Visual audio frequency comparator
United States Patent 91 Lester et al.  
 [ Apr.8, 1975 41 VISUAL AUDIO FREQUENCY COMPARATOR [75] Inventors: Theodore V. Lester, Schiller Park;  
 Donald C. Ryon, Carol Stream, both of III.  
 [73] Assignee: Motorola, Inc., Chicago, Ill.  
 [22] Filed: Sept. 17, 1973 211 Appl. No.: 398,027  
  521 u.s.c| 324/7911; 324/82; 324/88; 84/454 51 mu ..G0lr23/l4 [58] Field of Search... 324/78 R, 78 Q, 79 R, 82 X, 324/88, 121, 91; 84/454 [56] References Cited UNITED STATES PATENTS 3.074.054 l/l963 Ross 324/82 X 3.287.639 ll/l966 Vail 324/88 X OTHER PUBLICATIONS Tolley, IBM Tech. Disc. Bul., Vol. 14, No. 4. Sept.  
  OSCI LLATORS AND DIVIDERS OSCILLATORS AND D IVIDERS PATTERN GENERATOR l97l,p. 1266.  
 Primary Examiner-Alfred E. Smith Assistant ExaminerRolf I-Iille Attorney, Agent, or FirmEugene A. Parsons; Vincent J. Rauner 5 7 ABSTRACT A system for determining the frequency relationship between two notes of a musical instrument includes a split screen display device, and bar generating means connected to the instrument and display device for generating a first plurality of bars representative of the frequency of the first note on one portion of the display device and a second plurality of bars representative of the frequency of the second note on another portion of the display device. The vertical sweep circuit of the display device is synchronized to the frequency of one of the notes.  
 4 Claims, 6 Drawing Figures VERTICAL SYNC.  
 sumanrg SAME FREQUENCY AND PHASE SAME FREQUENCY-I80 OUT OF PHASE 2Il OCTAVE 514 A THIRD INTONE lNTERVAL VISUAL AUDIO FREQUENCY COMPARATOR BACKGROUND 1. Field of Invention This invention relates generally to frequency comparison systems, and more particularly to systems for determining the frequency relationship between two notes provided by a musical instrument.  
  There are many applications wherein it is necessary to compare the frequency of one signal to another. One such application is in an electronic musical instrument wherein it is necessary to compare the frequencies of various notes sounded by the instrument for tuning, demonstration or experimentation purposes.  
 2. Prior Art Several techniques for determining the frequencies of signals are known. Such techniques include the use of tuned reeds which vibrate when the frequency of the signal applied thereto is equal to the resonant frequency of the reed, beat note systems wherein the frequency of the note is compared with the frequency of an oscillator having a known frequency and visual comparison systems employing Lissajous figures.  
  Whereas these techniques provide a way to determine frequencies of signals, the reed and standard oscillator systems are costly and only provide an accurate frequency determination at discrete points and do not provide a direct frequency comparison between two signals. The Lissajous display systems provide a direct comparison, but the display cannot be readily interpreted unless the ratio between the signals applied thereto is an integral number, and interpretation of Lissajous figures becomes extremely difficult when frequencies of a musical scale, which have a non-integral frequency relationship therebetween, are compared.  
 SUMMARY It is an object of the present invention to provide an improved visual frequency comparison circuit for comparing the frequency of musical notes.  
  It is a further object of the invention to provide a frequency comparison circuit that readily provides a comparison between signals having a non-integral frequency relationship therebetween.  
  It is a further object of this invention to provide a visual frequency comparison system that has a low cost and is readily interpreted by personnel untrained in electronics.  
  In accordance with a preferred embodiment of the invention, a split screen video monitor is employed. A bar generator is used to generate a plurality of bars indicative of the frequency of one of the notes in one area of the split screen and a second plurality of bars indicative of the frequency of a second note in another portion of the split screen. The vertical sweep circuit of the video monitor is synchronized to the frequency of one of the notes, thereby providing a stationary bar pattern on one portion of the screen. The frequency of the signal displayed in the other portion of the screen is compared to the fixed pattern by comparing the relative number of bars and the position and movement of the bars with respect to the fixed bar pattern.  
