downsample

R2026b

Decrease sample rate by integer factor

Syntax

y = downsample(x,n)

y = downsample(x,n,phase)

Description

y = downsample(x,n) decreases the sample rate of x by keeping the first sample and then every nth sample after the first. If x is a matrix, the function treats each column as a separate sequence.

y = downsample(x,n,phase) specifies the number of samples by which to offset the downsampled sequence.

Examples

Decrease Sample Rates

Decrease the sample rate of a sequence by a factor of 3.

x = [1 2 3 4 5 6 7 8 9 10];
y = downsample(x,3)

y = 1×4

     1     4     7    10

Decrease the sample rate of the sequence by a factor of 3 and add a phase offset of 2.

y = downsample(x,3,2)

y = 1×3

     3     6     9

Decrease the sample rate of a matrix by a factor of 3.

x = [1  2  3;
     4  5  6;
     7  8  9;
    10 11 12];
y = downsample(x,3)

y = 2×3

     1     2     3
    10    11    12

Downsample Sine Waveform

Since R2026b

Generate a 100-second sine waveform with a fundamental frequency of 0.01 Hz.

t = (1:100)';
x = sin(2*pi*0.01*t);

Downsample the signal using a downsampling factor of 5. Compare the original and downsampled signals.

out = downsample([t x],5);
plot(t,x,".",out(:,1),out(:,2),"*")
legend(["Original" "Downsampled"])

Downsample the signal using a downsampling factor of 5 and a phase offset of 3. Compare the original and downsampled signals. Because of the specified phase offset, the first sample of the downsampled signal starts at the fourth sample on the original signal.

out3 = downsample([t x],5,3);
plot(t,x,".",out3(:,1),out3(:,2),"*")
legend(["Original" "Downsampled"])

Input Arguments

x — Input array

vector | matrix

Input array, specified as a vector or matrix. If x is a matrix, the function treats the columns as independent channels.

Tip

To avoid aliasing in the downsampled output, apply a lowpass filter with a normalized cutoff frequency pi/n rad/sample to x before passing it to the function. Alternatively, use the decimate function.

Example: cos(pi/4*(0:159)) + randn(1,160) specifies a sinusoid embedded in white Gaussian noise.

Example: cos(pi./[4;2]*(0:159))' + randn(160,2) specifies a two-channel noisy sinusoid.

n — Downsampling factor

positive integer

Downsampling factor, specified as a positive integer.

Data Types: single | double

phase — Offset

0 (default) | positive integer

Offset, specified as a positive integer from 0 to n – 1.

Data Types: single | double

Output Arguments

y — Downsampled array

vector | matrix

Downsampled array, returned as a vector or matrix.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

GPU Code Generation
Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

The downsample function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

This function fully supports GPU arrays. For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Version History

Introduced before R2006a

R2026b: Use thread-based environments

You can use downsample in thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

R2024a: downsample supports GPU code generation

The downsample function supports code generation for graphical processing units (GPUs). You must have MATLAB^® Coder™ and GPU Coder™ to generate CUDA^® code.