Replace dithering with noise

src/FilterModelConfig.h

@@ -54,45 +54,11 @@ class FilterModelConfig
     };
 
 protected:
-    static inline uint16_t to_uint16_dither(double x, double d_noise)
-    {
-        const int tmp = static_cast<int>(x + d_noise);
-        assert((tmp >= 0) && (tmp <= USHRT_MAX));
-        return static_cast<uint16_t>(tmp);
-    }
-
     static inline uint16_t to_uint16(double x)
     {
-        return to_uint16_dither(x, 0.5);
+        return static_cast<uint16_t>(x);
     }
 
-private:
-    /*
-     * Hack to add quick dither when converting values from float to int
-     * and avoid quantization noise.
-     * Hopefully this can be removed the day we move all the analog part
-     * processing to floats.
-     *
-     * Not sure about the effect of using such small buffer of numbers
-     * since the random sequence repeats every 1024 values but for
-     * now it seems to do the job.
-     */
-    class Randomnoise
-    {
-    private:
-        double buffer[1024];
-        mutable int index = 0;
-    public:
-        Randomnoise()
-        {
-            std::uniform_real_distribution<double> unif(0., 1.);
-            std::default_random_engine re;
-            for (int i=0; i<1024; i++)
-                buffer[i] = unif(re);
-        }
-        double getNoise() const { index = (index + 1) & 0x3ff; return buffer[index]; }
-    };
-
 protected:
     /// Capacitor value.
     const double C;
@@ -134,9 +100,6 @@ class FilterModelConfig
     /// Reverse op-amp transfer function.
     uint16_t opamp_rev[1 << 16]; //-V730_NOINIT this is initialized in the derived class constructor
 
-private:
-    Randomnoise rnd;
-
 private:
     FilterModelConfig(const FilterModelConfig&) = delete;
     FilterModelConfig& operator= (const FilterModelConfig&) = delete;
@@ -296,7 +259,7 @@ class FilterModelConfig
 
     inline uint16_t getNormalizedValue(double value) const
     {
-        return to_uint16_dither(N16 * (value - vmin), rnd.getNoise());
+        return to_uint16(N16 * (value - vmin));
     }
 
     template<int N>

src/resample/Resampler.h

@@ -40,6 +40,21 @@ class Resampler
 
     Resampler() {}
 
+    mutable uint32_t m_wnoise = 34653463u;
+    mutable int32_t m_bnoise = 0;
+
+    int32_t bnoise() const
+    {
+        // white noise
+        m_wnoise = m_wnoise * 1664525u + 1013904223u;
+
+        // low-passed noise
+        int32_t n = (int32_t)((m_wnoise >> 20) & 0x7f) - 0x80;
+        m_bnoise = m_bnoise + ((0x06*(n - m_bnoise)) >> 7);
+
+        return m_bnoise;
+    }
+
 public:
     virtual ~Resampler() = default;
 
@@ -59,7 +74,7 @@ class Resampler
     inline int16_t getOutput(int32_t scaleFactor) const
     {
         const int32_t out = (scaleFactor * output()) / 2;
-        return Limiter::softClip(out);
+        return Limiter::softClip(out + bnoise());
     }
 
     virtual void reset() = 0;