Digital to analogue converters
This is a continuation of application Ser. No. 764,104 filed Jan. 31, 1977 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to digital to analogue converters and more particular, though not exclusively, to such converters which are applicable to television or radio tuners in which the frequencies of resonant circuits are controlled by means of direct voltages applied to varicap diodes. The magnitude of the direct voltage is controlled by digital code signals applied to a digital to analogue converter. 
     One prior art type of digital to analogue converter comprises a register adapted for storing an applied digital signal, an oscillator connected to a counter and a comparator for providing an output signal upon coincidence between the values held in the storage register and the counter. This type of converter further comprises a bistable multivibrator circuit which is set by the coincidence signal and is reset upon the &#34;full count&#34; of the counter. The bistable circuit controls a switch that alternately couples one of two different potential sources to a smoothing circuit; and the direct voltage output from the smoothing circuit is representative of the applied digital signal. 
     In such a converter, the frequency of ripple on the output is dependent on the signal frequency from the oscillator and of the value of the maximum count of the counter. The time constant of the smoothing circuit must be such as to effectively remove the ripple from its output signal; however, large time constants have the disadvantage of reducing the response speed of the converter. For example, some configurations could require several seconds to attain a new value after the applied digital signal is changed. 
     SUMMARY OF THE INVENTION 
     A primary object of the subject invention is to provide an improved digital to analogue converter in which certain difficulties associated with known forms of digital to analogue converters are overcome or reduced. 
     A more specific object of the subject invention is to provide an economical and reliable digital to analogue converter which has a relatively fast response speed. 
     One embodiment of the subject invention comprises means for producing a first series of pulses whose average value is representative of the first N bits of an applied digital signal; means for producing a second series of pulses whose average value exceeds that of the first series of pulses by a preselected fixed amount; means for filtering the first and second series of pulses and switching means for alternately applying the filtered first or second series of pulses to a smoothing circuit such that the percentage of the time respective ones of said filtered series of pulses are applied to the smoothing circuit is proportional to the value of the remaining bits of the applied digital signal. The direct voltage output from the smoothing circuit is representative of the applied digital signal. 
     Compared to prior art converters of similar cost and complexity, those in accordance with the subject invention have ripple voltages of higher frequency and consequently the time constant in the smoothing circuit may be appropriately reduced and the speed of response of the circuit correspondingly increased. 
     In order that the invention may be clearly understood and readily carried into effect, an embodiment thereof will now be described with reference to the accompanying drawing in which like reference characters refer to like parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing showing a known form of digital to analogue converter, 
     FIGS. 2a and 2b show waveforms of voltages to be found therein, and 
     FIG. 3 is a schematic drawing of a digital to analogue converter in accordance with the present invention. 
    
    
     Referring to FIG. 1, which shows a digital to analogue converter for use with a touch-tune television receiver, a read only memory 10 is used to store in digital form signals representative of the direct voltage which is to be applied to a varicap diode (not shown) in order to tune specific transmitters. Thus in one example memory 10 provides twelve registers each capable of storing a 12 bit binary representation. Any of the registers can be selected by touch-terminals, three of which are shown and marked 1, 2 and 3, in a well known touch-switch manner, and when so selected a number stored in the appropriate register is transferred into a 12-bit store 11. An oscillator 12 is connected to a 12-bit counting register 13. Register 11 and counter 13 are connected to a comparator 14 the output of which is connected to a bistable multivibrator circuit 15 sometimes referred to as a &#34;flip-flop&#34; circuit and designated by F/F in FIGS. 1 and 3. The arrangement is such that after a number has been stored in register 11, after counting of the oscillations from oscillator 12 commences in counter 13, and when coincidence is achieved between the numbers stored in register 11 and counter 13 an output signal is applied from comparator 14 to switch the bistable circuit (F/F) 15 into its &#34;on&#34; condition. The counter 13 is also connected to bistable 15 and is arranged to reset the bistable when counter 13 has reached its maximum count. The output bistable 15 is shown in FIG. 2a and it will be appreciated that the ratio between the time for which the bistable is in its &#34;off&#34; condition to the cycle time is dependent directly on the number which has been stored in register 11. Bistable 15 is connected to a switch means 16, 16&#39; so that in the &#34;on&#34; condition the output of the switch is connected to terminal 18, a 0 volt supply line, and in the &#34;off&#34; condition of the bistable 15 the output is connected to terminal 17, a 30 volt supply line. Accordingly the output of the switch 16&#39; is alternately connected to a 30 volt supply and to a 0 volt supply, the relative durations of the connections being dependent on the number stored in register 11. The output is shown in FIG. 2b and is a voltage, the average value of which is proportional to the number stored in register 11, but which has a ripple superimposed thereon. The output from switch means 16&#39; is smoothed by a smoothing circuit shown at 19 (as a resistor capacitor type electrical filter) and the output from circuit 19 is obtained at terminal 20. 
