ZX beeper: set a more audible duty

TODO.md

@@ -5,9 +5,7 @@
   - input pad
   - settings
 - RF5C68 system
-- ZX beeper system
-- Y8950 system
-- maybe YMU759 ADPCM channel
+- ZX beeper system overlay percussion
 - ADPCM chips
 - Game Boy envelope macro/sequence
 - rewrite the system name detection function anyway

src/engine/platform/zxbeeper.h

@@ -62,7 +62,7 @@ class DivPlatformZXBeeper: public DivDispatch {
       vol(1),
       outVol(1),
       sPosition(0),
-      duty(3) {}
+      duty(64) {}
   };
   Channel chan[6];
   DivDispatchOscBuffer* oscBuf[6];