OK, really now, this time I mean it. :-P

[stargem2] / src / sound.cpp

diff --git a/src/sound.cpp b/src/sound.cpp
index b67659b0a4f7daeb9cda3e859820b134d8785638..8010d8985667c04bae056bcfe51c6f5d42cfa0fb 100755 (executable)
--- a/src/sound.cpp
+++ b/src/sound.cpp
@@ -10,6 +10,8 @@
 // ---  ----------  ------------------------------------------------------------
 // JLH  06/15/2006  Added changelog ;-)
 // JLH  06/15/2006  Scrubbed all BYTE, WORD & DWORD references from the code
+// JLH  02/10/2009  Fixed sound IRQ callback so that CPU samples are taken at
+//                  cycle exact boundaries
 //
 // Notes:
 //   The sound CPU (6808) runs at 894,750 (3,579,000 / 4) Hz.
@@ -85,13 +87,54 @@ void SDLSoundCallback(void * userdata, Uint8 * buffer, int length)
        extern uint8 sram[];
        int cnt = 0;
 
+#define CYCLE_EXACT_SOUND
+#ifdef CYCLE_EXACT_SOUND
+       static float overflow = 0.0;
+       static uint32 time = 20;
+#endif
+
        while (cnt != length)
        {
                // This is close, but not cycle exact (exact would be 20.289115646...)
 
-//Need to figure out how to get that fraction to execute... !!! FIX !!!                
+//Need to figure out how to get that fraction to execute... !!! FIX !!! [DONE]
+/*
+One way would be to use a fractional accumulator, then subtract 1 every
+time it overflows. Like so:
+
+double overflow = 0;
+uint32 time = 20;
+while (!done)
+{
+       Execute6808(&soundCPU, time);
+       overflow += 0.289115646;
+       if (overflow > 1.0)
+       {
+               overflow -= 1.0;
+               time = 21;
+       }
+       else
+               time = 20;
+}
+*/
+#ifdef CYCLE_EXACT_SOUND
+               Execute6808(&soundCPU, time);
+               soundCPU.clock -= time;
+#else
                Execute6808(&soundCPU, 20);
                soundCPU.clock -= 20;
+#endif
                buffer[cnt++] = sram[0x0400];                           // Fill the buffer with the PIA output value
+#ifdef CYCLE_EXACT_SOUND
+               overflow += 0.289115646;                                        // Fractional part of CPU cycles...
+
+               if (overflow > 1.0)
+               {
+                       overflow -= 1.0;
+                       time = 21;
+               }
+               else
+                       time = 20;
+#endif
        }
 }