Why do upsample and downsample commands give wrong results in start of sequence?

I have a code for upsampling an input sequnce x and then downsampling it back to original by a factor 3. When I look at the plot of input sequence x, it starts at 1 though in DSP it must start at n=0. Likewise when I look at the plot of upsampled sequence y, it must also start at n=0 and then insert two zeros in between every two samples of x. But its not so here in the plots. Why it is so?

% Up/Down Sampler
clear all
close all
clc
x=[1 2 3 4]; % Input Sequence
y=upsample(x,3); % Upsampled sequence y by a factor of 3
subplot(4,1,1)
stem(x) 
title('Input Sequence')

subplot(4,1,2)
stem(y)
title('Up sampled sequence')


c=downsample(y,3); % downsampled sequence y again by the same factor 3
subplot(4,1,3)
stem(c)
title('Down sampled sequence')

Answer 1

[Chunru]

All time axes start from 0.

% Up/Down Sampler
clear all
close all
clc
x=[1 2 3 4]; % Input Sequence
y=upsample(x,3); % Upsampled sequence y by a factor of 3
subplot(4,1,1)
stem(0:numel(x)-1, x) 
title('Input Sequence')
subplot(4,1,2)
stem(0:numel(y)-1, y)
title('Up sampled sequence')
c=downsample(y,3); % downsampled sequence y again by the same factor 3
subplot(4,1,3)
stem(0:numel(c)-1, c)
title('Down sampled sequence')

Answer 2

[dpb]

Don't know what the complaint is, precisely?

upsample and downsample are working as documented --

>> upsample(1:4,3)
ans =
     1     0     0     2     0     0     3     0     0     4     0     0
>> downsample(ans,3)
ans =
     1     2     3     4
>> 

"y = upsample(x,n) increases the sample rate of x by inserting n – 1 zeros between samples." from the documentation for upsample which is precisely what you see above -- and downsample recreates the original input series.

What's to complain about?

If the "issue" is that the plots are shown on the x axis as from 1:N instead of 0:N-1, that's because you plotted them without providing an abscissa vector in which case (also as documented) stem and all the other MATLAB plotting functions plot the provided y values against the ordinal position in the array.

Also as documented, MATLAB array indexing is one-based, NOT zero-based and is immutable/unchangeable by the user. If you want to plot versus some other x, then you must provide that

N=numel(y)-1;
stem(0:N,y)
xlim([0 N])
xticks([0:11])

may be more what you want.

Answer 3

[Mathieu NOE]

hello

Iprefered somehow to create an explicit x axis for each data

now I think the plot is better because evrything is aligned as expected

% Up/Down Sampler
clear all
close all
clc
y = [1 2 3 4]; % Input Sequence
x = (0:length(y)-1);

yu=upsample(y,3); % Upsampled sequence y by a factor of 3
xu = (0:length(yu)-1);
yc=downsample(yu,3); % downsampled sequence y again by the same factor 3
xc = (0:length(yc)-1);

subplot(3,1,1)
stem(x,y) 
title('Input Sequence')
subplot(3,1,2)
stem(xu,yu)
title('Up sampled sequence')
c=downsample(y,3); % downsampled sequence y again by the same factor 3
subplot(3,1,3)
stem(xc,yc)
title('Down sampled sequence')

[Sadiq Akbar]

Thanks to all of you. But I have to accept one answer. As the chunru has responded 1st, and that works too, so I am going to accept his answer. Thanks once again to all of you.

[Mathieu NOE]

ok

I will survive even though on my screen it appears that I answered 1 hour before Chunru

argh!!!