Pulse rate modulation type piezoelectric crystal driver device
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a piezoelectric crystal driver device, and more particularly to a pulse rate modulation type method and driver device for driving a piezoelectric crystal. 
     2. Description of the Related Art 
     Piezoelectric crystals are widely used in microphones, loudspeakers, sound pick-up devices, buzzers, etc. The piezoelectric crystal expands or bends whenever a signal voltage is applied thereto. This deflection results in the generation of an appropriate sound output. 
     The conventional method used in driving piezoelectric crystals is as follows: A digitized sound signal input (such as speech, music, etc.) is converted into an appropriate analog signal via a digital-to-analog converter means. The analog signal is then applied onto the terminals of the piezoelectric crystal, causing the piezoelectric crystal to vibrate and thereby produce sound. 
     SUMMARY OF THE INVENTION 
     Therefore, the objective of the present invention is to provide a piezoelectric crystal driver device which uses a different method to drive the piezoelectric crystal. 
     More specifically, the objective of the present invention is to provide a pulse rate modulation type piezoelectric crystal driver device having a pulse output that is to be applied to a piezoelectric crystal. The pulse density per unit time of the pulse output varies according to the magnitude of a digitized sound signal input. If the magnitude of the digitized sound signal input is higher, the pulse density is correspondingly denser. A decrease in the magnitude of the digitized sound signal input correspondingly reduces the pulse density. 
     The difference between the conventional piezoelectric crystal driver device and the driver device of the present invention is as follows: Referring to FIG. 1A, a digitized sound signal input is shown to comprise four four-bit data bytes: 0000, 1010, 0011 and 1111. Referring to FIG. 1B, the output of the conventional driver device is a varying analog voltage (0 V, 10/15 V, 3/15 V, 1 V) which corresponds to the magnitude of the data bytes. Referring to FIG. 1C, the output of the driver device of the present invention is a pulse train, the pulse density per unit time of the pulse train being varied in accordance with the magnitude of the data bytes. There are 0 pulses per unit time if the data byte is 0000, (n*10) pulses per unit time if the data byte is 1010, (n*3) pulses per unit time if the data byte is 0011, and (n*15) pulses per unit time if the data byte is 1111. 
     Referring to FIG. 2, the principle of the preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device according to the present invention is as follows: A sound source (S) has a digitized sound signal output (such as speech, music, etc.). The positive values of the digitized sound signal output are received by a first pulse rate modulator (M1), while the negative values of the digitized sound signal output are received by a second pulse rate modulator (M2). The first and second pulse rate modulators (M1, M2) vary the pulse density per unit time of the clock pulse output of a pulse generator (P) according to the magnitude of the digitized sound signal input. The outputs of the first and second pulse rate modulators (M1, M2) are then applied to the terminals of a piezoelectric crystal (B) to permit the latter to undergo mechanical strain and thereby produce sound. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the/accompanying drawings, of which: 
     FIG. 1A shows four four-bit data bytes of a sample digitized sound signal input used to illustrate the difference between the driver device of the present invention and the prior art; 
     FIG. 1B illustrates that the output of the conventional driver device is a varying analog voltage which corresponds to the magnitude of the data bytes shown in FIG. 1A; 
     FIG. 1C shows that the output of the preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device according to the present invention is a pulse train, the pulse density per unit time of the pulse train being varied according to the magnitude of the data bytes shown in FIG. 1A; 
     FIG. 2 is schematic block diagram of the preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device according to the present invention; 
     FIG. 3 is a schematic electrical circuit diagram of the first preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device of the present invention; and 
     FIG. 4 is a schematic electrical circuit diagram of the second preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, the first preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device receives an 8-bit digitized sound signal input (d7-d0) and comprises a pulse train generator means (10), a multi-step frequency divider means (20), a mixing circuit means (30) and an output select circuit means (40). The most significant bit (d7) of the digitized sound signal input is a sign bit. The magnitude of the digitized sound signal input is indicated by the remaining bits (d6-d0). 
     The pulse train generator means (10) produces a clock pulse output having a clock frequency (f). The multi-step frequency divider means (20) includes an inverter (210) serving as a unity divider circuit, and six cascaded flip-flop means (211-216). The input of the first flip-flop means (211) is connected to the pulse train generator means (10). The multi-step frequency divider means (20) has seven pulse outputs (R0-R6), each of which corresponds to one of the magnitude bits (d6-d0) of the digitized sound signal input. The values of the pulse outputs (R1-R6) as a function of the clock frequency (f) are as follows: 
     R1=1/2 f Hz 
     R2=1/4 f Hz 
     R3=1/8 f Hz 
     R4=1/16 f Hz 
     R5=1/32 f Hz 
     R6=1/64 f Hz 
     The pulse output (R0) is equal to the clock frequency (f), but is 180° out of phase. The pulse outputs (R0-R6) of the multi-step frequency divider means (20) provide the different pulse signals required to accomplish the pulse rate modulation technique used by the present invention. 
