How can I mark points of the period length on stem of cos?

Question

nastia korzoun 2021-7-25

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/885659-how-can-i-mark-points-of-the-period-length-on-stem-of-cos

评论： nastia korzoun 2021-7-26

Hi,

I have two subplots of two specific functions we are supposed to plot using stem (for discrete signal) (so far this went great).

Now we need to mark by red dots, the points of the period length starting from n=0.

I can't understand why it would only mark the origin point of n=0 with the red dot, and won't mark the other period length points.

The code I used:

n = linspace (0, 36 , 200);
%
X1 = cos((n.*pi)/4);
X2 = cos((3.*n.*pi)/8);
%
subplot(2, 1, 1);
stem (n, X1,"Marker","none",'Color','b');
hold;
ind1 = n==8.*((n.*pi)./4);
stem(n(ind1),X1(ind1),'Color','r','Marker',".","LineStyle","none")
xlim([0,36]);
ylim([-1.25,1.25]);
xlabel('Time(s)');
ylabel('Amplitude');
title('cos((npi)/4)');
grid;
%
subplot (2, 1, 2);
stem (n, X2,"Marker","none",'Color','b');
hold;
ind2 = n==(16./3).*((3.*n.*pi)./8);
stem(n(ind2),X2(ind2),'Color','r','Marker',".","LineStyle","none")
xlim([0,36]);
ylim([-1.25,1.25]);
xlabel('Time(s)');
ylabel('Amplitude');
title('cos((3npi)/8)');
grid;

The screenshot of the stem:

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

dpb 2021-7-25

在 MATLAB Online 中打开

>> n = linspace (0, 36 , 200);
>> ind2 = n==(16./3).*((3.*n.*pi)./8);
>> whos ind2
  Name      Size             Bytes  Class      Attributes
  ind2      1x200              200  logical              
>> sum(ind2)
ans =
     1
>> 

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

DGM 2021-7-26

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/885659-how-can-i-mark-points-of-the-period-length-on-stem-of-cos#answer_753654

编辑：DGM 2021-7-26

在 MATLAB Online 中打开

You're running into problems because your sampling rate means that your samples (other than the first one) don't neatly line up with the period of the underlying sinusoid. How you want to handle this is up to you. Consider the first plot alone. I'm using a different marker to make it easier to see.

n = linspace (0, 36 , 200);
X1 = cos((n.*pi)/4); % period is 8
stem (n, X1,"Marker","none",'Color','b');
hold on;
xlim([0,36]);
ylim([-1.25,1.25]);
xlabel('Time(s)');
ylabel('Amplitude');
title('cos((npi)/4)');
grid;

Since the samples don't exactly line up (zoom in at the crest near x=8), you might choose to mark the nearest sample:

% find minima by simple thresholding
%ind1 = rem(n,8) < 0.15; 
%stem(n(ind1),X1(ind1),'Color','r','Marker',"*","LineStyle","none")
% a bit more robust minima finding method
[~,ind1] = findpeaks([0 8-rem(n,8)]);
stem(n(ind1-1),X1(ind1-1),'Color','r','Marker',"*","LineStyle","none")

Or you might choose to mark the exact period of the sinusoid:

% plot exact period instead of closest sample
xx = 0:8:36;
stem(xx,ones(size(xx)),'Color','r','Marker',"*","LineStyle",":")

If the latter case is appropriate, and your requirements allow it, a different method of visualization might be a bit easier to read:

% plot exact period using background
px = [0 8 8 0];
py = [-1.5 -1.5 1.5 1.5];
for pn = 1:ceil(36/16)
    a = patch(px,py,'r','facealpha',0.1,'edgecolor','none');
    uistack(a,'bottom')
    px = px + 16;
end