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Unexpected result for sin()

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Lorenzo
Lorenzo 2012-12-1
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!

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回答（4 个）

Wayne King
Wayne King 2012-12-1
编辑：Wayne King 2012-12-1
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)
Matt Fig
Matt Fig 2012-12-1
编辑：Matt Fig 2012-12-1
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)
Azzi Abdelmalek
Azzi Abdelmalek 2012-12-1
编辑：Azzi Abdelmalek 2012-12-1
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)
Lorenzo
Lorenzo 2012-12-1
Thank you very much everybody! I figured that out eventually!

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