Hi, lately I've stumbled upon an old article that sings praise of Akai S1000 samplers and the resampling algorithm it used. This piqued my curiosity and inspired me to try implementing a sinc resampler into LS for comparison.
I attach my code to the message, as patch against svn trunk. Link to the article: http://martin78.com/samplers/akai-s1000-s1100/ -- some implementation notes: * this is a N-point sinc resampler, where N is defined in code by parameter SINC_NPTOTAL=SINC_NPBEFORE+SINC_NPAFTER * code uses optimization to reduce the number of trig operations/sample from N sin() to 1 sin() + 1 cos() As default I have used parameters NPBEFORE=1 NPAFTER=7, the number of past and future samples used respectively. This happens in some situations, with some samples of my set, to crash LS with segfault, presumably because of out-of-bounds access by getSample(). The question is: where in LS is defined how many past/future samples are available to the resampling? I note LS already has linear and cubic which require 2 and 4. How to control this number? Akai S1000 supposedly has N=8, but I'd like to also to try larger lengths.
Index: src/engines/common/Resampler.h
===================================================================
--- src/engines/common/Resampler.h (revision 2878)
+++ src/engines/common/Resampler.h (working copy)
@@ -30,10 +30,15 @@
#include "../../common/global_private.h"
// TODO: cubic interpolation is not yet supported by the MMX/SSE(1) version though
-#ifndef USE_LINEAR_INTERPOLATION
-# define USE_LINEAR_INTERPOLATION 1 ///< set to 0 if you prefer cubic interpolation (slower, better quality)
-#endif
+// # define USE_LINEAR_INTERPOLATION 1 ///< set if you prefer linear interpolation
+//# define USE_CUBIC_INTERPOLATION 1 ///< set if you prefer cubic interpolation (slower, better quality)
+# define USE_SINC_INTERPOLATION 1 ///< set if you prefer sinc interpolation (slower, better quality)
+/// parameters of sinc interpolation: number of points used before/after the fractional sample position
+#define SINC_NPBEFORE 1
+#define SINC_NPAFTER 7
+#define SINC_NPTOTAL (SINC_NPBEFORE+SINC_NPAFTER)
+
namespace LinuxSampler {
/** @brief Stereo sample point
@@ -165,15 +170,55 @@
}
}
+#ifdef USE_SINC_INTERPOLATION
+
+#if 0 // given for reference
+ template <unsigned N>
+ static inline void fastsins(float (&sintab)[N], float start, float step) {
+ /// fast sin computation using identity
+ /// sin(a+b)=sin(a)cos(b)+cos(a)sin(b)
+ /// cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
+ const float a = cosf(step), b = sinf(step);
+ float s = sinf(start), c = cosf(start);
+ sintab[0] = s;
+ for (unsigned i = 1; i < N; i++) {
+ const float ns = b*c + a*s, nc = a*c - b*s;
+ c = nc;
+ s = ns;
+ sintab[i] = s;
+ }
+ }
+#endif
+
+ template <unsigned N>
+ static inline void fastsincs(float (&sinctab)[N], float start) {
+ /// compute normalized sinc(x): x=0 => 1, otherwise => sin(pi*x)/(pi*x)
+ /// same tactic as fastsins using step=pi
+ const float a = -1.0f, b = 0.0f; // cos(step), sin(step)
+ start *= (float)M_PI;
+ float s = sinf(start), c = cosf(start);
+ float denom = start;
+ sinctab[0] = (denom == 0) ? 1.0f : (s / denom);
+ for (unsigned i = 1; i < N; i++) {
+ const float ns = b*c + a*s, nc = a*c - b*s;
+ c = nc;
