Sine generation with variable frequency and amplitude

Hi everyone,

I have been trying to write a code that generates series of sine wave with changing frequency and amplitude; for example a 20 Hz sine wave for 20 seconds, after it reaches to 30 Hz for 10 seconds and so on.

When with constant ouput values, there is no problem. The code I wrote for it is below

k=zeros(1,100);
t=zeros(1,100);
t=0:99;

for i=1:100;
    if i<=30;
        k(i)=1;
    else if i<=60;
            k(i)=3;
        else i<=100;
            k(i)=2;
        end
    end
end

plot(t,k)

It gives correct results. Try this one and imagine I want to change these constant values with sine waves.

When I changed k(i) values with sine, like below;

k=zeros(1,100);
t=zeros(1,100);
t=0:99;
a=linspace(0,2*pi,30);

for i=1:90;
    if i<=30;
        k(i)=2*sin(a);
    else if i<=60;
            k(i)=sin(a);
        else i<=90;
            k(i)=3*sin(a);
        end
    end
end

plot(t,k)

It gives that error;

??? In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in ==> Untitled3 at 9 k(i)=2*sin(a);

The dimensions match, Can anyone give me a hand about the code above or show any other way?

Thanks,

Ozgur Palaz

Answer 1

[Ozgur]

I think i solve the problem. Here is the code if you are interested;

num=length(t);

k=zeros(1,num);

f1=0.8;
f2=1;
f3=0.5;
fs=100;
T=1/fs;

for i=1:num;
       if i<=num/5;
               k(i)=2*sin(2*pi*f1*t(i));

         else if i<=2*num/5;
                     k(i)=0;

             else if i<=3*num/5;           
                     k(i)=sin(2*pi*f2*t(i));

                 else if i<=4*num/5;
                     k(i)=0;

                     else   i<=num;
                     k(i)=3*sin(2*pi*f3*t(i));

                     end
                 end
             end
         end
  end

plot(t,k)

[Chaithra Kc]

I have tried this the code is working but, I'm not able to generate it in the scope. The waveform is as shown in the picture...please help

Answer 2

[Friedrich]

Hi,

You get this error because sin(a) is a vector of size 1x30 and you want assign this to a single field k(i). Maybe give k an other size

k=zeros(100,30)

and than do

k(i,:)=2*sin(a)

[Ozgur]

Thanks Friedrich, but unfortunately that didn't worked. Here is the plot of your suggestion;

<<http://imageshack.us/photo/my-images/543/resultsd.jpg/>>

[Friedrich]

Sorry my fault this would be correct:

k=zeros(1,100);

t=zeros(1,100);

t=0:99;

a=linspace(0,2*pi,30);

for i=1:90;

if i<=30;

k(i)=2*sin(a(i));

else if i<=60;

k(i)=sin(a(i-30));

else i<=90;

k(i)=3*sin(a(i-60));

end

end

end

but the code from andrei is much better.

[Ozgur]

hanks, it results like Andrei suggested. Now I'm tryin on setting the sine frequency in the form of Sin(2*pi*f*t)

Time vector t should have been put into the for loop for this I guess.

Do you have any suggestion about that? Check this link to get what i am trying to manage.

<http://imageshack.us/photo/my-images/20/signalq.jpg/>

Thanks again.

Ozgur

Answer 3

[Andrei Bobrov]

k= zeros(1,100);
k(1:90) = sin(linspace(0,2*pi,30))'*[2 1 3];

EDIT

k= zeros(30,100);
k(:,1:90) = reshape(permute(repmat(sin(linspace(0,2*pi,30))'*[2 1 3],[1,1,30]),[1 3 2]),30,[]);

answer on comment

1. '*[2 1 3]

eg.:

>> a = (1:3)'
a =
     1
     2
     3
>> b = [2 1 3]
b =
     2     1     3
>> a*b
ans =
     2     1     3
     4     2     6
     6     3     9

or

>> bsxfun(@times,a,b)
ans =
     2     1     3
     4     2     6
     6     3     9

2. Please specify question

[Ozgur]

Thank you for your reply, this creates a sine wave and the error is gone, can you explain more detailed about this '*[2 1 3] expression?

Do you have any idea about how can I set the frequencies of the sine waves, can i do it with "t" and a "f" variable that are put into the format of sine SIN(wt)?

Thanks for your help...

[Ozgur]

.

.

.

.

.

Your edit resulted different as Friedrich did before; like below;

http://imageshack.us/photo/my-images/20/signalq.jpg/

The signal i want to get is alike below figure please check it.

http://imageshack.us/photo/my-images/20/signalq.jpg/

Now, the question is about the frequencies. Let me say the first freq of the sine is 10 hz, second and the third is 50 and 20 Hz respectively.

how can i get the values into the code?

As you know, sinus is in the format of SIN(2*pi*f*t) where f is freq in Hz and t is time in sec. Should be there a sampling freq? I'm trying hard on this and i couldn't manage.

Ozgur

Answer 4

[Ozgur]

The sine waves i want to realize is alike below figure, I made it on Microsoft paint, so it is a little rough..

http://imageshack.us/photo/my-images/20/signalq.jpg/

Ozgur