the frequency divider circuit 28 will work with this input but other channels may provide musically useful effects. The gain of amplifier 15 can be adjusted so the peak signal will be comparable in amplitude to the output of transistors 16 and 17.
The output of source 11 is also applied directly to outputcircuit 36. This allows the signal to be used directly in the output, or through tone forming networks, or input to amplitude control means which may also be part of output circuit 36. The amplitude control means would be responsive to the amplitude of the input signal and would in turn control the amplitude of another signal correspondingly so it would follow the input signal in amplitude.
1. In an electronic musical instrument apparatus havinga power supply and adapted to receive an input of musical tone signals in electrical form and provide an output to a sound transducer, the combination of:
a. an NPN transistor having its emitter connected to the negative side of said power supply;
b. a PNP transistor having its emitter connected to the positive side of said power supply and its collector to the collector of said NPN TRANSISTOR:
c. a first capacitor connected between the base of said NPN transistor and said input for musical tone signals;
d. a second capacitor connected between the base of said PNP transistor and said input for musical tone signals;
e. means for forward biasing said NPN and PNP transistors; f. and means for connecting the collectors of said NPN and PNP transistors to said output for said sound transducer.
2. The combination according to claim 1 wherein said biasing means consists of a resistor connected. between the bases of said NPN and PNP transistors.
3. The combination according to claim 1 wherein said connecting means includes a frequency divider circuit.
4. The combination according to claim 1 wherein said con necting means includes atone forming circuit. 5. The combination according to claim 4 wherein said tone forming circuit consists of a capacitor and inductor connected in series across the output from the collectors of said NPN and PNP transistors.
7. The combination according to claim 1 wherein said connecting means includes an emitter follower having its input I connected to the collectors of said NPN and PNP transistors.
8 The combination according to claim 1 wherein said connecting means includes a high impedance input circuit.