libs/ardour/interpolation.cc

   1 #include <stdint.h>
   2 #include "ardour/interpolation.h"
   3
   4 nframes_t
   5 LinearInterpolation::interpolate (nframes_t nframes, Sample *input, Sample *output)
   6 {
   7         // the idea behind phase is that when the speed is not 1.0, we have to
   8         // interpolate between samples and then we have to store where we thought we were.
   9         // rather than being at sample N or N+1, we were at N+0.8792922
  10         // so the "phase" element, if you want to think about this way,
  11         // varies from 0 to 1, representing the "offset" between samples
  12         uint64_t    phase = last_phase;
  13
  14         // acceleration
  15         int64_t     phi_delta;
  16
  17         // phi = fixed point speed
  18         if (phi != target_phi) {
  19                 phi_delta = ((int64_t)(target_phi - phi)) / nframes;
  20         } else {
  21                 phi_delta = 0;
  22         }
  23
  24         // index in the input buffers
  25         nframes_t   i = 0;
  26
  27         for (nframes_t outsample = 0; outsample < nframes; ++outsample) {
  28                 i = phase >> 24;
  29                 Sample fractional_phase_part = (phase & fractional_part_mask) / binary_scaling_factor;
  30
  31         // Linearly interpolate into the output buffer
  32         // using fixed point math
  33                 output[outsample] =
  34                         input[i] * (1.0f - fractional_phase_part) +
  35                         input[i+1] * fractional_phase_part;
  36                 phase += phi + phi_delta;
  37         }
  38
  39         last_phase = (phase & fractional_part_mask);
  40
  41         // playback distance
  42         return i;
  43 }