     The frequency of the voltage ripple is dependent on the frequency of the oscillator 12 and of the total count of counter 13. Thus if the frequency of oscillator 12 is 2.048 MHz, and the counter 13 is a 12 stage counter counting up to 2 12 , that is 4096, then the frequency of the ripple is (2.048×10 6 )/4096=0.5 kHz. The time constants of the smoothing circuit 19 must be such as to effectively remove this ripple from the output and this means that when the number stored in register 11 changes the voltage obtained at terminal 20 will be relatively slow to change, for example, in some embodiments taking several seconds before it attains the new value. 
     In order to overcome this disadvantage, instead of the single stage of chopping performed by switch 16, two stages are used as shown in the arrangement of FIG. 3. In FIG. 3 parts which are the same as those shown in FIG. 1 have been given the same reference numerals. In place of the register 11 storing 12 bits of information, two registers 21 and 31 are provided each storing 6 bits of information. It will be appreciated of course that the same 12 bit register may be used but for the purpose of this description it is to be regarded as consisting of two parts which are accordingly designated separately. When a transmitter is to be selected, the first six bits of the information from memory 10 are applied to register 21 and the remaining six bits to register 31. In a similar manner counter 13 is divided into two parts 23 and 33 each comprising a 6 bit counter. Comparator 14 is divided into two parts 24 and 34 the first part 24 being used to establish coincidence between the number stored in register 21 and counter 23, and the second part 34 being used to determine coincidence between the numbers stored in register 31 and counter 33. Bistable 25 is arranged to be turned to its &#34;on&#34; condition by coincidence established by comparator 24 and reset when counter 23 has attained its maximum count. The bistable 25 is arranged to operate a switch means 26, 26&#39; which is arranged to select alternately the output from the 30 volt line 17 and the 0 volt line 18 as in FIG. 1. However a switch means 27, 27&#39; is also connected to bistable 25 and the lines 17 and 18, but means are provided for inserting a delay to extend the pulse width of the &#34;off&#34; signal from 25 by one oscillation period of oscillator 12. 
     The means for extending the &#34;off&#34; signal could include a disable gate (not shown) in switch 27 which responds to a signal &#34;F-1&#34; applied from counter 23 during the count just prior to its maximum count. In response to the F-1 signal, switch 27 is gated off for a portion of the pulse from bistable 25 which equals one oscillation period. For example, the disable gate could be at the input of switch 27 in series with the signal from bistable circuit 25 and implemented so as to pass said signal except for one oscillation period following the F-1 signal. Alternately the means for extending the &#34;off&#34; signal could be implemented by having switch 27 controlled by a separate bistable circuit (not shown) instead of bistable circuit 25. This additional bistable circuit would be set by the coincidence signal from comparator 24 and reset by the &#34;F-1&#34; signal from counter 23. In yet another example of means for extending the &#34;off&#34; signal, switch 27 would be controlled by a separate bistable circuit (not shown) instead of bistable circuit 25 and this additional bistable circuit would be set by the coincidence signal from comparator 24 after said signal has passed through a one stage digital shift register (not shown) and said bistable circuit would be reset by the &#34;maximum count&#34; signal from counter 23. The shift register would be clocked by the signals from oscillator 12 and hence provide a one oscillation period extension to the &#34;off&#34; signal controlling switch means 27. 
     Resistor capacitor type filters (sometimes hereinafter referred to as smoothing circuits) 41 and 43 in the lines 37 and 38, respectively, smooth the output signals from switches 26&#39; and 27&#39; before these signals are applied to a switch 36&#39;. Accordingly the output signals from filters 41 and 43 are representative of the number specified by the first six digits of the number stored in register 21, and that number increased by one unit, respectively. The required voltage accordingly will lie between those provided on the lines 37 and 38. The fine selection of the required voltage is accomplished by a further switch means 36, 36&#39; operated by a bistable 35 connected to comparator 34 and counter 33. This switch is operated in exactly the same fashion as the switch 16, 16&#39; of FIG. 1, but selects the voltages on lines 37 and 38 with a relative duration determined by the second six bits of the information. The resulting output from switch 36&#39; is passed through a smoothing circuit 19 to the output terminal 20. 
     If it is assumed that the oscillator frequency from oscillator 12 is, as before, 2.048 MHz, and the counter 23 can count up to 2 6 , that is 64, then the frequency of the ripple present in the outputs from switches 26&#39; and 27&#39; will be (2.048×10 6 ) /64=32 kHz. It will be seen that this frequency is 64 times that obtained with the FIG. 1 arrangement, and accordingly the time constant in the smoothing circuit 19 may be appropriately reduced and the speed of response of the circuit correspondingly increased. 
     It will be appreciated that many modifications are possible in the arrangement shown. Thus the register 21, 31 has been shown as a 12-bit register divided into two parts. Other numbers may be selected as desired. It will also be appreciated that while a two stage chopping arrangement has been shown, with the registers divided into two parts, it would be possible to divide the registers into a different number of parts and to increase the number of stages of chopping. Thus with the registers divided into three parts then three stages of chopping could be used instead of the two stages shown. Further it will be understood that although for clarity of explanation switches 26, 26&#39;; 27, 27&#39; and 36, 36&#39; are shown as including mechanically switched members, that preferably electrical gating arrangements, e.g. implemented by solid state circuits, would be used.