Unexpected result for sin()

Dear all;

I think I'm getting crazy or something. Fact is that Matlab is giving me some strange results for a simple sin function.

If I try and run the following:

f=30;
x=0:0.01:1;
y=0.7*sin(2*pi*f*x);
plot(x,y)

I get a simple wave plot.

Now, if I change f to 50, then I get this strange plot:

https://dl.dropbox.com/u/2459663/test.png

How is that possible?

Thank you!

Answer 1

[Wayne King]

Because you are evaluating sin() at multiples of pi

2*pi*k/2

Each one of your steps in x is a multiple of 1/100, then you are multiplying that by 50 when f=50 so you are evaluating sin() at multiples of 2*pi*k/2 and they will all be essentially zero. Look at the y-axis of your plot. Those numbers are all essentially zero.

Actually even your 30-Hz sine does not look that good because you are sampling your x-vector too coarsely, compare for example

   x = 0:0.01:1;
   xprime = 0:0.001:1;
   y = sin(2*pi*30*x);
   yprime = sin(2*pi*30*xprime);
   subplot(211)
   plot(x,y)
   subplot(212)
   plot(xprime,yprime)

Answer 2

[Matt Fig]

In general, you need to make your sample frequency sensitive to the signal frequency... (Nyquist)

f = 50; 
x = 0:0.1/(2*f):1; % Sample frequency
y = 0.7*sin(2*pi*f*x); 
plot(x,y)

Answer 3

[Azzi Abdelmalek]

f=50;
np=5         % number of period
t1=np/f      % final time
ns=20        % number of sample per period
ts=1/(ns*f)  % sample time
x=0:ts:t1;
y=0.7*sin(2*pi*f*x); 
plot(x,y)

Answer 4

[Lorenzo]

Thank you very much everybody! I figured that out eventually!