     The mixing circuit means (30) includes positive and negative mixer circuits (30a, 30b). The positive mixer circuit (30a) receives the magnitude bits (d6-d0) of the digitized sound signal input. The logic states of the magnitude bits (d6-d0) are inverted before they are received by the negative mixer circuit (30b). The pulse outputs (R0-R6) are also received by both the positive and negative mixer circuits (30a, 30b). 
     Each of the positive and negative mixer circuits (30a, 30b) includes a first circuit stage (31) and a second circuit stage (32). The first circuit stage (31) comprises seven, two-input NAND logic means (310-316). Each of the NAND logic means (310-316) has one of the magnitude bits (d6-d0) and one of the pulse outputs (R0-R6) as inputs thereto. The second circuit stage (32) comprises a seven-input NAND logic means (321) which is connected to the outputs of the NAND logic means (310-316). The magnitude bits (d6-d0) control which of the pulse outputs (R0-R6) should be present at the input ports of the NAND logic means (321). The NAND logic means (321) superimposes the pulse outputs (R0-R6) present at its input ports, thereby generating a corresponding pulse arrangement for a particular value of magnitude bits (d6-d0). This illustrates how the preferred embodiment accomplishes pulse rate modulation. 
     The input select circuit means (40) includes first and second two-input AND logic means (41, 42). The first AND logic means (41) receives the output of the positive mixer circuit (30a) and the sign bit (d7) of the digitized sound signal input. The second AND logic means (42) receives the output of the negative mixer circuit (30b) and the inverted sign bit (d7) of the digitized sound signal input. 
     When the sign bit (d7) is a logic &#34;1&#34;, the first AND logic means (41) sends the output of the positive mixer circuit (30a) to the positive terminal (ol) of the piezoelectric crystal. The output of the second AND logic means (42) is alogic &#34;0&#34;. 
     When the sign bit (d7) is a logic &#34;0&#34;, the second AND logic means (42) sends the output of the negative mixer circuit (30b) to the negative terminal (o2) of the piezoelectric crystal. The output of the first AND logic means (41) is alogic &#34;0&#34;. The electric signals applied to the piezoelectric crystal permit the latter to undergo mechanical strain and thereby produce sound. 
     The value of the magnitude bits (d6-d0) determines the density and spacing of the driving pulse signal output of the driver device. The sign bit (d7) is used to indicate whether or not the polarity of the digitized sound signal input is positive or negative. Assuming that the digitized sound signal input is 00101101, the corresponding driving signal generated by the preferred embodiment is as follows: (0*1 f Hz)+(1*1/2 f Hz)+(0*1/4 f Hz)+(1*1/8 f Hz)+(1 * 1/16 f Hz)+(0*1/32 f Hz)+(1*1/64 f Hz)=45/64 f Hz applied to the negative terminal (o2) of the piezoelectric crystal. 
     The piezoelectric crystal requires a minimum operating frequency (fo Hz) in order to produce sound. When selecting the clock frequency (f) for the pulse train generator means (10), it is important to note that the smallest pulse output (R6), which is 1/64 f Hz, must be sufficient to drive the piezoelectric crystal so as to control the latter to generate sound. Thus, the clock frequency (f) must be greater than 64 fo Hz. The formula for finding the required clock frequency (f) for an N-bit digitized sound signal input is as follows: 
     
         f=2 exp (N-2) fo Hz 
    
     Referring to FIG. 4, the second preferred embodiment of a pulse rate modulation type piezoelectric crystal driver device is shown to be substantially similar to the first preferred embodiment and thus, its construction and operation will not be detailed herein. The main difference between the first and second preferred embodiments resides in the configuration of the mixing circuit means (30, 30&#39;). In the second preferred embodiment, the mixing circuit means (30&#39;) comprises seven exclusive NOR logic means (330-336). Each of the exclusive NOR logic means (330-336) has one of the magnitude bits (d6-d0) and the sign bit (d7) as inputs thereto. The outputs of the exclusive NOR logic means (330-336) are the inverse of the corresponding magnitude bits (d6-d0) when the sign bit (d7) is a logic &#34;0&#34;. The mixing circuit means (30&#39;) further comprises seven two-input NAND logic means (310&#39;-316&#39;). Each of the logic means (310&#39;-316&#39;) has the output of one of the exclusive NOR logic means (330-336) and one of the pulse outputs (R0-R6) as inputs thereto. As with the first preferred embodiment, a seven-input NAND logic means (321) is connected to the outputs of the logic means (310&#39;-316&#39;). The output of the mixer circuit means (30&#39;) is received by the AND logic means (41, 42) of the output select circuit means 40. 
     While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments, but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.