一、任务解析：

         以前用51单片机制作了一个可演奏的电子琴，学习了EDA技术之后，感觉FPGA与单片机有许多类似的地方。于是，启发我用FPGA做一个可随意演奏的音乐播放器。设计低、中、高21个音，几乎能够实现一般歌曲的演奏。要实现该功能，就必须考虑到一首歌曲的产生应该考虑的两个因素：音符的产生频率和音的节拍（即声音持续的时间）。只要控制输出到扬声器的激励信号频率的高低和持续的时间，就可以使扬声器发声产生优美的歌曲。

二、方案论证：

1、音调的控制：

         频率的高低决定了音调的高低。通过查阅资料，得到下面的音符名与频率的关系表：

分频比预置数的计算：

         分频比就是从6Mhz基准频率通过二分频得到的3Mhz频率基础上计算得到的。对于乐曲中的休止符，只需将其分频系数设为0，将分频预置数设为16383即可。例如：低音3的频率为330hz，分频比为3M/330hz=3000000/330=9091,则其分频预置数为：16383-9091=7292。其他的音符对应的分频比和分频比预置数均按此法计算可得到。依次计算出低、中、高3X7=21个音的预置数。

各音阶对应的预置数如下表：

2、音长的控制：

         音符的持续时间必须根据乐曲的速度及每个音符的节拍数来确定。本试验中，设定最短的音符为4分音符，若将全音符的持续时间设为1s的话，则需要提供一个4Hz的时钟频率即可产生4分音符的时间，每1/4个节拍的时间为0.25s，一个完整的节拍为1s。

以上即为音乐播放器的原理框图。控制音调通过设置计数器的预置数来实现，预置不同的数可以使计数器产生不同频率的信号，从而产生不同的音调。控制音长则是通过控制计数器预置数的停留时间来实现的，预置数停留的时间越长，则该音符演奏的时间越长。每个音符的演奏时间都是0.25s的整数倍，对于节拍较长的音符，例如2分音符，占了2/4个节拍，在记谱的时候将其连续记录2次即可。

三、实验步骤：

1、分频产生6Mz的基准频率：

module div_clk6mhz(clk24m,clk6m);

input clk24m;

output clk6m;

reg clk6m;

reg cnt;

always @(posedge clk24m)

         if(cnt<1)cnt=cnt+1;            //（24m/6m=4分频,cnt<[4/2-1=1]即可）

         else begin cnt=0;clk6m=!clk6m;end

endmodule

由于此分频电路简单，就不在这里给出仿真波形了。

2、分频产生4hz的节拍控制频率：

module div_clk4hz(clk24m,clk4);

input clk24m;

output clk4;

reg clk4;

reg [21:0]cnt;

always @(posedge clk24m)

         if(cnt<2999999)cnt=cnt+1;//(24m/4hz=6000000,cnt<[6000000/2-1=2999999]）

         else begin cnt=0;clk4=!clk4;end

endmodule

此分频电路也很简单，就不在这里给出仿真波形了。

3、乐曲产生模块：

         核心模块就在这里，这里：（1）首先罗列了低、中、高21个音的分频比预置数（还有休止符的预置数16383），通过6Mhz的基准频率，在其上升沿的时候，计数器累加，当计满到16383之后就取反speaker(speaker<=~speaker非阻塞赋值)，产生对应音符的频率，这里和单片机的定时计数器溢出中断差不多。

（2）产生了21个音调之后，我们要产生流畅的歌曲，还必须根据歌曲的谱子，调用相应音符并且根据节拍控制其持续的时间，例如：若一个音持续4个1/4节拍，则在clk_4hz的上升沿，连续调用4次它的音符就可以了。用counter来计数，计满了（何时计满取决于歌曲长度）就将counter返回为0,这样就可以实现循环演奏了。代码如下：

module song(clk_6mhz,clk_4hz,speaker);

input clk_6mhz,clk_4hz;

output speaker;

reg [3:0]high,med,low;

reg [13:0]divider,origin;

reg [7:0]counter;

reg speaker;

wire carry;

assign carry=(divider==16383);

always @(posedge clk_6mhz)

         begin if(carry)divider<=origin;

                else divider<=divider+1;

         end

always @(posedge carry)

         begin speaker<=~speaker;end

always @(posedge clk_4hz)

         begin

              case({high,med,low})

              'b000000000001:origin<=4933;

