Frequency following circuit

A plurality of a signal peak detection circuits connected in cascade for operation on a complex waveform input signal, for generating a reference signal having peaks occurring in time with the peaks of the fundamental frequency component of the input signal. The reference signal is processed for producing a voltage proportional to the period between successive signal peaks, which voltage is successively stored and monitored at select times for comparison of the relative magnitude changes in the voltage, for updating an output control voltage.

BACKGROUND OF THE INVENTION 
The invention relates to electronic music generation and, more 
particularly, to a frequency following circuit for deriving a control 
voltage proportional to the fundamental frequency (pitch) of a periodic 
input signal of a complex waveform. 
An electrical circuit which produces a control voltage which follows the 
fundamental frequency of an electrical input signal, generated, for 
example, by a musical sound wave, may be utilized to determine the 
frequencies of one or more appropriately scaled voltage 
controlled-oscillators. The oscillators may be connected to drive musical 
sound producers so that the frequencies of the oscillators retain a fixed 
musical interval relationship with respect to the input signal. 
When using such a frequency following circuit for musical purposes, it is 
desirable that the frequency control voltage be derived as soon as 
possible upon input of the musical signal. Changes in the frequency of the 
musical signal should create corresponding changes in the control voltage 
with a minimum delay. If the delay is too long, the listener will receive 
a disturbing time lag between the change in pitch of the input signal and 
the change in pitch of the output of the voltage-controlled oscillators. 
Also, it is desirable that the control voltage should not change in 
response to random changes in the input signal, that is, the control 
voltage should not respond to noise such as the breathy starting transient 
of a wind instrument tone. 
The problem presented in deriving a control voltage proportional to the 
pitch of the musical tone of complex waveform, is the preparation of the 
wave form so that its fundamental frequency can be extracted. The prior 
art has employed a method wherein the input signal is passed through one 
or more low pass filters which attenuate the higher harmonics of the 
signal faster than the filters attenuate the fundamental frequency. This 
scheme has the disadvantage that it introduces a time delay in the output 
by virtue of the phase shift of the low pass filters. It also boosts line 
hum and other unwanted low frequency signals. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an improved 
frequency following circuit which does not utilize low pass filters. 
It is another object of the present invention to provide a novel frequency 
following circuit which provides noise discrimination. 
These and other objects of the invention are achieved by converting a 
complex input signal to a signal bearing a reference point in each cycle 
of the fundamental frequency component of the input signal. The reference 
points of the converted signal are utilized to generate a voltage 
proportional to the period of the fundamental frequency component, which 
voltage is monitored at select times for relative magnitude changes in 
order to update a voltage output representative of the fundamental 
frequency of the input signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the single FIGURE, an electromagnetic or other direct pickup 
11 transduces an aural signal, or the like, into an electrical signal of 
complex waveform which includes a fundamental frequency component plus a 
plurality of harmonic components. The signal is fed from pickup 11 
successively through three peak pickers 13, 15, 17 for detection of the 
highest peak in each cycle. As will suggest itself from the following 
description, one or more peak pickers may be utilized in place of the 
three illustrated in the single FIGURE. 
Each peak picker is comprised of like circuit components as illustrated by 
peak picker 13. A series-connected capacitor 19 and resistor 21 connect 
pickup 11 to the inverting input of an operational amplifier (op amp) 23. 
A parallel-connected capacitor 25 and resistor 27 connect the inverting 
input of op amp 23 to the op amp output via the anode-cathode junction of 
a diode 29. A second diode 31 has its cathode-anode junction connected 
directly between the inverting input and the output of op amp 23. The 
non-inverting input of op amp 23 is connected to ground via a resistor 33. 
A circuit node 35 located at the junction between the anode of diode 29 and 
resistor 27 is connected to ground via a series-connected circuit of a 
capacitor 37 and a resistor 39. The signal developed at the junction 
between capacitor 37 and resistor 39 is fed as an input to the second peak 
picker 15. 
