How to make a signal have a slower sample rate

Hi, I'm sure this has an easy answer but I'll ask anyway.

I have a signal (in this case vehicle speed) on a sample test with a sample rate of 2ms (the system sample rate). The real system being simulated has a can signal which updates every 40ms, or 20 samples in this case.

How can I alter the signal I have, which changes every sample, to only change every 20 samples?

Thank you!

Answer 1

[Mathieu NOE]

so this would be the code :

samples = 1000;
input_signal = 1 + sin(2*pi*(1:samples)/samples)+ 0.025*rand(1,samples);
decim_factor = 20;    % nb of samples / decimation
output_signal = zeros(1,samples); % pre allocation

% zero overlap mean averaging
for ci=1:floor(samples/ decim_factor)
    start_index = 1+(ci-1)*decim_factor;
    stop_index = min(start_index+ decim_factor,length(input_signal));
    output_signal(start_index:stop_index) = input_signal(start_index);
end

figure(1),
plot((1:samples),input_signal,'b',(1:samples),output_signal,'-r');

[João Araújo]

I also thought I would have to do a cycle for this. Thank you for taking the time to write the code, you've been very helpful!

[Mathieu NOE]

you're welcome !

Answer 2

[Mathieu NOE]

hello

for integer factor downsampling use : decimate

for non integer factor resampling use resample (not needed in your case)

EXAMPLE: Decimate a signal by a factor of four

t = 0:.00025:1; % Time vector

x = sin(2*pi*30*t) + sin(2*pi*60*t);

y = decimate(x,4);

subplot(1,2,1);

stem(x(1:120)), axis([0 120 -2 2]) % Original signal

title('Original Signal')

subplot(1,2,2);

stem(y(1:30)) % Decimated signal

title('Decimated Signal')

[João Araújo]

Thank you for the answer, this is not exactly what i need but maybe I haven't explained myself well.

In your example we get the original vector with 4001 samples and the decimated vector with 1001 samples.

What I need is that result but still with 4001 samples, and the values only change every 20 samples.

[Mathieu NOE]

so you mean you would like to pick 1 sample every 20 and hold the value on the 19 next iterations ?

it's like applying a zero hold sampler

[João Araújo]

Exactly this yes.