Digital tone generator
BACKGROUND OF THE INVENTION 
     This invention relates to tone generators and more particularly to tone generators for providing single or multiple tones. 
     Telephone signaling requirements often include the use of a number of tone frequencies used singly or in combination over transmission links such as in a Touch Tone telephone system. Thus, both telephone and telegraph carrier systems require multiple tone sources. 
     Presently telephone and telegraph signaling systems employ both RC (resistor capacitor) and LC (inductor capacitor) type oscillators of a variety of configurations to produce the desired tones. RC type tone generators are often not stable enough for some applications, although they do have the advantage that integrated circuit techniques can be employed with this type of circuit for miniaturization. In contrast, the LC type circuits can be designed to be stable enough, however, the coils are often bulky, and do not lend themselves to miniaturization by integrated circuit techniques. Both types of generators suffer in that in production, there is required the individual tuning of each tone frequency and that they require alignment or retuning as part of the maintenance procedure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a digital generator of one or more tones that may be used singly or in combination, where such tones are sinusoidal in shape, and stable in frequency. 
     Another object of the present invention is to provide a digital stable, multi-frequency sine wave generator that can be miniaturized by means of integrated circuit techniques. 
     Still another object of the present invention is to provide a digital generator for two or more sine wave frequencies derived from a single, stable oscillator or generator, and these frequencies will not require either initial adjustment in production, nor will they require alignment or retuning as part of the maintenance procedure. 
     A further object of the present invention is to provide a digital single or multitone generator which is adaptable to integrated circuit techniques, either conventional, or LSI (large scale integrated circuit) (where production quantities warrent having the absence of tuning requirements and which represents a cost effective approach to provide a multifrequency tone generator. 
     A feature of the present invention is the provision of a digital tone generator comprising: first means to generate a reference square wave signal having a stable given frequency; second means coupled to the first means to provide a plurality of square wave signals each having a different predetermined frequency, each of the predetermined frequencies having a different predetermined relationship to the given frequency; third means coupled to the second means to select at least one of the plurality of square wave signals; and fourth means coupled to the second means to provide as an output signal of the generator one sine wave tone having a frequency equal to the predetermined frequency divided by a given factor of the selected one of the plurality of square wave signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which the single FIGURE is a logic diagram of the digital tone generator in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIGURE, the digital tone generator of the present invention includes a tone frequency selection pad 10 including therein a plurality of buttons which activate a pair of switches when depressed. For instance, when button 1 is depressed, switches E and A are closed to provide a potential which is a logic &#34;1&#34;. By depressing this one button, two frequencies are simultaneously selected due to the closing of switches A and E. The other buttons also actuate two switches when depressed to select two frequencies and provide a &#34;1&#34; for application to logic circuitry to be discussed hereinbelow. The frequencies for purposes of explanation herein associated with the horizontal row of buttons is selected when any of these buttons are depressed, while the buttons in the vertical rows each select the same frequency associated therewith in the drawing. 
     The digital tone generator illustrated in the FIGURE is for the generation of two sine wave tones. However, if one of the banks switches associated with each of the buttons of pad 10 are rendered inoperative and only one half of the circuit illustrated in the Figure is employed a single tone may be generated. 
     The digital tone generator illustrated in the Figure includes a square wave crystal controlled generator 11 to generate a stable frequency of 411230 Hz (hertz). Generator 11 feeds its output signal to two logic circuits 12 and 13. Logic circuit 12 includes binary counter divider 14, AND gates 15 - 18, OR gate 19 and a one-shot multivibrator 20. Logic circuit 13 includes binary counter divider 21, NAD gates 22 - 25, OR gate 26 and one-shot multivibrator 27. The output signal of one-shot multivibrator 20 resets divider 14 and also provides the driving pulse for a Johnson or ring counter 28 which has certain outputs of the JK flip-flop stages thereof coupled to a resistor tone mixing circuit 29 including resistors 30 - 33, while the output signal of one-shot multivibrator 27 resets binary counter-divider 21 and also provides the driving pulse for the Johnson or ring counter 34. The outputs of the predetermined ones of the JK flip-flop stages of counter 34 are coupled to resistors 35 - 38 of circuit 29. The output from circuit 29 is coupled to a filter 39 illustrated as capacitor 40 and to a resistive load R L . The desired sine wave tone having the selected frequency is provided by the output terminal 41. 