During the positive portion of the input signal at peak picker 13, the 
output of op amp 23 goes negative and capacitor 37 charges very rapidly 
providing a negative voltage level at node 35. As the input signal 
increases in magnitude, op amp 23 forces the voltage at node 35 to 
maintain a fixed voltage relationship with the voltage appearing at the 
inverting input of op amp 23, for producing a current through resistor 27 
substantially equal to the current through resistor 21. When the positive 
portion of the input signal has peaked and begins decreasing in magnitude, 
the voltage at junction node 35 becomes more negative than necessary to 
provide proper feedback voltage and capacitor 37 discharges at a slow time 
constant rate through resistor 27 and diode 31. 
During the negative portion of the input signal to peak picker 13, op amp 
23 produces a positive output and diode 31 conducts. A relatively 
insubstantial voltage drop appears across resistor 39, the effect of which 
is relatively negligible on the voltage output of the peak picker. 
The voltage output of the peak picker is substantially proportional to the 
current charging capacitor 37. The magnitude of the charging current is a 
function of the rate of increase (slope) of the positive portion of the 
input signal. The peaks in the voltage output occur during charging of the 
capacitor, whereas the valleys between peaks occur during discharge of the 
capacitor and during the negative portion of the input signal. Thus, the 
output signal carries an indication of the occurrence of the positive 
peaks of the input signal. 
The effective operation of the peak picker is to generate an output 
waveform which represents a sharpening of the positive peaks of the input 
signal and an exaggeration of the difference in height between the highest 
and lower positive peaks. Thus, with a periodic waveform appearing at 
pickup 11, the use of three or four such peak pickers in cascade will 
effectively suppress all but the highest peak of each cycle, passing the 
fundamental frequency component of the complex waveform. 
Capacitor 25 of the peak picker serves to keep the circuit from oscillating 
by reducing the gain of op amp 23 at high frequencies, and resistor 33 
serves to supply an input voltage at the noninverting input of op amp 23. 
The following pertinent circuit component values are given as illustrative 
of an operative preferred embodiment: 
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Capacitor 19 .22 microfarads 
Resistor 21 15 kilo ohms 
Op Amp 23 operation Amplifier No. 4558 
Capacitor 25 500 picofarads 
Resistor 27 33 kilo ohms 
Capacitor 37 .22 microfarads 
Resistor 39 330 ohms 
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Since the output of peak picker 13 is formed of negative voltage peaks, 
diodes 29, 31 of peak picker 15 have their respective anode and cathode 
connections interchanged for detection of negative going peaks in the 
input signal to peak picker 15. The output signal of peak picker 17 is a 
signal formed of peaks occurring substantially in time with the peaks of 
the fundamental component of the input signal. The peaks of the output 
signal of picker 17 serve as a reference point in each cycle of the 
fundamental frequency of the input signal. 
The successive peaks are transmitted from the third peak picker 17 to a 
converting circuit 41 for deriving a control voltage output proportional 
to the period between successive peaks. Converting circuit 41 
discriminates against noise and random variations in the period by 
monitoring the relative change in magnitude of the period at selected 
times, and updating the control voltage output accordingly. 
Converting circuit 41 includes a period-to-voltage converter 43, an output 
circuit 45, and a counter 47 which controls the operation of circuit 41. 
Counter 47 is driven by a Schmitt trigger 49 which receives the periodic 
peak signals from peak picker 17 and produces a counting pulse in sync 
with each received peak. The counting pulses are fed to counter 47 along a 
lead 50. 
In order for counter 47 to count twice for each full cycle, a Schmitt 
trigger 51 generates another counting pulse along a lead 53, occurring in 
time between each counting pulse generated by Schmitt trigger 49. In the 
preferred embodiment, Schmitt trigger 51 receives an input signal from a 
negative cycle peak detector 55 which is comprised of three cascaded peak 
detectors, each similar to peak pickers 13, 15, 17 except that diodes 29, 
31 of each picker are reversed in direction, in order for the cascaded 
peak pickers to detect the negative peaks of the input signal from pickup 
11. 
Period-to-voltage converter 43 receives successive input peak signals from 
peak picker 17 and using each peak as a reference point in each cycle, 
converts the period between successive peaks to a proportional voltage 
level output along an output lead 57 to output circuit 45. As the period 
changes, the voltage level generated by period-to-voltage converter 43 
will change accordingly. 