+ s = ns;
+ denom += (float)M_PI;
+ sinctab[i] = (denom == 0) ? 1.0f : (s / denom);
+ }
+ }
+#endif // USE_SINC_INTERPOLATION
+
inline static float Interpolate1StepMonoCPP(sample_t* __restrict pSrc, double* __restrict Pos, float& Pitch) {
int pos_int = (int) *Pos; // integer position
float pos_fract = *Pos - pos_int; // fractional part of position
- #if USE_LINEAR_INTERPOLATION
+ #if defined(USE_LINEAR_INTERPOLATION)
int x1 = getSample(pSrc, pos_int);
int x2 = getSample(pSrc, pos_int + 1);
float samplePoint = (x1 + pos_fract * (x2 - x1));
- #else // polynomial interpolation
+
+ #elif defined(USE_CUBIC_INTERPOLATION)
float xm1 = getSample(pSrc, pos_int);
float x0 = getSample(pSrc, pos_int + 1);
float x1 = getSample(pSrc, pos_int + 2);
@@ -182,8 +227,18 @@
float b = 2.0f * x1 + xm1 - (5.0f * x0 + x2) * 0.5f;
float c = (x1 - xm1) * 0.5f;
float samplePoint = (((a * pos_fract) + b) * pos_fract + c) * pos_fract + x0;
- #endif // USE_LINEAR_INTERPOLATION
+ #elif defined(USE_SINC_INTERPOLATION)
+ float samplePoint = 0.0f;
+ const float sincStart = (-SINC_NPBEFORE + 1) - pos_fract;
+ float sincValues[SINC_NPTOTAL];
+ fastsincs(sincValues, sincStart);
+ for (int i = 0; i < SINC_NPTOTAL; ++i) {
+ float inputSample = getSample(pSrc, pos_int + i - SINC_NPBEFORE + 1);
+ samplePoint += inputSample * sincValues[i];
+ }
+ #endif
+
*Pos += Pitch;
return samplePoint;
}
@@ -195,7 +250,7 @@
stereo_sample_t samplePoint;
- #if USE_LINEAR_INTERPOLATION
+ #if defined(USE_LINEAR_INTERPOLATION)
// left channel
int x1 = getSample(pSrc, pos_int);
int x2 = getSample(pSrc, pos_int + 2);
@@ -204,7 +259,8 @@
x1 = getSample(pSrc, pos_int + 1);
x2 = getSample(pSrc, pos_int + 3);
samplePoint.right = (x1 + pos_fract * (x2 - x1));
- #else // polynomial interpolation
+
+ #elif defined(USE_CUBIC_INTERPOLATION)
// calculate left channel
float xm1 = getSample(pSrc, pos_int);
float x0 = getSample(pSrc, pos_int + 2);
@@ -224,8 +280,21 @@
b = 2.0f * x1 + xm1 - (5.0f * x0 + x2) * 0.5f;
c = (x1 - xm1) * 0.5f;
samplePoint.right = (((a * pos_fract) + b) * pos_fract + c) * pos_fract + x0;
- #endif // USE_LINEAR_INTERPOLATION
+ #elif defined(USE_SINC_INTERPOLATION)
+ samplePoint.left = 0.0f;
+ samplePoint.right = 0.0f;
+ const float sincStart = (-SINC_NPBEFORE + 1) - pos_fract;
+ float sincValues[SINC_NPTOTAL];
+ fastsincs(sincValues, sincStart);
+ for (int i = 0; i < SINC_NPTOTAL; ++i) {
+ float leftSample = getSample(pSrc, 2*i*(pos_int - SINC_NPBEFORE + 1));
+ float rightSample = getSample(pSrc, 2*i*(pos_int - SINC_NPBEFORE + 1)+1);
+ samplePoint.left += leftSample * sincValues[i];
+ samplePoint.right += rightSample * sincValues[i];
+ }
+ #endif
+
*Pos += Pitch;
return samplePoint;
}
Index: src/linuxsampler.cpp
===================================================================
--- src/linuxsampler.cpp (revision 2878)
+++ src/linuxsampler.cpp (working copy)
@@ -118,6 +118,7 @@
}
dmsg(1,("LinuxSampler %s\n", VERSION));
+ dmsg(1,("-- using Sinc interpolation patch (experimental)\n"));
dmsg(1,("Copyright (C) 2003,2004 by Benno Senoner and Christian Schoenebeck\n"));
dmsg(1,("Copyright (C) 2005-2016 Christian Schoenebeck\n"));
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Description: OpenPGP digital signature
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