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Fix sound resampling low pass filter.

Subversion-branch: /trunk/chocolate-doom
Subversion-revision: 1951

--- a/src/i_sdlsound.c

+++ b/src/i_sdlsound.c

@@ -244,12 +244,15 @@

             // (maximum frequency, by nyquist)

             dt = 1.0f / mixer_freq;

-            rc = 1.0f / (3.14f * samplerate);

+            rc = 1.0f / (2 * 3.14f * samplerate);

             alpha = dt / (rc + dt);

-            for (i=1; i<expanded_length; ++i) 

+            // Both channels are processed in parallel, hence [i-2]:

+

+            for (i=2; i<expanded_length * 2; ++i)

             {

-                expanded[i] = (Sint16) (alpha * expanded[i] + (1 - alpha) * expanded[i-1]);

+                expanded[i] = (Sint16) (alpha * expanded[i]

+                                      + (1 - alpha) * expanded[i-2]);

             }

         }

 #endif /* #ifdef LOW_PASS_FILTER */