From be62fba2e093ec7131c2734199d2b1648fc108cd Mon Sep 17 00:00:00 2001
From: Holger Wirtz <wirtz@parasitstudio.de>
Date: Fri, 14 Jun 2019 10:28:41 +0200
Subject: [PATCH] Slightly better simple interpolation code.

---
 effect_modulated_delay.cpp | 49 +++++++++++++++++++++++++-------------
 1 file changed, 33 insertions(+), 16 deletions(-)

diff --git a/effect_modulated_delay.cpp b/effect_modulated_delay.cpp
index 42fec14..6d738c1 100644
--- a/effect_modulated_delay.cpp
+++ b/effect_modulated_delay.cpp
@@ -87,6 +87,8 @@ void AudioEffectModulatedDelay::update(void)
 
   if (block && modulation)
   {
+    float _tmp;
+
     for (uint16_t i = 0; i < AUDIO_BLOCK_SAMPLES; i++)
     {
       // write data into circular buffer
@@ -100,9 +102,9 @@ void AudioEffectModulatedDelay::update(void)
 
 #ifdef INTERPOLATE_MODE
       // Generate a an array with the size of INTERPOLATION_WINDOW_SIZE of x/y values around mod_idx for interpolation
-      uint8_t c = 0;
+      uint8_t c;
       int16_t c_mod_idx = _circ_idx - int(mod_idx + 0.5); // This is the pointer to the value in the circular buffer at the current modulation index
-      for (j = ~(INTERPOLATION_WINDOW_SIZE >> 1) | 0x01; j <= INTERPOLATION_WINDOW_SIZE >> 1; j++) // only another way to say: from -INTERPOLATION_WINDOW_SIZE/2 to INTERPOLATION_WINDOW_SIZE/2
+      for (j = ~(INTERPOLATION_WINDOW_SIZE >> 1) | 0x01, c = 0; j <= INTERPOLATION_WINDOW_SIZE >> 1; j++, c++) // only another way to say: from -INTERPOLATION_WINDOW_SIZE/2 to INTERPOLATION_WINDOW_SIZE/2
       {
         int16_t jc_mod_idx = (c_mod_idx + j) % _delay_length; // The modulation index pointer plus the value of the current window pointer
         if (jc_mod_idx < 0)
@@ -110,25 +112,40 @@ void AudioEffectModulatedDelay::update(void)
         else
           y[c] = float(_delayline[jc_mod_idx]);
         x[c] = float(j);
-        c++; // because 42 is the answer! ;-)
       }
-      *bp = int(s.value(modff(mod_idx, NULL)) + 0.5);
+      *bp = int(s.value(modff(mod_idx, &_tmp)) + 0.5);
 #else
-      // No interpolation - should sound really bad...
+      // Simple interpolation
       int16_t c_mod_idx = (_circ_idx - int(mod_idx + 0.5)) % _delay_length;
+      float fraction = modff(mod_idx, &_tmp);
+      float value1, value2;
       if (c_mod_idx < 0)
-        *bp = _delayline[_delay_length + c_mod_idx];
+      {
+        if (fraction < 0.5)
+        {
+          value1 = _delayline[_delay_length + c_mod_idx - 1];
+          value2 = _delayline[_delay_length + c_mod_idx];
+        }
+        else
+        {
+          value1 = _delayline[_delay_length + c_mod_idx];
+          value2 = _delayline[_delay_length + c_mod_idx + 1];
+        }
+      }
       else
-        *bp = _delayline[c_mod_idx];
-      /*      Serial.print("mod_idx=");
-            Serial.print(mod_idx, 3);
-            Serial.print(" c_mod_idx=");
-            Serial.print(c_mod_idx, DEC);
-            Serial.print(" MODULATION=");
-            Serial.print(*mp, DEC);
-            Serial.print(" DATA=");
-            Serial.print(*bp, DEC);
-            Serial.println();*/
+      {
+        if (fraction < 0.5)
+        {
+          value1 = _delayline[c_mod_idx - 1];
+          value2 = _delayline[c_mod_idx];
+        }
+        else
+        {
+          value1 = _delayline[c_mod_idx];
+          value2 = _delayline[c_mod_idx + 1];
+        }
+      }
+      *bp = int ((fraction * value1) + ((1.0 - fraction) * value2) + 0.5);
 #endif
 
       bp++; // next audio data