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#include <stdint.h>
#include "ardour/interpolation.h"
nframes_t
LinearInterpolation::interpolate (nframes_t nframes, Sample *input, Sample *output)
{
// the idea behind phase is that when the speed is not 1.0, we have to
// interpolate between samples and then we have to store where we thought we were.
// rather than being at sample N or N+1, we were at N+0.8792922
// so the "phase" element, if you want to think about this way,
// varies from 0 to 1, representing the "offset" between samples
uint64_t phase = last_phase;
// acceleration
int64_t phi_delta;
// phi = fixed point speed
if (phi != target_phi) {
phi_delta = ((int64_t)(target_phi - phi)) / nframes;
} else {
phi_delta = 0;
}
// index in the input buffers
nframes_t i = 0;
for (nframes_t outsample = 0; outsample < nframes; ++outsample) {
i = phase >> 24;
Sample fractional_phase_part = (phase & fractional_part_mask) / binary_scaling_factor;
// Linearly interpolate into the output buffer
// using fixed point math
output[outsample] =
input[i] * (1.0f - fractional_phase_part) +
input[i+1] * fractional_phase_part;
phase += phi + phi_delta;
}
last_phase = (phase & fractional_part_mask);
// playback distance
return i;
}
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