 In the drawingsf FIG. 1 is a block diagram of the visual frequency comparison circuit according to the invention shown connected to an electronic organ;  
  FIG. 2 is a diagram, partially in schematic form and partially in block diagram form, of the visual display device according to the invention; and  
  FIGS. 3-6 are drawings showing the bar patterns obtained for various frequency ratios of signals applied thereto.  
 DETAILED DESCRIPTION Referring to greater detail to FIG. 1, there is shown a portion of a musical instrument including oscillators and dividers 10, 20 and keyboards 12, 22 connected, respectively, thereto. A pair of amplifiers 14, 24 have inputs thereof connected to the keyboards 12,22 and outputs connected to a pattern generator 16. The pattern generator 16 is connected to display means such as a video monitor 18 and generates bars in first and second portions of a split screen 26 and 28. Vertical synchronization for the monitor 18 is applied thereto from the pattern generator 16, and horizontal synchronization is applied from the monitor 18 to the pattern generator 16.  
  In operation, the keyboard 12 passes a first tone from the oscillators and dividers 10 to the input of the amplifier 14. The amplified signal from amplifier 14 is processed by the pattern generator 16 and a video signal which is substantially a square or rectangular wave having the same frequency as the frequency of the first tone is applied to the monitor 18 to generate the bar pattern in the area 26. Similarly, a signal from the oscillators and dividers 20 is passed by the keyboard 22 to the amplifier 24. The pattern generator 16 generates a video signal in response to the signal from the amplifier 24 to generate the bar pattern in the area 28. A vertical synchronization signal is applied to the monitor 18 to synchronize the vertical sweep of the monitor to a submultiple of the frequency of the signal from the amplifier 14. A horizontal synchronization signal is obtained from the horizontal synchronization circuitry of the monitor 18 and controls a gate circuit in the pattern generator 16 to alternately pass signals from the amplifiers l4 and 24 to the video portion of the monitor 18 to generate the patterns appearing in areas 26 and 28. A more detailed explanation of the operation of the pattern generator 16 and the monitor 18 follows.  
  Referring to FIG. 2, there is shown a schematic diagram of one embodiment of the pattern generator 16 and a more detailed block diagram of a typical monitor 18. The pattern generator 16 has an input point 34 which is connected to the output of theamplifier 14. The input point 34 is also connected to the base of a transistor 36 through a capacitor 38 and a resistor 40. Similarly, an input point 44 is connected to the output of the amplifier 24 and to the base of a transistor 46 through a capacitor 48 and a resistor 50. The collectors of the transistors 36 and 46 are connected to a power supply A+ through resistors 42 and 52, respectively, and to the input of a video amplifier in the monitor 18 through a pair of resistors 63 and 64, respectively. The collector of the transistor 36 is also connected to the vertical oscillator 54. A pair of transistors 62 and 72 have the collectors and emitters thereof connected to the collectors and emitters of the transistors 36 and 46, respectively, and the emitters of the transistors 36, 46, 62 and 72 are connected to a ground or common potential. The base of the transistor 62 is connected by means of a resistor 68 to the collectors of a pair of transistors 64, 66, each having an emitter connected to ground. Similarly, the base of the transistor 72 is connected through a resistor 78 to the collectors of a pair of transistors 74 and 76, whose emitters are also connected to ground. The collectors of the transistors 74 and 76 are connected to the power supply A+ through a resistor 79. The collectors of the transistors 64 and 66 are also connected to the power supply A+ through a resistor 69 and to the bases of the transistors 74 and 76 through resistors 80 and 82, respectively. The bases of the transistors 64 and 66 are connected to the output of a horizontal oscillator 90 in the monitor 18 by means of resistors 84 and 86, respectively, and a capacitor 88. Resistor 87 is adjusted to split the screen in the exact center of the raster.  
  The output of the horizontal oscillator 90 is also connected to a horizontal output circuit 92 which drives the horizontal sweep coils of a yoke 94 and a high voltage circuit 96 which is connected to the anode of a display device such as a cathode ray tube 98. A vertical output circuit 100 of the monitor 18 has an input connected to the output of the. vertical oscillator 54 and an output connected to the vertical sweep windings of the yoke 94. Thea-output of the video amplifier 60 is connected to the grid 102 of the cathode ray tube 98 to modulate the raster produced on the face of the cathode ray tube 98 by the horizontal output circuit 92 and the vertical output circuit 100.  
