module ProLogicDecoder( ss, L, R, Left, Right, Center, Surround );
// input specifications
input ss;
input [5:0] L, R;
// output specifications
output [6:0] Left, Right, Center, Surround;
// declarations
wire [6:0] Left, Right, Center, Surround;
wire [6:0] LpR, LmR, mC, mS;
// assignments for Left and Right
//
// Left and Right are padded with a zero
// to make them the same size as Center
// and Surround
assign Left = {1'b0, L};
assign Right = {1'b0, R};
// calculate sum and difference between
// Left and Right.
//
// the subtraction must be handled specially
// to avoid clipping in the audio output,
// or other types of discontinuities
mapLpR adder( .L(Left), .R(Right), .out(LpR) );
mapLmR subtracter( .L(Left), .R(Right), .out(LmR) );
// calculate the Center and Surround outputs
// from the prior sum and difference.
//
// the root2encoder calculates the binary value
// that most closely represents dividing the
// input by the square root of 2
root2encoder Croot2Encoder( .in(LpR), .out(mC) );
root2encoder Sroot2Encoder( .in(LmR), .out(mS) );
// mux output depending on ss input flag
//
// Center and Surround should be 0 if ss is 0
mux2to1x7 Cmux( .in1(mC), .in0(7'b000_0000), .s(ss), .out(Center) );
mux2to1x7 Smux( .in1(mS), .in0(7'b000_0000), .s(ss), .out(Surround) );
endmodule
// multiplexer
module mux2to1x7( in1, in0, s, out );
input [6:0] in1, in0;
input s;
output [6:0] out;
wire [6:0] out;
assign out = s? in1 : in0;
endmodule
// special adder
module mapLpR( L, R, out );
input [6:0] L, R;
output [6:0] out;
wire [6:0] out;
assign out = L + R;
endmodule
// special subtractor
module mapLmR( L, R, out );
input [6:0] L, R;
output [6:0] out;
wire [6:0] sum, out;
assign sum = L + 63; // add 63 to avoid subtraction clipping
assign out = sum - R;
endmodule
// divide by square root of 2
module root2encoder( in, out );
input [6:0] in;
output [6:0] out;
reg [6:0] out;
always @ ( in )
case( in )
0 : out <= 0;
1 : out <= 1;
2 : out <= 1;
3 : out <= 2;
4 : out <= 3;
5 : out <= 4;
6 : out <= 4;
7 : out <= 5;
8 : out <= 6;
9 : out <= 6;
10 : out <= 7;
11 : out <= 8;
12 : out <= 8;
13 : out <= 9;
14 : out <= 10;
15 : out <= 11;
16 : out <= 11;
17 : out <= 12;
18 : out <= 13;
19 : out <= 13;
20 : out <= 14;
21 : out <= 15;
22 : out <= 16;
23 : out <= 16;
24 : out <= 17;
25 : out <= 18;
26 : out <= 18;
27 : out <= 19;
28 : out <= 20;
29 : out <= 21;
30 : out <= 21;
31 : out <= 22;
32 : out <= 23;
33 : out <= 23;
34 : out <= 24;
35 : out <= 25;
36 : out <= 25;
37 : out <= 26;
38 : out <= 27;
39 : out <= 28;
40 : out <= 28;
41 : out <= 29;
42 : out <= 30;
43 : out <= 30;
44 : out <= 31;
45 : out <= 32;
46 : out <= 33;
47 : out <= 33;
48 : out <= 34;
49 : out <= 35;
50 : out <= 35;
51 : out <= 36;
52 : out <= 37;
53 : out <= 37;
54 : out <= 38;
55 : out <= 39;
56 : out <= 40;
57 : out <= 40;
58 : out <= 41;
59 : out <= 42;
60 : out <= 42;
61 : out <= 43;
62 : out <= 44;
63 : out <= 45;
64 : out <= 45;
65 : out <= 46;
66 : out <= 47;
67 : out <= 47;
68 : out <= 48;
69 : out <= 49;
70 : out <= 49;
71 : out <= 50;
72 : out <= 51;
73 : out <= 52;
74 : out <= 52;
75 : out <= 53;
76 : out <= 54;
77 : out <= 54;
78 : out <= 55;
79 : out <= 56;
80 : out <= 57;
81 : out <= 57;
82 : out <= 58;
83 : out <= 59;
84 : out <= 59;
85 : out <= 60;
86 : out <= 61;
87 : out <= 62;
88 : out <= 62;
89 : out <= 63;
90 : out <= 64;
91 : out <= 64;
92 : out <= 65;
93 : out <= 66;
94 : out <= 66;
95 : out <= 67;
96 : out <= 68;
97 : out <= 69;
98 : out <= 69;
99 : out <= 70;
100 : out <= 71;
101 : out <= 71;
102 : out <= 72;
103 : out <= 73;
104 : out <= 74;
105 : out <= 74;
106 : out <= 75;
107 : out <= 76;
108 : out <= 76;
109 : out <= 77;
110 : out <= 78;
111 : out <= 78;
112 : out <= 79;
113 : out <= 80;
114 : out <= 81;
115 : out <= 81;
116 : out <= 82;
117 : out <= 83;
118 : out <= 83;
119 : out <= 84;
120 : out <= 85;
121 : out <= 86;
122 : out <= 86;
123 : out <= 87;
124 : out <= 88;
125 : out <= 88;
126 : out <= 89;
127 : out <= 90;
endcase
endmodule