Abstract:
A sound synthesizing circuit has means for synthesizing sound digital data in a pitch period of a sound to be synthesized and means for producing a sound analog signal according to the synthesized sound digital data. The synthesized sound digital data are sequentially transferred to the producing means according to a sequential sampling pulse. The sequential sampling pulse has a predetermined interval and a corrected interval different from the predetermined interval in one pitch period of a sound to be synthesized. Thus, a sound of good quality with an arbitrary interval can be synthesized in a wide scale by a simple hardware integrated circuit in a small semiconductor chip.

Description:
This is a continuation, of application Ser. No. 671,353, filed Nov. 14, 1984 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sound synthesizing circuit, and particularly to a circuit for synthesizing a musical sound, for example the sound of a piano, a violin, a flute, a whistle, or the like. 
     In general, conventional musical sound synthesizers are very expensive because of the need for a large number of sound oscillators. However, by employing a semiconductor techniques, a low cost digital synthesizer has been provided in recent years. This prior art digital synthesizer uses a sound waveform synthesizing method which is used in a conventional speech synthesizer. In this synthesizer a waveform of a musical sound to be synthesized is sampled a plurality of times along a time axis, and the amplitude level at each sampling point is digitized according to a predetermined bit number. Thus, waveform data is stored in a memory and is read out of the memory when the musical sound is to be synthesized. In a musical sound waveform, the same waveform is repeated for all pitch periods. Therefore, a representative waveform in only one pitch period usually is used repeatedly to synthesize a desired musical sound. Thus, since memory capacity can be reduced, an integrated circuit on a semiconductor chip is realizable. 
     The waveform data stored in the memory are sequentially read out of the memory and are synthesized in a synthesizing circuit. The synthesized digital data is sequentially sent to a digital-to-analog converter and is converted to an analog signal in response to a sampling pulse. Thereafter, the analog signal is provided to a speaker. 
     However, the digital synthesizer of the prior art synthesizes a musical sound according to a sampling pulse which is generated at a predetermined fixed interval. That is, the digital to analog conversion is operated at the fixed same interval as the sampling pulse which is generated at an equal time interval. The sampling pulse is produced by dividing a clock signal generated by a clock generator. 
     On the other hand, the pitch period must be changed in order to synthesize a different tone from the fundamental tone stored in the memory. In other words, a frequency of the representative waveform has to be changed. The change of the pitch period can be performed by changing the interval of the sampling pulse. As described above, the sampling pulse is produced by a frequency divider which divides the clock signal by a dividing ratio corresponding to a tone to be synthesized. 
     However, in the case where a waveform in one pitch is divided into 32 sampling points, the following equations are to be satisfied: 
     
         Ts=N×T.sub.1                                         ( 1) 
    
     
         Tp=32×Ts=32×N×T.sub.1                    (2) 
    
     Ts: a period of a sampling pulse 
     N: a dividing ratio 
     T 1  : a period of a clock signal 
     Tp: a pitch period of the waveform to be synthesized 
     As clear in the above equations (1) and (2), the pitch period Tp of the music sound waveform to be synthesized is changed by only the unit of 32×T 1 , if the dividing ratio N is changed by 1. Consequently, the number of tones which can be synthesized by the prior art synthesizer employing a waveform synthesis method has been limited to a small number. Further, when a desired tone frequency (pitch frequency) does not correspond to the integer magnifications of 32×T 1 , a frequency lag will be occurred. This lag is a maximum 16×T 1  in one pitch period and is very offensive to the ear. 
     In order to reduce the frequency lag, the number of divisions of a waveform may be decreased. However, the decrease of the division number introduces inaccuracies into a musical sound with a complex waveform, such as a piano, a trumpet, etc. For these sounds a division number higher than a predetermined number is required. 
     Therefore, the prior art sound synthesizer has used a high clock frequency. However, an operating speed of an integrated circuit on a semiconductor chip must be increased according to the high clock frequency utilized. As a result, circuit design and also fabrication become very complex, and chip size becomes large. Moreover, in a semiconductor circuit having a complementary type transistor pair (CMOS) power consumption is in proportion to the square of the operating frequency (clock frequency), so that increasing clock frequency is unsuitable for the musical sound synthesizer including an integrated circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a musical sound synthesizing circuit which can synthesize a musical sound wherein frequency lag is reduced. 
     Another object of the present invention is to provide a sound synthesizing circuit which can be integrated on a small semiconductor chip. 
     A further object of the present invention is to provide a sound synthesizer which can synthesize a complex sound without increasing a clock frequency rate. 
     Still another object of the present invention is to provide a sound synthesizing circuit employing a waveform synthesis method by which a musical sound of good quality can be synthesized. 
     A musical sound synthesizing circuit according to the present invention comprises a synthesizing circuit having a memory wherein sampling data of a waveform in a fundamental pitch period are stored and a synthesizing means synthesizing a musical sound waveform according to the sampling data stored in the memory, a clock generator generating a clock signal to control an operation of the synthesizing circuit, a sampling pulse generating circuit having a frequency divider generating a sampling pulse according to the clock signal, a digital to analog converter receiving the musical sound waveform data from the synthesizing means and producing a musical sound signal according to the sampling pulse, and a correcting circuit correcting a period of at least one selected sampling pulse among a plurality of sampling pulses generated during a predetermined pitch period corresponding to a pitch frequency of a musical sound to be synthesized. 