  In operation, a tone signal applied to the input point 34 is amplified and amplitude limited by the transistor 36 to provide a substantially rectangular wave to the input of the amplifier 60 when the transistor 62 is rendered nonconductive. Similarly the transistor 46 amplifies signals from the input point 44 for application to the amplifier 60 when the transistor 72 is nonconductive. The transistors 62 and 72 serve to shunt the signals from the respective transistors 36 and 46 to ground when either of the transistors 62 and 72 is conductive.  
  The horizontal oscillator 90 provides oscillations at a relatively high horizontal sweep frequency which may be 15,750 Hz or other suitable frequency to the horizontal output circuit 92 which provides a ramp function to the horizontal coils of the yoke 94 to horizontally sweep the beam across the face of the cathode ray tube 98. The flyback pulses from the horizontal output stage 92 are, transformed to a high voltage and rectified by the high voltage circuit 98 to provide the anode voltage for the cathode ray tube 98 in a conventional fashion. The output of the horizontal oscillator 90 is also coupled through the capacitor 88 to the bases of the transistors 64 and 66 which are effectively connected in parallel. A single transistor may be used in place of the transistors 64 and 66, however because integrated circuit comprising four dual input gates was used in the fabrication of the pattern generator 16, one of the gates consisting of transistors 64 and 66 was connected as an amplifier as shown.  
  Because the output of the horizontal oscillator 90 is capacitively coupled to the transistors 64 and 66, both transistors are rendered conductive when the value of the alternating current component of the signal from the horizontal oscillator 90 is sufficient to forward bias both transistors. When the value of the alternating current component is less than the value required to forward bias the base-emitter junctions of the transistors 64 and 66, both transistors 64 and 66 are rendered nonconductive. The output at the collectors of the transistors 64 and 66 is therefore substantially a square or rectangular wave having a frequency equal to the frequency of the signal generated by the horizontal oscillator 90.  
  The square wave signal from the transistors 64 and 66 is applied to the base of the transistor 62 to render the transistor 62 conductive when the square wave is at its high state and to render the transistor 62 nonconductive when the square wave is in its low state. The  
 signal from the transistors 64 and 66 is also applied to the bases of the transistors 74 and 76 which have a similar configuration to the transistor pair 64, 66, and which serve as a phase inverter to generate a second wave 180 out of phase with the square wave applied thereto. The signal from the collectors of the transistors 74 and 76 is applied to the base of the transistor 72 to render the transistor 72 alternately conductive and non-conductive, transistor 72 being conductive when transistor 62 is non-conductive and vise versa.  
  Since the transistors 72 and 62 are alternately rendered conductive, one of the signals from the transistors 46 and 36 is alternately shunted to ground while the signal from the other of the transistors 46 and 36 is applied to the video amplifier 60. The aforementioned action causes the beam of the cathode ray tube 98 to be modulated by the video amplifier 60 in response to the signal from the transistor 36 during the first portion of each horizontal sweep, and in response to the signal from the transistor 46 during the remainder of the sweep, thereby generating the split screen effeet.  
  The signal from the transistor 36 is also applied to the vertical oscillator 54 to provide a synchronization pulse thereto which causes the vertical oscillator to operate at a frquency which is a submultiple of the frequency of the signal from the transistor 36. The vertical oscillator 54, which generally operates at a lower frequency than the frequency of the horizontal oscillator 90, causes the vertical output circuit to apply a ramp signal to the vertical winding of the yoke 94 to cause the beam of the cathode ray tube 98 to be vertically swept at a submultiple of the frequency of one of the tones applied to the frequency comparator. The synchronization causes the bar display corresponding to the tone to which the system is synchronized to appear as a series of stationary horizontal bars, the number of bars being determined by the frequency ratio of the tone signal and the operating frequency of the vertical oscillator 54. The number of bars appearing in the display area for the other of the signals is determined by the frequency ratio between that signal and the frequency of the vertical oscillator 54 and serves to provide a frequency comparison between the two signals applied to the comparator according to the invention.  
