plot3 gives strange plot

Question

Raady 2017-1-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/320526-plot3-gives-strange-plot

评论： Walter Roberson 2017-1-16

采纳的回答： Raady

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I want to plot 3 signal for comparision as in 3d view, so that I can clearly observe the change in different signals.

For an example I want to plot 3 sin signals with different frequencies.

t = [0:0.1:20];
A = 1;
f = 10000;
y1 = A*sin(1000*t);
y2 = A*sin(1250*t);
y3 = A*sin(1500*t);
plot3(y1,y2,y3);

the plot is very strange. where have I gone wrong ?

If i do as plot3(y1,y2,y3,t); it gives error as no enough arguments. Please suggest me how can i plot as the first figure

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Answer 1

Raady 2017-1-16

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John thanks for your reply, but I was concerning about plot as first figure not the second ones i misunderstood the plot3, here is my code and what i wanted to plot that as

 t  = 1:100;
 x1 = ones(1,100);
 x2 = 2*x1;
 x3 = 4*x1;
 s1 = sin(t);
 s2 = sin(2*t);
 s3 = sin(4*t);
 figure(1);
 hold on;
 plot3(t,x1,s1);
 plot3(t,x2,s2);
 plot3(t,x3,s3);
 hold off;

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John BG 2017-1-16

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Raady

thanks for your fast response.

The reason why you get quirky signals is: Alias

Your are not sampling enough samples per cycle, so the 3 plots miss some peaks and some valleys.

Use the values I have given you:

1.

Shorter time window: from what you have shown there is not need for more than the few cycles you show in the first image. 2 seconds is excessive.

2.

Higher sampling frequency: Is same as smaller time step.

Replace

t = [0:0.1:20];

with

t = [0:0.00001:0.1];

The 3D plotting is nice but, correct me if wrong, the reason why you posted the question is that your plot doesn't look as smooth as the one you show in the question, and the one show by other contributors to your question.

The reason is: not enough samples

Once you have enough samples per cycle, any plot you choose will resemble sin(wt) cos(wt), as expected.

This is why I kindly ask you to consider marking my answer as the accepted answer.

Regards

John BG <mailto:jgb2012@sky.com jgb2012@sky.com>

Walter Roberson 2017-1-16

No, the problem is not to do with aliasing. Raady was trying to plot three separate plots with a single plot3() call, thinking that each component would get drawn as a separate 2D layer.

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Answer 2

John BG 2017-1-16

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refine, your plot is suffering alias.

What about this

format long;t = [0:0.00001:0.1];
A = 1;
y1 = A*sin(1000*t);
y2 = A*sin(1250*t);
y3 = A*sin(1500*t);
plot3(y1,y2,y3);

.

It doesn't seem you are using this line

f = 10000;

would you please be so kind to consider marking my answer as Accepted Answer?

thanks for time and attention, awaiting answer

John BG <mailto:jgb2012@sky.com jgb2012@sky.com>

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Answer 3

Star Strider 2017-1-16

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The ribbon plot may do what you want.

The Code —

t = [0:0.1:20];
A = 1;
f = 10000;
y1 = A*sin(1000*t);
y2 = A*sin(1250*t);
y3 = A*sin(1500*t);
figure(1)
hr = ribbon(t, [y1; y2; y3]', 0.1)
grid on
set(hr, 'EdgeColor','none')
axis([xlim    ylim    [-2  2]])
view([-50    45])