Add 3xxx effect to set echo delay buffer length. Add documentation

papers/doc/7-systems/qsound.md

@@ -0,0 +1,18 @@
+# Capcom QSound (DL-1425)
+
+This chip was used in Capcom's CP System Dash, CP System II and ZN arcade PCBs.
+
+It supports 16 PCM channels and uses the patented (now expired) QSound stereo expansion algorithm, as the name implies.
+
+Because the chip lacks sample interpolation, it's recommended that you try to play samples at around 24038 Hz to avoid aliasing. This is especially important for e.g. cymbals.
+
+The QSound chip also has a small echo buffer, somewhat similar to the SNES, although with a very basic (and non-adjustable) filter. It is however possible to adjust the feedback and length of the echo buffer. The initial values can be set in the "Configure system" dialog.
+
+There are also 3 ADPCM channels, however they cannot be used in Furnace yet. They have been reserved in case this feature is added later. ADPCM samples are limited to 8012 Hz.
+
+# effects
+
+- `08xx`: Set panning. Valid range is 00-20. 00 for full left, 10 for center and 20 for full right. It is also possible to bypass the QSound algorithm by using the range 30-50.
+- `10xx`: Set echo feedback level. This effect will apply to all channels.
+- `11xx`: Set echo level.
+- `3xxx`: Set the length of the echo delay buffer.

src/engine/dispatch.h

@@ -98,6 +98,7 @@ enum DivDispatchCmds {
   DIV_CMD_SAA_ENVELOPE,
 
   DIV_CMD_QSOUND_ECHO_FEEDBACK,
+  DIV_CMD_QSOUND_ECHO_DELAY,
   DIV_CMD_QSOUND_ECHO_LEVEL,
 
   DIV_ALWAYS_SET_VOLUME,

src/engine/platform/qsound.cpp

@@ -257,6 +257,9 @@ const char* DivPlatformQSound::getEffectName(unsigned char effect) {
     case 0x11:
       return "11xx: Set channel echo level (00 to FF)";
       break;
+    default:
+	  if((effect & 0xf0) == 0x30)
+		  return "3xxx: Set echo delay buffer length (000 to AA5)";
   }
   return NULL;
 }
@@ -433,6 +436,9 @@ int DivPlatformQSound::dispatch(DivCommand c) {
     case DIV_CMD_QSOUND_ECHO_FEEDBACK:
 	  immWrite(Q1_ECHO_FEEDBACK, c.value << 6);
       break;
+	case DIV_CMD_QSOUND_ECHO_DELAY:
+	  immWrite(Q1_ECHO_LENGTH, (c.value > 2725 ? 0xfff : 0xfff - (2725 - c.value)));
+      break;
     case DIV_CMD_PITCH:
       chan[c.chan].pitch=c.value;
       chan[c.chan].freqChanged=true;

src/engine/playback.cpp

@@ -110,6 +110,7 @@ const char* cmdName[DIV_CMD_MAX]={
   "SAA_ENVELOPE",
 
   "QSOUND_ECHO_FEEDBACK",
+  "QSOUND_ECHO_DELAY",
   "QSOUND_ECHO_LEVEL",
 
   "ALWAYS_SET_VOLUME"
@@ -230,7 +231,10 @@ bool DivEngine::perSystemEffect(int ch, unsigned char effect, unsigned char effe
           dispatchCmd(DivCommand(DIV_CMD_QSOUND_ECHO_LEVEL,ch,effectVal));
           break;
         default:
-          return false;
+		  if((effect & 0xf0) == 0x30)
+			  dispatchCmd(DivCommand(DIV_CMD_QSOUND_ECHO_DELAY,ch,((effect & 0x0f) << 8) | effectVal));
+		  else
+			  return false;
       }
       break;
     default: