Resampling

sf_resample() converts an NDArray from one sample rate to another using libsamplerate.

Full Signature

function sf_resample(
    NDArray $input,
    int $inputRate,
    int $outputRate,
    ResampleQuality $quality = ResampleQuality::Best,
    ?int $chunkSize = 2048,
): NDArray

Two Modes

Chunked progressive (default)

When $chunkSize is non-null, the function processes the input signal in chunks. A single output buffer is pre-allocated and filled chunk by chunk, then wrapped in a single NDArray at the end. This is safe for large signals.

$y = sf_resample($x, 44100, 16000); // default chunkSize = 2048

You can control chunk size (in frames):

$y = sf_resample($x, 44100, 16000, chunkSize: 8192);

One-shot simple

When $chunkSize is null, the function process the input signal in a single pass. This is best for small signals where the convenience outweighs the memory trade-off.

$resampled = sf_resample($audio, 44100, 8000, chunkSize: null);

Both modes produce identical output for the same input and ratio.

Quality Levels

Four quality levels trade CPU time for frequency response accuracy:

Level	Description	Use Case
ResampleQuality::Best	Band-limited sinc, highest quality	Final output, mastering
ResampleQuality::Medium	Band-limited sinc, medium quality	General purpose
ResampleQuality::Fastest	Band-limited sinc, fastest	Preview, real-time
ResampleQuality::Linear	Linear interpolation	Minimum latency, low quality

use PhpMlKit\SoundFile\Enums\ResampleQuality;

$high = sf_resample($audio, 44100, 48000, quality: ResampleQuality::Best);
$fast = sf_resample($audio, 44100, 48000, quality: ResampleQuality::Fastest);

DType Handling

libsamplerate operates on 32-bit float internally. The input is automatically converted to Float32 if it has a different dtype:

// Int16 input — automatically converted to Float32
$audio = NDArray::array([[100], [200], [300]], DType::Int16);
$resampled = sf_resample($audio, 8000, 16000);
// $resampled->dtype() === DType::Float32

The output is always Float32.

Multi-channel

Stereo and multi-channel inputs are resampled with all channels preserved:

$stereo = NDArray::array([[0.1, 0.2], [0.3, 0.4], [0.5, 0.6]], DType::Float32);
$resampled = sf_resample($stereo, 8000, 16000);
// Shape: [6, 2] — both channels resampled independently

Frame Count Accuracy

The output frame count is computed as ceil(inputFrames × outputRate / inputRate). For exact ratios this is precise. For non-integer ratios, the output may have one extra frame due to ceiling.