              'b000000000010:origin<=6179;

              'b000000000011:origin<=7292;

              'b000000000100:origin<=7787;

              'b000000000101:origin<=8730;

              'b000000000110:origin<=9565;

              'b000000000111:origin<=10310;

              'b000000010000:origin<=10647;

              'b000000100000:origin<=11272;

              'b000000110000:origin<=11831;

              'b000001000000:origin<=12085;

              'b000001010000:origin<=12556;

              'b000001100000:origin<=12974;

              'b000001110000:origin<=13347;

              'b000100000000:origin<=13515;

              'b001000000000:origin<=13830;

              'b001100000000:origin<=14107;

              'b010000000000:origin<=14236;

              'b010100000000:origin<=14470;

              'b011000000000:origin<=14678;

              'b011100000000:origin<=14858;

              'b000000000000:origin<=16383;

              endcase

         end

always @(posedge clk_4hz)

         begin

              if(counter==113)counter<=0;

              else counter=counter+1;

              case(counter)

              0: {high,med,low}<='b000000000101;

              1: {high,med,low}<='b000000000101;//

              2: {high,med,low}<='b000000000110;

              3: {high,med,low}<='b000000000110;//

              4: {high,med,low}<='b000000000101;

              5: {high,med,low}<='b000000000101;//

              6: {high,med,low}<='b000100000000;

              7: {high,med,low}<='b000100000000;//

              8: {high,med,low}<='b000001110000;

              9: {high,med,low}<='b000001110000;

              10:{high,med,low}<='b000001110000;

              11:{high,med,low}<='b000001110000;//

              12:{high,med,low}<='b000001010000;

              13:{high,med,low}<='b000001010000;//

              14:{high,med,low}<='b000001100000;

              15:{high,med,low}<='b000001100000;//

              16:{high,med,low}<='b000001010000;

              17:{high,med,low}<='b000001010000;//

              18:{high,med,low}<='b001000000000;

              19:{high,med,low}<='b001000000000;//

              20:{high,med,low}<='b000100000000;

              21:{high,med,low}<='b000100000000;

              22:{high,med,low}<='b000100000000;

              23:{high,med,low}<='b000100000000;//

              24:{high,med,low}<='b000001010000;

               25:{high,med,low}<='b000001010000;//

              26:{high,med,low}<='b010100000000;

              27:{high,med,low}<='b010100000000;//

              28:{high,med,low}<='b001100000000;

              29:{high,med,low}<='b001100000000;//

              30:{high,med,low}<='b000100000000;

              31:{high,med,low}<='b000100000000;//

              32:{high,med,low}<='b000001110000;

              33:{high,med,low}<='b000001110000;//

              34:{high,med,low}<='b011000000000;

              35:{high,med,low}<='b011000000000;//

              36:{high,med,low}<='b010000000000;

              37:{high,med,low}<='b010000000000;//

              38:{high,med,low}<='b001100000000;

              39:{high,med,low}<='b001100000000;//

              40:{high,med,low}<='b000100000000;

              41:{high,med,low}<='b000100000000;//

              42:{high,med,low}<='b001000000000;

              43:{high,med,low}<='b001000000000;//

              44:{high,med,low}<='b000100000000;

              45:{high,med,low}<='b000100000000;

              46:{high,med,low}<='b000100000000;

              47:{high,med,low}<='b000100000000;

              48:{high,med,low}<='b000100000000;

              49:{high,med,low}<='b000100000000;///////祝你生日快乐，完毕。

              50: {high,med,low}<='b000000000011;/////////梁祝，开始

              51: {high,med,low}<='b000000000011;

              52: {high,med,low}<='b000000000011;

              53: {high,med,low}<='b000000000011;

              54: {high,med,low}<='b000000000101;

              55: {high,med,low}<='b000000000101;

              56: {high,med,low}<='b000000000101;

              57: {high,med,low}<='b000000000110;

              58: {high,med,low}<='b000000010000;

              59: {high,med,low}<='b000000010000;

              60:{high,med,low}<='b000000010000;

              61:{high,med,low}<='b000000100000;

              62:{high,med,low}<='b000000000110;

              63:{high,med,low}<='b000000010000;

              64:{high,med,low}<='b000000000101;

              65:{high,med,low}<='b000000000101;

              66:{high,med,low}<='b000001010000;

              67:{high,med,low}<='b000001010000;

