I've already posted about this and phi.lho was kind enough to impart some of his knowledge in answering some of my questions. One of the unanswered problems though was that of my drum samples being triggered multiple times in quick succession, making them sound distorted. I think I now understand why this is happening, but still cant come up with a solution. Here's my code so far:

2. Grid [][] grid;
3. import controlP5.*;
4. import ddf.minim.*;
5. ControlP5 gui;
6. Minim minim;
7. AudioSample kick, snare, hihat, crash;
8. Slider bpmSlider;
9. 
   
10. int columns = 20, rows = 9;
11. float speed;//total time across grid
12. float bpm;
13. float initial;
14. float current;
15. float next;
16. float gridd;
17. int step;//column along grid
18. static int clock;
19. 
    
20. boolean on = false;
21. 
    
22. void setup(){
23.   size(735, 350); 
24.   background(0);
25.   gui = new ControlP5(this);
26.   bpmSlider = gui.addSlider("bpm", 80, 200, 228, 265, 160, 15);
27.    
28.   minim = new Minim(this);
29.   kick = minim.loadSample("BD.mp3", 128);
30.   snare = minim.loadSample("SN.wav", 128);
31.   hihat = minim.loadSample("HH.wav", 128);
32.   crash = minim.loadSample("CR.wav", 128);
33.    
34.   fill(255);
35.   text("hihat", 55, 115);
36.   fill(255);
37.   text("kick", 55, 140);
38.   fill(255);
39.   text("snare", 55, 165);
40.   fill(255);
41.   text("crash", 55, 190);
42. 
    
43.  grid = new Grid[columns][rows];
44.   for (int c = 4; c < columns; c++) {
45.     for (int r = 4; r < rows; r++) {
46.       grid[c][r] = new Grid(c * 25, r * 25, 20, 20);
47.     }
48.   }
49. }
50. 
    
51. void draw() {
52. speed =  (((60 / bpm) * 1000) * 4);//60 seconds divided by bpm times by a thousand (to make it milli seconds) times by 4 for the 4 beats on the grid
53.   for (int c = 4; c < columns; c++) {
54.     for (int r = 4; r < rows; r++) {
55.       if((c == 4) || (c == 8) || (c == 12) || (c == 16)){
56.         grid[c][r].display(210);//darker shade to signify beat
57.      }else{       
58.         grid[c][r].display(255);
59.       }
60.     }
61.     
62.    if(on == true){
63.    int r;
64.    clock = millis();
65.    current = clock - initial;
66.    next = speed / current;
67.    println(next);
68.    gridd = (4 + (16 / next));
69.         
70.     if(current < speed){ 
71.      step = floor(gridd);
72.       
73.       for(r = 4; r < rows; r++){
74.       grid[step][r].display(0);
75.       grid[step][r].trigger(step, r);
76. 
    
77.     }
78.     }else{
79.       initial = clock;
80.     } 
81.    }
82.   }
83. }
84. 
    
85. void mousePressed() {
86.    if(mouseX > 100 && mouseX < 500 && mouseY > 100 && mouseY < 225){// 'if mouse is clicked within grid'
87.     int colBox = int(mouseX)/25; //uses mouse coordinates to determine grid number
88.     int rowBox = int(mouseY)/25;   
89.    grid[colBox][rowBox].checked();
90.    }
91. }
92. 
    
93. void keyPressed(){ 
94.   switch(key){
95.   case 'm':
96.     initial = millis();
97.     if(on == true){
98.     on = false;
99.     }else{
100.     on = true;}
101.     break;
102.   }
103. }
104. 
     
105. void stop(){
106.   kick.close();
107.   hihat.close();
108.   snare.close();
109.   crash.close();
110.   minim.stop();
111.   super.stop();
112. }
113. 
     
114. class Grid {
115.   float x, y, wd, ht;
116.   boolean active = false;
117.   
118.   
119. Grid(float X, float Y, float WD, float HT) {
120.     x = X;
121.     y = Y;
122.     wd = WD;
123.     ht = HT;
124.  }
125.  
126. void display(int col) {
127.     stroke (255);
128.     if(active ==  true){
129.        fill(0);
130.     }else{
131.        fill(col);
132.     }
133.     rect(x, y, wd, ht);
134.   }
135.   
136. void checked(){
137.    if(active == true){
138.      active = false;
139.   }else{
140.      active = true;
141.   } 
142. }
143. 
     
144. void trigger(float x, float y){
145.   if (active == true){
146.     if (y == 4){
147.       hihat.trigger();
148.     }
149.     if (y == 5){
150.       kick.trigger();
151.     }
152.     if (y == 6){
153.       snare.trigger();
154.     }
155.     if (y == 7){
156.       crash.trigger();
157.     }
158.   }
159.  }
160. }

I think the reason the samples are being triggered in really quick succession, rather than just once, is 'active == true' (line 145 of the code) is true for multiple frames before moving onto the next column. For example, if the BPM is 120 then each checked block will be activated (active == true) for 125ms before moving on to the next column. I'm assuming that in that 125ms, every time the frame resets, it will trigger the sample - hence the quick succession of samples. Obviously what I want is for the sample to only trigger once. Is there a way of doing this?

Any help or suggestions on this would be greatly appreciated!

Thanks for your time