     According to the present invention, the musical sound synthesizing circuit has the correcting circuit which corrects a frequency of at least one sampling pulse at a selected sampling point. Namely, this correcting circuit is provided to change an interval between a selected sampling pulse and the next sampling pulse following the selected sampling pulse from that of the remaining sampling pulses in a desired pitch period of a musical sound to be synthesized. As a result, according to the present invention, at least two sampling pulses which have a different frequency from each other are involved in a pitch period of a musical sound to be synthesized. Therefore, in comparison with the prior art synthesizing circuit wherein a sampling pulse in a pitch period is fixed at the same frequency, the synthesizing circuit of the present invention has advantages as follows: 
     (1) Frequency lag between a musical sound to be desired and a synthesized musical sound can be remarkably reduced because of correcting a pitch period of a musical sound to be synthesized. 
     (2) A musical sound of good quality can be synthesized without increasing a clock frequency rate. 
     (3) A musical sound synthesizing circuit can be integrated in a small semiconductor chip. 
     (4) A musical sound with a wide scale and many intervals can be synthesized by a simple hardware circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a block diagram of the prior art synthesizing circuit employing a waveform synthesis method; 
     FIG. 2 shows a block diagram of an embodiment of the present invention indicating a musical sound synthesizing circuit employing a waveform synthesis method; 
     FIG. 3 shows a timing chart indicating an operation of the synthesizing circuit shown in FIG. 2; 
     FIG. 4 shows a block diagram of a principle part (i.e. a sampling pulse generating section) of FIG. 2; 
     FIG. 5 shows a timing chart indicating an operation of the sampling pulse generating section in FIG. 4; and 
     FIG. 6 shows a block diagram of another embodiment according to the present invention. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     A block diagram of FIG. 1 shows the prior art synthesizing circuit employing a waveform synthesis method, in which a clock generator 1, a frequency dividing circuit 2, a waveform synthesizing circuit 3, a digital to analog converter 4 and a dividing ratio setting circuit 6 are integrated in a single semiconductor chip. The clock generator 1 generates a clock signal and applies the clock signal to the other circuits 2, 3, 4 and 6 to control operations of these circuits. The frequency dividing circuit 2 divides the clock signal into a sampling pulse ts with a constant frequency (the same interval) according to the dividing ratio applied from the setting circuit 6. The setting circuit 6 prepares a dividing ratio corresponding to a pitch frequency of a musical sound to be synthesized and applies it to the dividing circuit 2 in every pitch period. The waveform synthesizing circuit 3 sequentially reads digitized waveform data out of the memory according to the address information designating a group of a musical sound to be synthesized and transfers the read out waveform data (e.g. 8-bit, 10-bit, or the like per one address) to the digital to analog converter 4 in response to the sampling pulse ts. The sampling pulse ts is used to synchronize transferring timing of the waveform data with the operation timing of the D/A converter 4. Namely, the waveform data indicating an amplitude level of the musical sound to be synthesized at each sampling point is transferred to the D/A converter 4 in synchronism with a period of the sampling pulse produced by the dividing circuit 2 according to the pitch period of the musical sound to be synthesized. Thus, a musical sound signal with the required pitch frequency is applied to a speaker 5 through the D/A converter 4. The synthesizing circuit 3 may modify the read out waveform data to change an amplitude level of the musical sound if required. 
     Now, when a musical sound with 440 Hz pitch frequency (2.27 ms pitch period) is required, a pitch frequency of a musical sound capable of synthesizing in the synthesizing circuit of FIG. 1 is 426 Hz according to the abovementioned equations (1) and (2). 
     Because, ##EQU1## 
     In this case, it is assumed that a waveform is divided into 32 sampling points and that 150 kHz is used as the clock frequency. 
     Therefore, a pitch period error of 3.2% occurs between the required musical sound and the synthesized musical sound. This is because the sampling pulse rate depends on only the dividing ratio N. It may be easily understood that the error can be corrected by increasing in the clock frequency rate or decreasing in the number of sampling points. However, these operations have shortcomings as described before. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows a block diagram of an embodiment according to the present invention, in which a different section from FIG. 1 is a frequency dividing section and a correcting circuit 14 is newly added. The frequency dividing section comprises a register 11 and a counter 12. The other circuits, that is, the clock generator 1, the waveform synthesizing circuit 3, the digital to analog converter 4 and a dividing ratio setting circuit 6 and their operations are the same as FIG. 1. Therefore, explanation of these circuits and operations will be omitted. 