              68:{high,med,low}<='b000001010000;

              69:{high,med,low}<='b000100000000;

              70:{high,med,low}<='b000001100000;

              71:{high,med,low}<='b000001010000;

              72:{high,med,low}<='b000000110000;

              73:{high,med,low}<='b000001010000;

              74:{high,med,low}<='b000000100000;

              75:{high,med,low}<='b000000100000;

              76:{high,med,low}<='b000000100000;

              77:{high,med,low}<='b000000100000;

              78:{high,med,low}<='b000000100000;

              79:{high,med,low}<='b000000100000;

              80:{high,med,low}<='b000000100000;

              81:{high,med,low}<='b000000100000;

              82:{high,med,low}<='b000000100000;

              83:{high,med,low}<='b000000100000;

              84:{high,med,low}<='b000000100000;

              85:{high,med,low}<='b000000110000;

              86:{high,med,low}<='b000000000111;

              87:{high,med,low}<='b000000000111;

              88:{high,med,low}<='b000000000110;

              89:{high,med,low}<='b000000000110;

              90:{high,med,low}<='b000000000101;

              91:{high,med,low}<='b000000000101;

              92:{high,med,low}<='b000000000101;

              93:{high,med,low}<='b000000000110;

              94:{high,med,low}<='b000000010000;

              95:{high,med,low}<='b000000010000;

              96:{high,med,low}<='b000000100000;

              97:{high,med,low}<='b000000100000;

              98:{high,med,low}<='b000000000011;

              99:{high,med,low}<='b000000000011;

              100:{high,med,low}<='b000000010000;

              101:{high,med,low}<='b000000010000;

              102:{high,med,low}<='b000000000110;

              103:{high,med,low}<='b000000000101;

              104:{high,med,low}<='b000000000110;

              105:{high,med,low}<='b000000010000;

              106:{high,med,low}<='b000000000101;

              107:{high,med,low}<='b000000000101;

              108:{high,med,low}<='b000000000101;

              109:{high,med,low}<='b000000000101;

              110:{high,med,low}<='b000000000101;

              111:{high,med,low}<='b000000000101;

              112:{high,med,low}<='b000000000101;

              113:{high,med,low}<='b000000000101; 

              endcase

         end

endmodule

4、三个模块都完成之后，生成相应的电路原理图，并连接起来，组成一个完整的音乐播放器电路，如下：

5、完成以上步骤之后，分配引脚到试验箱上的芯片，连接好引脚到扬声器等，并且接好相应的短路帽。然后烧写程序到芯片中去，测试音乐播放器是否可以产生预期已经谱好了曲子的歌曲。经过硬件测试，能够产生《梁祝》和《祝你生日快乐》，在这里我只写了2首简单的歌曲。

四、实验分析

         1、所有不同频率的信号都是从同一个基准频率分频得到。由于音阶频率多为分整数，而分频比又不能为小数，故必须将计算得到的分频数四舍五入取整。如果基准频率过低，则如果分频比太小，四舍五入取整后的误差较大。若基准频率过高，虽然误差小，但分频数将会变大。设计时，综合考虑，在尽量减小频率误差的前提下取合适的基准频率。我在这里就选取了6Mhz的时钟频率。

         2、由于最大分频预置数为16383（休止符），2的14次方=16384>16383,故只需取14bit即可，14位的二进制计数器可满足要求。分频比都是从6Mhz频率二分频得到3Mhz频率基础上计算得到。

         3、如何分频得到想要的频率？总结了下，如果想由已知频率得到N分频的频率，计数器count<(N/2-1)为分界线，将要得到的频率在分界线处取反即可。

比如：上面要从24Mhz得到4hz的频率，N=24000000/4=6000000,则cnt<(6000000/2-1)即cnt<2999999即可。

五、实验总结;

         其实，音乐播放器用FPGA来实现是很简单的。核心部分就是抓住了乐曲的两个要点：音调的控制和音长的控制。因此，涉及到两个频率，一个用来控制产生相应的音调，一个是用来控制该音调持续的时间（即节拍）。我做的这个简易音乐播放器，硬件电路也很简单。其实，可以做的完善和人性化一点，因为声音不只21个音，还有升半调#和降调b，可以加些按键控制，选中某一首歌曲播放。或者还可以通过按键调整模式，可以有自己弹奏乐曲的模式，和音乐自动播放模式，两种模式进行切换。