One of the successive voltage outputs generated by period-to-voltage 
converter 43 is stored in a sample-hold circuit 59 of output circuit 45, 
for providing a control voltage output to a conventional sound producing 
circuit 61 which responds to the magnitude of the control voltage by 
producing an associated musical note. The voltage stored in sample-hold 
circuit 59 is updated by output circuit 45; a comparator 63 of the output 
circuit correlates the magnitude of successive voltage levels produced by 
the period-to-voltage converter for controlling the update function. 
Period-to-voltage converter 43 produces a voltage which is proportional to 
the period between peaks in a conventional manner by generating a ramp 
voltage signal beginning at one peak and ending at the next peak. A 
charging of a capacitor is utilized to generate the ramp voltage during 
one complete cycle. During the following half cycle the charging is 
discontinued permitting the capacitor voltage to be sampled. During the 
next half cycle the capacitor is discharged. Thus, the period of every 
other full cycle is converted to a voltage level representative of the 
period between its respective peaks. 
Lead 57 is connected to the input of a sample-hold circuit 65 which in 
response to a store command placed along its sample lead 67 stores the 
period voltage developed at its input from lead 57. The voltage stored by 
sample-hold circuit 65 is fed to the input of sample-hold circuit 59 and 
to the input of a third sample-hold circuit 69. Sample-hold circuits 59, 
69 store the voltage appearing at their inputs responsive to a control 
signal placed along their respective sample leads, 71, 73. 
The voltages stored in sample hold circuits 65, 69, are fed to voltage 
comparator 63 for comparison of the stored voltages. Comparator 63 
determines whether successive periods, as represented by the stored 
voltages, lie within a certain small percentage of one another and 
produces a logic output along a lead 75 indicative of the determination. 
Comparator 63 may comprise a conventional window comparator which produces 
a logic output whenever its inputs are a certain percentage of one 
another. 
Lead 75 is connected to a gating circuit 77 for gating the output of 
comparator 63 onto sample lead 71. If the correlation by comparator 63 
indicates that the two period voltages fall within the preset range, then 
sample-hold circuit 59 is actuated along lead 71 for updating the voltage 
stored in circuit 59 with the period voltage stored in sample-hold circuit 
65. 
Thus, only when there is a close correlation between successive period 
voltages is the control voltage output as stored in sample hold circuit 59 
permitted to change for updating the output of the system. 
Counter 47 controls both the generation of the period voltage and the 
updating of the control voltage output. Command signals are generated on 
half cycles as the counter counts through the first three of its four 
separate count outputs. 
A pair of counter output leads 79, 81 transmit command signals on count two 
and count three, respectively, to period-to-voltage converter 43 for 
controlling capacitor charging and discharging within converter 43. Sample 
lead 67 of sample-hold circuit 65 is connected to the counter for 
receiving a command signal on count one. 
On count three, the timing capacitor of converter 43 is permitted to 
charge. One full cycle later on count one, the voltage on the timing 
capacitor is stored in sample hold circuit 65. One half cycle later on 
count two the timing capacitor is discharged to make ready for the next 
charging cycle. As will suggest itself, the charging of the capacitor may 
be discontined responsive to a count one command signal. 
Counter 47 controls the updating of the control voltage output, by 
generating command signals along sample lead 73 and along a gating lead 83 
which controls gating circuit 77. Command signals are generated along 
leads 73, 83 on count three and count two respectively, for commanding 
update of the period voltage. 
On count three, sample-hold circuit 69 stores the immediate period voltage 
as stored in sample-hold circuit 65. One cycle later on count one, a new 
period voltage is stored in sample-hold circuit 65 and comparator 63 
compares the new period voltage with the prior period voltage. On count 
two, the new period voltage is stored in sample-hold circuit 59 according 
to the output of comparator 63 via operation of gating circuit 77. 
By using three sample-hold circuits 59, 65, 69, initial storage of the 
control voltage in sample-hold circuit 59 is delayed by comparator 63 
during the start of a new note by the musician until the tone assumes a 
definite pitch. Thus, a control voltage will not be produced in response 
to noise such as the breathy starting transient prevalent with wind 
instruments. 
It should be understood, of course, that the foregoing disclosure relates 
to a preferred embodiment of the invention and that other modifications or 
alterations may be made therein without departing from the spirit or scope 
of the invention as set forth in the appended claims.