downsample — Decrease a signal's sample rate by keeping every Nth sample.
y = downsample(x, n) keeps every nth sample of x, starting at the first sample, along the first non-singleton dimension. y = downsample(x, n, phase) starts at the zero-based phase offset.
Syntax
Y = downsample(X, N)
Y = downsample(X, N, PHASE)Inputs
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
X | Any | Yes | — | Input signal array. |
N | NumericScalar | Yes | — | Integer sample-rate factor. |
PHASE | NumericScalar | No | 0 | Integer phase offset in the range 0 <= PHASE < N. |
Returns
| Name | Type | Description |
|---|---|---|
Y | NumericArray | Sample-rate-adjusted signal. |
Errors
| Identifier | When | Message |
|---|---|---|
RunMat:sampleRate:ArgCount | Required arguments are missing or more than three inputs are provided. | sample-rate builtin: expected X, N, and optional PHASE |
RunMat:sampleRate:InvalidInput | X is not numeric, logical, complex, or a gpuArray containing numeric values. | sample-rate builtin: X must be numeric or logical |
RunMat:sampleRate:InvalidFactor | N is not a finite positive integer scalar. | sample-rate builtin: N must be a positive integer scalar |
RunMat:sampleRate:InvalidPhase | PHASE is not an integer scalar in the range 0 <= PHASE < N. | sample-rate builtin: PHASE must be an integer scalar with 0 <= PHASE < N |
RunMat:sampleRate:GatherFailed | A gpuArray input cannot be gathered for host sample-rate conversion. | sample-rate builtin: failed to gather GPU input |
RunMat:sampleRate:SizeOverflow | The requested output shape overflows host allocation limits. | sample-rate builtin: requested output is too large |
RunMat:sampleRate:BuildOutput | Output tensor construction fails. | sample-rate builtin: failed to build output |
How downsample works
- The documented input surface is vectors and matrices. RunMat also accepts N-D inputs; inputs with more than two dimensions are a compatibility-gated RunMat extension.
- Operates along the first non-singleton dimension, matching MATLAB's column-wise behavior for matrices and row-wise behavior for row vectors.
nmust be a finite positive integer-valued single or double scalar. Typed-integernvalues are a separately gated RunMat extension and are parsed exactly.phaseis optional, defaults to 0, and must be an integer-valued single or double scalar in the range0 <= phase < n. Typed-integerphasevalues are a separately gated RunMat extension and are parsed exactly.- Integer input samples preserve all eight signed and unsigned integer classes and their exact element values; downsampling performs structural selection rather than floating-point conversion.
- An input handle whose device has no proven owner is rejected before any provider hook or gather. For a proven owner, a provider hook error or unsupported hook triggers owner-resolved gathering and the host fallback. A provider result that violates owner, device, shape, storage, integer/logical class, or precision invariants is treated as a provider-contract error instead of silently falling back.
- Malformed resident outputs are freed only when their owner can be resolved. RunMat never sends an unknown-owner handle to the input provider for cleanup.
- The output keeps samples at zero-based positions
phase,phase + n,phase + 2*n, and so on. - No anti-aliasing filter is applied; use filtering operations separately when decimating sampled signals.
Examples
Keep every second row-vector sample
x = [1 2 3 4 5];
y = downsample(x, 2)Expected output:
y = [1 3 5]Start at the second sample with phase 1
x = [1 2 3 4 5];
y = downsample(x, 2, 1)Expected output:
y = [2 4]Downsample matrix columns
X = [1 3; 2 4; 5 7];
Y = downsample(X, 2)Expected output:
Y = [1 3; 5 7]Using downsample with coding agents
Open a RunMat example with live inputs, then ask the agent to explain how downsample changes the result.
Run a small downsample example, explain the result, then change one input and compare the output.
FAQ
Which dimension does downsample use?⌄
RunMat follows MATLAB's default and operates along the first non-singleton dimension.
Does downsample apply an anti-aliasing filter?⌄
No. downsample only selects samples. Apply a low-pass filter first when anti-aliasing is required.
Can I pass GPU tensors?⌄
Yes. Inputs stay resident through a proven owning provider's gather_linear hook when it returns a result with the required owner, device, shape, storage, class, and precision invariants. Unknown-device inputs reject before provider work. Unsupported hooks from a proven owner fall back through an owner-resolved host gather; malformed successful results raise a provider-contract error.
Related Math functions
Signal
blackman · butter · buttord · cheb2ord · conv · conv2 · deconv · envelope · filter · filtfilt · fir1 · freqz · gauspuls · hamming · hann · hilbert · periodogram · pulstran · pwelch · rectpuls · resample · sawtooth · sinc · spectrogram · square · tripuls · unwrap · upsample · zplane
Elementwise
abs · angle · bsxfun · complex · conj · double · erf · erfcinv · exp · expm1 · factorial · flintmax · gamma · gammaln · heaviside · hypot · idivide · imag · intmax · intmin · ldivide · log · log10 · log1p · log2 · minus · nextpow2 · plus · pow2 · power · rdivide · real · realmax · realmin · realsqrt · rescale · sign · single · sqrt · swapbytes · times · typecast · uint16 · uint32 · uint8
Trigonometry
acos · acosh · asin · asinh · atan · atan2 · atanh · cos · cosd · cosh · cospi · deg2rad · pol2cart · rad2deg · sin · sind · sinh · sinpi · tan · tand · tanh
Reduction
all · any · bounds · cummax · cummin · cumprod · cumsum · cumtrapz · diff · gradient · max · maxk · mean · median · min · mink · movmax · movmean · movmedian · movmin · movprod · movstd · movsum · movvar · nnz · prod · rms · std · sum · trapz · var
Structure
bandwidth · isdiag · ishermitian · issymmetric · istril · istriu · symrcm
Optim
coneprog · fminbnd · fminunc · fsolve · fzero · integral · linprog · lsqcurvefit · lsqnonlin · optimoptions · optimset · quad · secondordercone
Ops
cross · ctranspose · dot · mldivide · mpower · mrdivide · mtimes · pagemtimes · pagetranspose · trace · transpose
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how downsample is executed, line by line, in Rust.
- View the source for downsample in Rust on GitHub
- Learn how the RunMat runtime works
- Found a bug? Open an issue with a minimal reproduction.
About RunMat
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