     In order to choose the frequency of generator 11, a number is first chosen that is an integral multiple M of each of the desired sine wave frequencies. Where the sine wave tone frequencies are not harmonically related, the number chosen can be the integral number of frequencies within the allowable tolerance of these desired frequencies. Upon establishing the common multiplier as described, the frequency of generator 11 will be a frequency ten times higher than the common multiplier and is a square wave output signal of this frequency value. 
     The square signal of the predetermined frequency of generator 11 drives the frequency dividers 14 and 21, each of which programmable to recycle at any desired integral number from 2 to 2 n  -1, where n equals the number of stages in the frequency dividers 14 and 21. 
     The dividing or recycling rate is selected by the keyboard selector switches of pad 10 or other external selection switches that are provided. 
     AND gates 15 - 18 and AND gates 22 - 25 are respectfully coupled to the output of the stages of dividers 14 and 21 to provide the integral multiple M, the value of which is shown at the output of each of these AND gates. The value of the integral multiple M is determined by the sum of the weights of the stages of the dividers 14 and 21 to which the input of the AND gates are connected. The frequencies at the output of the AND gates 15 - 18 and 22 - 25 are the desired selected frequency within a ± 1% tolerance of the selected frequency. However, the actual value of the frequency of the square wave output of these AND gates are ten times higher as shown due to the multiplying of the common multiplier determined for generator 11 by 10. The frequency of the output of OR gate 19 is equal to the frequency of generator 11 divided by the multiple M. For instance, if switch A is closed, AND gate will be enabled and will produce a frequency of 411230 divided by 59. If switch C is closed, AND gate 17 will produce an output at OR gate 19 equal to 411230 divided by 48. Thus, the outputs from OR gates 19 and 26 are equal to F (the frequency output of generator 11) divided by M of the enabled AND gate. 
     Each pulse received from generator 11 by dividers 14 and 21 increments the divider by one count. Upon reaching the desired, selected count, all legs of the AND gate enabled by a switch of pad 10 will be at &#34;1&#34; providing a &#34;1&#34; at the output of the enabled AND gate. This signal when obtained and reaching the OR gates 19 and 26 will cause a &#34;1&#34; to be generated at the output of these OR gates. The output signal of the OR gate is coupled to a one-shot multivibrator 20 or 27, or any other pulse-stretcher stage in order to provide a reset pulse of adequate width to reliably reset the dividers 14 and 21 to zero. Other presettable counter configurations may be used and are workable. 
     The reset pulse from multivibrators 20 and 27 resetting dividers 14 and 21 to zero will occur at ten times the value of the desired tone wave frequency. In addition to resetting dividers 14 and 21, the reset pulse also drives the associated Johnson or ring counters 28 and 34. The property of a Johnson counter, a five-stage shift register using JK flip-flops fed back on itself through NOR gates 42 and 43, is that during the first five pulses the counters 28 and 34 will successively load up with &#34;1&#34;&#39;s, then the reverse will take place, i.e., the counters 28 and 34 will load all &#34;0&#34;&#39;s. This loading operation is illustrated in the Johnson counter state diagram shown in the Figure. Ten steps are required to complete the loading cycle at which time the cycle will continue to repeat. 
     If resistor taps are provided for each stage of the Johnson counter, and the resistor values of the network 29 are chosen to represent a total current flow to approximate a sine wave (in 1/10 or 36° steps) then, if these resistor values are chosen, and the current flows into a resistor whose value is relatively small in relation to the resistor tap values, the voltage across the load resistor R L  is proportional to the current flow, and the waveshape approximates a sine wave quantized in ten steps. f.sub.˜out is equal to f.sub.˜1 + f.sub.˜2, where f.sub.˜1 is equal to 1/10 of the pulse rate of the frequency of the signal applied to the Johnson counter 28 and f.sub.˜2 is equal to 1/10 of the pulse rate of the frequency of the input signal applied to Johnson counter 34. As a result, the two selected tones processed by the Johnson counters 28 and 34 will appear at the output terminal 41. 
     A small value bypass capacitor 40 will remove the quantization ripple and filter out the high order harmonics leaving a relatively clean sine wave tone signal. For multiple tones, additional Johnson counters and resistor mixing circuits may be added to accomodate the mixing of three or more tones. 
     Because the output of the dividers 14 and 21 is ten times the selected frequency and the Johnson counters require ten steps per cycle, the output sine wave tone will have the frequency selected by the depressed button in pad 10. 
     The stability and accuracy of the calculated tones will be equal to that of the crystal conrolled generator 11, and no further adjustments are required. 
     While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.