Fix DspDither

It's used in ->PCM16 downsampling. Error feedback coefficients were
wrong and it was causing audible noise. Also switch to TPDF dither while at it.

2 files changed, 18 insertions, 11 deletions

diff --git a/src/DspDither.cpp b/src/DspDither.cpp
index c0c91a5..c8682b0 100644..100755
--- a/src/DspDither.cpp
+++ b/src/DspDither.cpp
@@ -6,8 +6,13 @@ namespace SaneAudioRenderer
     void DspDither::Initialize(DspFormat outputFormat)

     {

         m_active = (outputFormat == DspFormat::Pcm16);

-        m_error1 = {};

-        m_error2 = {};

+

+        for (size_t i = 0; i < 18; i++)

+        {

+            m_previous[i] = 0.0f;

+            m_generator[i].seed((uint32_t)(GetPerformanceCounter() + i));

+            m_distributor[i] = std::uniform_real_distribution<float>(0, 1.0f);

+        }

     }

 

     bool DspDither::Active()

@@ -32,12 +37,14 @@ namespace SaneAudioRenderer
         {

             for (size_t channel = 0; channel < channels; channel++)

             {

-                // Rectangular dither with simple second-order noise shaping.

-                float inputSample = inputData[frame * channels + channel] * (INT16_MAX - 4);

-                float noise = (float)m_rand() / m_rand.max() + 0.5f * m_error1[channel] - m_error2[channel];

-                float outputSample = round(inputSample + noise);

-                m_error2[channel] = m_error1[channel];

-                m_error1[channel] = outputSample - inputSample;

+                float inputSample = inputData[frame * channels + channel] * (INT16_MAX - 1);

+

+                // High-pass TPDF, 2 LSB amplitude.

+                float r = m_distributor[channel](m_generator[channel]);

+                float noise = r - m_previous[channel];

+                m_previous[channel] = r;

+

+                float outputSample = std::round(inputSample + noise);

                 assert(outputSample >= INT16_MIN && outputSample <= INT16_MAX);

                 outputData[frame * channels + channel] = (int16_t)outputSample;

             }

diff --git a/src/DspDither.h b/src/DspDither.h
index b41c37e..6fe5422 100644..100755
--- a/src/DspDither.h
+++ b/src/DspDither.h
@@ -25,8 +25,8 @@ namespace SaneAudioRenderer
     private:

 

         bool m_active = false;

-        std::array<float, 18> m_error1;

-        std::array<float, 18> m_error2;

-        std::minstd_rand m_rand;

+        std::array<float, 18> m_previous;

+        std::array<std::minstd_rand, 18> m_generator;

+        std::array<std::uniform_real_distribution<float>, 18> m_distributor;

     };

 }