  Referring to FIG. 3, the bar pattern generated in the areas 26 and 28 of the monitor 18 indicates that the frequencies being compared by the visual frequency comparison system according to the invention have both the same frequency and phase because the number of bars in both the regions 26 and 28 is equal and the bars lie in the same horizontal line.  
  The pattern shown&#39; in FIG. 4 indicates that the two signals have the same frequency since the spacing between the bars in each of the areas 26 and 28 is equal,  
 in the area 26. Similarly, FIG. 6 shows a 5:4 ratio, five bars being present in area 28 for every four present in the area 26.  
  The display provided by the system according to the invention is also useful for comparing signals having only a slight frequency offset therebetween. Since the vertical sweep of the monitor 18 is synchronized to the signal displayed in the area 26, the bars in the area 26 will remain stationary. The bars in the area 28 will remain stationary when the frequency of the signal displayed in area 28 is related to the frequency of the signal displayed in area 26 by a ratio of small whole numbers such as, for example, 1:1, 2:1 or 5:4 as shown in FIGS. 3-6. If the aforementioned ratio does not occur, the bars in the area 28 will roll upward or downward with respect to the bars in the area 26. An upward movement shows that the signal displayed in area 28 has a slightly higher frequency, or a sharp relationship to the signal displayed in area 26, while a downward movement shows a lower or flat pitch.  
  For example, an upward movement of the bars in area 28 of FIG. 3 shows that the frequency of the signal displayed in area 28 is slightly higher than the frequency of the signal displayed in area 26. Similarly, an upward movement of the bars in area 28 of FIG. 5 shows that the frequency ratio is slightly greater than 2:l while an upward movement in the area 28 of FIG. 6 indicates a slightly greater ratio than 5:4. Thus, the frequency relationship between the two signals may be readily ascertained by counting the ratio of the bars displayed in area 26 and 28 and noting whether the bars in area 28 move upward or downward with respect to the bars in area 26.  
  The inventors have found that the system according to the invention provides an effective and accurate way to make frequency comparisons particularly of the type of comparisons necessary for tuning musical instruments.  
  Whereas the embodiment of the invention described in the foregoing shows apparatus for generating a plurality of horizontal bars, it should be obvious to those skilled in the art that vertical bars can also be generated by simply rotating the display tube by 90, by causing the horizontal rather than the vertical oscillator to operate at a submultiple frequency of one of the tone frequencies applied thereto, or by other means. Also, another display device such as a solid state display device may be used in place of the cathode ray tube 98. It should, however, be noted that these and other modifications readily apparent to those skilled in the art still fall within the scope and spirit of the invention.  
 We claim:  
  1. A system for comparing the frequencies of first and second musical notes including in combination:  
 means for receiving a first alternating current signal having a predetermined frequency relationship to the frequency of said first musical note; means for receiving a second alternating current signal having a predetermined frequency relationship to the frequency of said second musical note;  
 display means for visually displaying bars in response to alternating current signals applied thereto, the number and position of the bars displayed being proportional to the frequency and phase, respectively, of the alternating current signals applied to said display means said display means including a display device, and a high frequency sweep circuit and a low frequency sweep circuit coupled to said display device, one of said sweep circuits being horizontal and the other being vertical for generating a raster on said display device; and  
 gating means connected to said first and second alternating current signal receiving means and to said display means for alternately applying signals, representative of one of said first and second alternating current signals, to said display means an output of said high frequency sweep circuit being connected to said gating means for causing said gating means to alternately apply one of said first and second alternating current representative signals to said display means for a portion of each high frequency sweep.  
  2. A system as recited in claim 1 wherein said low frequency sweep circuit is connected to said first signal receiving means, said low frequency sweep circuit being responsive to said first signal receiving means for synchronizing the frequency of operation of said low frequency sweep circuit 0 a submultiple of the frequency of said first signal.  
  3. A system as recited in claim 2 wherein said first and second signal receiving means include a limiting amplifier for generating signals having a substantially rectangular wave shape in response to said first and second alternating current signals applied thereto.  
  4. A system as recited in claim 3 further including means for modulating the raster generated by said horizontal and vertical sweep circuits, said modulating means being connected to said gating means and to said display device for modulating the raster in response to said first and second alternating current signals.