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- Simplified low-pass filter
- Eliminated accidentally applied resonance from low-pass filter
- Silenced a C89 error by adding a static cast

--- a/dumb/src/helpers/fir_resampler.c

+++ b/dumb/src/helpers/fir_resampler.c

@@ -59,8 +59,8 @@

     int read_pos, read_filled;

     unsigned short phase;

     unsigned int phase_inc;

-    float a0, a1, a2, b0, b1, b2;

-    float xn1, xn2, yn1, yn2;

+    float a1, a2, b02, b1;

+    float yn1, yn2;

     short buffer_in[fir_buffer_size * 2];

     int buffer_out[fir_buffer_size];

 } fir_resampler;

@@ -76,9 +76,8 @@

     r->read_filled = 0;

     r->phase = 0;

     r->phase_inc = 0;

-    r->a0 = 0; r->a1 = 0; r->a2 = 0;

-    r->b0 = 0; r->b1 = 0; r->b2 = 0;

-    r->xn1 = 0; r->xn2 = 0;

+    r->a1 = 0; r->a2 = 0;

+    r->b02 = 0; r->b1 = 0;

     r->yn1 = 0; r->yn2 = 0;

     memset( r->buffer_in, 0, sizeof(r->buffer_in) );

     memset( r->buffer_out, 0, sizeof(r->buffer_out) );

@@ -103,14 +102,10 @@

     r_out->read_filled = r_in->read_filled;

     r_out->phase = r_in->phase;

     r_out->phase_inc = r_in->phase_inc;

-    r_out->a0 = r_in->a0;

     r_out->a1 = r_in->a1;

     r_out->a2 = r_in->a2;

-    r_out->b0 = r_in->b0;

+    r_out->b02 = r_in->b02;

     r_out->b1 = r_in->b1;

-    r_out->b2 = r_in->b2;

-    r_out->xn1 = r_in->xn1;

-    r_out->xn2 = r_in->xn2;

     r_out->yn1 = r_in->yn1;

     r_out->yn2 = r_in->yn2;

     memcpy( r_out->buffer_in, r_in->buffer_in, sizeof(r_in->buffer_in) );

@@ -139,8 +134,6 @@

     r->read_pos = 0;

     r->read_filled = 0;

     r->phase = 0;

-    r->xn1 = 0;

-    r->xn2 = 0;

     r->yn1 = 0;

     r->yn2 = 0;

     memset( r->buffer_in, 0, sizeof(r->buffer_in) );

@@ -155,14 +148,15 @@

         double omega = 2 * PI * 0.5 / new_factor;

         double cs = cos(omega);

         double sn = sin(omega);

-        double a1pha = sn / 20.0;

+        double alpha = sn / 2.0;

+        double a0_inv = 1.0 / (1.0 + alpha);

-        r->b0 =  (1.0 - cs) / 2.0 ;

-        r->b1 =   1.0 - cs ;

-        r->b2 =  (1.0 - cs) / 2.0 ;

-        r->a0 =   1.0 + a1pha ;

-        r->a1 =  -2.0 * cs ;

-        r->a2 =   1.0 - a1pha ;

+        double b1_temp = (1.0 - cs) * a0_inv;

+

+        r->b02 = b1_temp * 0.5;

+        r->b1 =  b1_temp;

+        r->a1 =  -2.0 * cs * a0_inv;

+        r->a2 =  (1.0 - alpha) * a0_inv;

     }

 }

@@ -174,14 +168,14 @@

 	{

         if ( r->phase_inc > 65536 )

         {

+            /* The filter is implemented in Direct-II form. */

             int s32;

-            float in, out;

+            float in, out, dsp_centernode;

             in = s;

-            out = (r->b0 * in + r->b1 * r->xn1 + r->b2 * r->xn2 - r->a1 * r->yn1 - r->a2 * r->yn2) / r->a0;

-            r->xn2 = r->xn1;

-            r->xn1 = in;

+            dsp_centernode = in - r->a1 * r->yn1 - r->a2 * r->yn2;

+            out = r->b02 * (dsp_centernode + r->yn2) + r->b1 * r->yn1;

             r->yn2 = r->yn1;

-            r->yn1 = out;

+            r->yn1 = dsp_centernode;

             s32 = out;

             if ( (unsigned)(s32 + 0x8000) >= 0x10000 ) s32 = (s32 >> 31) ^ 0x7FFF;

             s = (short) s32;

@@ -210,7 +204,7 @@

     double const filter = (ratio_ < 1.0) ? 1.0 : 1.0 / ratio_;

     double pos = 0.0;

     //int input_per_cycle = 0;

-    short* out = fir_impulses;

+    short* out = (short*) fir_impulses;

     int n;

     for ( n = res; --n >= 0; )

     {