     In FIG. 2, the register 11 receives a dividing ratio information indicating a pitch period of a musical sound to be synthesized from the setting circuit 6 and supplies it to the counter 12 at every pitch period. A down-counter is used as the counter 12, but an up-counter may be used. At the beginning of a synthesis operation, a dividing ratio information N prepared in the setting circuit 6 is preset into the down-counter 12 from the register 11 and is sequentially decreased by 1 in response to the clock signal. Namely, when n clock pulses have been applied to the downcounter 12, the content of the down-counter 12 becomes zero, thus the first sampling pulse ts 1  is generated. The downcounter 12 receives the dividing ratio information N again in response to the first sampling pulse and down counts the information N. Upon termination of this down counting operation, the second sampling pulse ts 2  is generated. By repeating these operations, the third sampling pulse, the fourth sampling pulse, . . . , the N-th sampling pulse, . . . are sequentially generated and applied to the synthesizing circuit 3 to control an output timing of the synthesizing circuit 3. 
     In this case, intervals between each sampling pulses are the same as each other, so that a musical sound with a pitch period (= the number of sampling points x the dividing ratio N× the period of the clock signal) can be synthesized. 
     On the other hand, when the correcting circuit 14 is activated, this circuit 14 sends a control signal 13 to the down-counter 12 at a predetermined timing. The downcounter 12 generates a sampling signal having a period determined by dividing the clock frequency into N+1 whenever the control signal 13 is applied to the counter 12. This operation can be performed by stopping the counting operation during one-downcount according to the control signal 13, so that the downcounter generates the sampling pulse divided into N+1. 
     According to the embodiment, a pitch period of a musical sound can be changed by the span of the period of the clock signal. If the control signal 13 is generated i times (1&lt;i&lt;K-1 when the number of sampling points is K), the pitch period Tp of a musical sound to be synthesized is as follows: ##EQU2## 
     In the synthesizing circuit according to this embodiment, the error of a pitch period can be reduced to 1/K. 
     In FIG. 3, an example of a sine waveform synthesis is shown. In this example, it is assumed that k is 8, i is 4 and that the control signal 13 is generated at a period of an odd number, so that the pitch period Tp of the sine wave, that is Tp=4×N×T1+4×(N+1)×T1=8×N×T1+4×T1=8×(N+1/2)×T1, can be obtained. Namely, this pitch period is in the middle of the pitch periods obtained when the dividing ratio N and the dividing ratio N+1 are set in the down-counter respectively, in the prior art of FIG. 1. 
     A preferred block diagram of the frequency dividing section and the correcting section according to the embodiment of FIG. 2 is shown in FIG. 4 in which an 8-bit register 100 and an 8-bit down-counter 101 are used as the register 11 and the down-counter 12, respectively. The 8-bit register 100 receives a dividing ratio information consisting of 8 bits. Each stage of the 8-bit down counter 101 has an exclusive OR gate 101-1 and a flip-flop circuit (101-2, 101-3 and 101-4) shown in block A. An input end T receives an output of AND gate corresponding to the respective stage. The AND gate receives two signals, one is an output Q of the preceding stage, the other is an output of the preceding AND gate. However, the first stage receives an output of an NOR gate 102 at an input end T. An output Q of th first stage and outputs Q of the second through eighth stages are applied to an NAND gate 103 in common. An output of the NAND gate 103 is sent to a one-bit delay circuit 105 via an inverter 104. An output of the delay circuit 105 is used as the sampling pulse ts and is transferred to a group of AND gates inserted betweeen the 8-bit register 100 and the 8-bit down counter 101 through an OR gate 106 which receives a RESET signal at the other input end. The NOR gate 102 receives three input signals, one is a HALT signal, another is a control signal 13, and the other is the sampling pulse ts. The control signal 13 is generated by a correcting circuit 107 having a programmable counter 107-1, a one-bit delay circuit 107-2 and an AND gate 107-3. A data designating an output timing of the control signal 13 is set in the programmable counter 107-1 and is counted down in response to the sampling pulse ts. 
     An operation of FIG. 4 will be described referring to FIG. 5. Two phase clock signals φ1 and φ2 are generated by the clock generator. A dividing ratio information N is preset into the down-counter 101 and is counted down in response to clock signals φ1 and φ2 as shown in FIG. 5. When the content of the down-counter 101 becomes 1, that is, 00000001, the output of the NAND gate becomes a low level. Therefore, a high level signal is represented at an output of the invertor 104 at a timing t1. The sampling pulse ts is generated after one-bit delay time at a timing t2. Now, if a counting operation of the programmable counter 107-1 is terminated at the timing t2, a high level signal is applied to the AND gate 107-3. This high level signal is sent to the NOR gate 102 at a timing t3 after one bit delay operation and is used as a control signal 13. Thus, an output of the NOR gate 102 becomes a low level, so that the AND gates coupled to input ends T generate a low level signal. As the result, the counting operation of the down-counter 101 is stopped during one bit time at the timing t3. Therefore, the counter 101 counts N+1 at this condition. 
     Besides stopping the counting operation, a dividing ratio information to be set in the register 11 or 100 may be changed as shown in FIG. 6. In this case, an arithmetic circuit 21 (e.g. an adder) is required. A frequency dividing circuit 2 can be used the same as that used in the prior art. To the dividing circuit 2, a dividing ratio N is set when the control signal 13 is not generated, while a dividing ratio N+1 is set when the control signal 13 is generated from the correcting circuit 14 because of adding the dividing ratio N to 1 which is transferred from the correcting circuit 14.