1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
|
function z = ssbdemod(y, Fc, Fs, varargin)
// This function performs Single Side Band Amplitude Demodulation
//
// Calling Sequence
// Z = SSBDEMOD(Y,Fc,Fs)
// Z = SSBDEMOD(Y,Fc,Fs,INI_PHASE)
// Z = SSBDEMOD(Y,Fc,Fs,INI_PHASE,NUM,DEN)
//
// Description
// Z = SSBDEMOD(Y,Fc,Fs)
// demodulates the single sideband amplitude modulated signal Y
// with the carrier frequency Fc (Hz).
// Sample frequency Fs (Hz). zero initial phase (ini_phase).
// The modulated signal can be an upper or lower sideband signal.
// A lowpass butterworth filter is used in the demodulation.
//
// Z = SSBDEMOD(Y,Fc,Fs,INI_PHASE)
// adds an extra argument the initial phase (rad) of the modulated signal.
//
// Z = SSBDEMOD(Y,Fc,Fs,INI_PHASE,NUM,DEN)
// adds extra arguments about the filter specifications
// i.e., the numerator and denominator of the lowpass filter.
//
// Fs must satisfy Fs >2*(Fc + BW), where BW is the bandwidth of the
// modulating signal.
//
//
// Examples
//
// Fs =200;
// t = [0:2*Fs+1]'/Fs;
// ini_phase = 5;
// Fc = 20;
// fm1= 2;
// fm2= 3
// x =sin(2*fm1*%pi*t)+sin(2*fm2*%pi*t);
// y = ssbmod(x,Fc,Fs,ini_phase);
// o = ssbdemod(y,Fc,Fs,ini_phase);
// z = fft(y);
// zz =abs(z(1:length(z)/2+1 ));
// axis = (0:Fs/length(zz):Fs -(Fs/length(zz)))/2;
//
// figure
// subplot(3,1,1); plot(x);
// title(' Message signal');
// subplot(3,1,2); plot(y);
// title('Amplitude modulated signal');
// subplot(3,1,3); plot(axis,zz);
// title('Spectrum of amplitude modulated signal');
// z1 =fft(o);
// zz1 =abs(z1(1:length(z1)/2+1 ));
// axis = (0:Fs/length(zz1):Fs -(Fs/length(zz1)))/2;
// figure
// subplot(3,1,1); plot(y);
// title(' Modulated signal');
// subplot(3,1,2); plot(o);
// title('Demodulated signal');
// subplot(3,1,3); plot(axis,zz1);
// title('Spectrum of Demodulated signal');
//
// Authors
// Pola Lakshmi Priyanka, IIT Bombay
//*************************************************************************************************************************************//
// Check number of input arguments
[outa,inpa]=argn(0)
if(inpa > 6)
error("comm:ssbdemod:Too Many Input Arguments");
end
//funcprot(0) //to protect the function
//Check y,Fc, Fs.
if(~isreal(y)| ~or(type(y)==[1 5 8]) )
error("comm:ssbdemod: Y must be real");
end
if(~isreal(Fc) | ~isscalar(Fc) | Fc<=0 | ~or(type(Fc)==[1 5 8]) )
error("comm:ssbdemod:Fc must be Real, scalar, positive");
end
if(~isreal(Fs) | ~isscalar(Fs) | Fs<=0 | ~or(type(Fs)==[1 5 8]) )
error("comm:ssbdemod:Fs must be Real, scalar, positive");
end
// Check if Fs is greater than 2*Fc
if(Fs<=2*Fc)
error("comm:ssbdemod:Fs<2Fc:Nyquist criteria");
end
// Check initial phase
if(inpa<4 )
ini_phase = 0;
else
ini_phase = varargin(1);
end
if(~isreal(ini_phase) | ~isscalar(ini_phase)| ~or(type(ini_phase)==[1 5 8]) )
error("comm:ssbdemod:Initial phase shoould be Real");
end
// Filter specifications
if(inpa<5)
H = iir(5,'lp','butt',[Fc/Fs,0],[0,0]);
num = coeff(H(2));
den = coeff(H(3));
num = num(length(num):-1:1);
den = den(length(den):-1:1);
// Check that the numerator and denominator are valid, and come in a pair
elseif( (inpa == 5) )
error("comm:ssbdemod:NumDenPair: Filter error :Two arguments required");
// Check to make sure that both num and den have values
elseif( bitxor( isempty(varargin(1)), isempty(varargin(2))))
error(message('comm:ssbdemod:Filter specifications'));
elseif( isempty(varargin(1)) & isempty(varargin(2)) )
H = iir(7,'lp','butt',[Fc/Fs*2*%pi,0],[0,0]);
num = coeff(H(2));
den = coeff(H(3));
num = num(length(num):-1:1);
den = den(length(den):-1:1);
else
num = varargin(1);
den = varargin(2);
end
// check if Y is one dimensional
wid = size(y,1);
if(wid ==1)
y = y(:);
end
// Demodulation
t = (0 : 1/Fs :(size(y,1)-1)/Fs)';
t = t(:, ones(1, size(y, 2)));
z = y .* cos(2*%pi * Fc * t + ini_phase);
for i = 1 : size(z, 2)
z(:, i) = filter(num, den, z(:, i)) ;
z=z(length(z):-1:1)
z(:, i) = filter(num, den, z(:, i)) ;
z=z*-2;
end;
// restore the output signal to the original orientation
if(wid == 1)
z = z';
end
endfunction
// End of function
|
