# single plot, dft graph?

조회 수: 6 (최근 30일)
david hanna . 2016년 3월 8일
댓글: Star Strider . 2016년 5월 10일
Hi folks just asking how you can a single point graph plotted, so that its x against y. the x plot should be 0:120, every 5 seconds. and y plot should be these values: 62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67 These values represent a reading every 5 seconds
in the format of the link below is what i'm trying to get.
hope this makes sense
##### 댓글 수: 2없음 표시없음 숨기기
Star Strider 2016년 3월 8일
See the R2015a documentation for fft. Pay special attention to the code between the top two figure images. Everything you need is there.
david hanna 2016년 3월 9일
Star Strider
i need an activation license to see the document you mention. I'm using matlab at my college,i.e. their computers, when i type license into the matlab prompt for the code i get: 347765 when i type this in for access onto the document,it doesnt allow me. any ideas? thanks

댓글을 달려면 로그인하십시오.

### 답변 (5개)

Star Strider 2016년 4월 13일
This is how I would code it:
xdat = 0:5:120;
ydat = [62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67];
Ts = mean(diff(xdat)); % Sampling Interval
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
L = length(ydat); % Signal Length (Obviously)
ft_y = fft(ydat)/L; % Fourier Transform (Normalised)
Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
figure(1)
plot(Fv, 2*abs(ft_y(Iv)))
grid
xlabel('Frequency (Arbitrary Units)')
ylabel('Amplitude (Arbitrary Units)')
There’s a relatively high d-c offset. You can eliminate that by subtracting the mean of ‘ydat’ before you take the fft. This will make the other frequencies more apparent.
##### 댓글 수: 6이전 댓글 4개 표시이전 댓글 4개 숨기기
davy hanna 2016년 5월 10일
Star Strider
Are they though, the ydat for that example is pulse rates every 5 seconds, during walking. when the fft is plotted, the y-scale is from 0 to a maximum of 5? so am thinking it can't be the ydat of the original form?
Star Strider 2016년 5월 10일
The Fourier transform is what it is, and I stand by my previous statements. It may not be appropriate for your analysis, since your data plotted as a function of time demonstrate a certain periodicity with respect to heart rate, with an increasing heart rate over time. This may be an artefact of your experimental conditions (for example, treadmill velocity, if you are studying heart rate over certain fixed periods of specific treadmill velocities). Since I don’t know what you’re doing, I can’t say with any certainty what analysis techniques are appropriate for your data.
If my ‘treadmill’ guess is correct, perhaps you should be regressing heart rate against treadmill velocity instead of doing a Fourier transform.
I have no idea.

댓글을 달려면 로그인하십시오.

Ilham Hardy 2016년 3월 9일
Perhaps this is what you want,
xdat = 0:5:120;
ydat = [62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67];
scatter(xdat,ydat,'filled')
set(gca,'xtick',0:5:120)
grid on
##### 댓글 수: 1이전 댓글 -1개 표시이전 댓글 -1개 숨기기
david hanna 2016년 3월 14일
llham Hardy
not exactly, but it does do the trick i suppose for my case, and just working through matlab. am having difficulty regarding doing a fourier transform or short-time fourier transform, on this signal. having problems relating to the tutorials compared to my own signal :(

댓글을 달려면 로그인하십시오.

david hanna 2016년 3월 14일
Fs = 1000; T = 1/Fs; L = 1000; t = (0:L-1)*T; x = 0:5:120; y = [62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67]; plot(Fs*t(1:50),y(1:50)) xlabel('time (seconds)')
Index exceeds matrix dimensions.
Getting that index error, maybe i'm not on the right track here for getting the fft even?
##### 댓글 수: 1이전 댓글 -1개 표시이전 댓글 -1개 숨기기
david hanna 2016년 3월 21일
BUMP

댓글을 달려면 로그인하십시오.

davy hanna 2016년 4월 12일
xdat = 0:5:120;
ydat = [62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67];
scatter(xdat,ydat,'filled')
set(gca,'xtick',0:5:120)
grid on
any idea on how to an fft on that plot above?
##### 댓글 수: 0이전 댓글 -2개 표시이전 댓글 -2개 숨기기

댓글을 달려면 로그인하십시오.

davy hanna 2016년 4월 13일
편집: davy hanna 님. 2016년 4월 13일
xdat= 0:5:120;
ydat=[62,57,56,55,57,56,58,62,62,63,61,61,62,62,62,61,60,60,60,61,65,68,67,66,67];
plot(xdat,ydat);
nfft=length(ydat);
nfft2=2.^nextpow2(nfft);
ffty=fftshift(fft(ydat,nfft2));
r=abs(ffty);
plot(r);
plot(r); ↑ Error: The input character is not valid in MATLAB statements or expressions.
can anyone tell me how/why im going wrong when trying to plot the FFT of this?

댓글을 달려면 로그인하십시오.

### 카테고리

Help CenterFile Exchange에서 Discrete Fourier and Cosine Transforms에 대해 자세히 알아보기

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by