Merge pull request #137 from bcc2528/master

Added PCM Linear interpolation.
captainys · Oct 5, 2024 · ed760f6 · ed760f6
2 parents 979235b + fe669fb
commit ed760f6
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Showing 2 changed files with 36 additions and 7 deletions.
diff --git a/src/rf5c68/rf5c68.cpp b/src/rf5c68/rf5c68.cpp
@@ -47,6 +47,20 @@ static inline int Gain(int a,int b)
 	}
 }
 
+// Linear interpolation
+static inline int lerp_int(int start, int end, float t)
+{
+	if (t >= 1.0)
+	{
+		return end;
+	}
+	else if(t <= 0.0)
+	{
+		return start;
+	}
+	float ret = (1.0 - t) * start + t * end;
+	return (int)ret;
+}
 
 
 RF5C68::RF5C68()
@@ -82,6 +96,8 @@ void RF5C68::Clear(void)
 	state.IRQBank=0;
 	state.IRQBankMask=0;
 	state.timeBalance=0;
+	state.Lout_prev=0;
+	state.Rout_prev=0;
 }
 
 RF5C68::StartAndStopChannelBits RF5C68::WriteControl(unsigned char value)
@@ -455,10 +471,16 @@ unsigned int RF5C68::AddWaveForNumSamples(unsigned char waveBuf[],unsigned int n
 			;
 		}
 
+		float time = 0;
+		float time_add = static_cast<float>(SAMPLING_RATE) / state.timeBalance;
+
 		while(0<=state.timeBalance && nFilled<numSamples)
 		{
-			WordOp_Add(wavePtr  ,Lout);
-			WordOp_Add(wavePtr+2,Rout);
+			//WordOp_Add(wavePtr  ,Lout);
+			//WordOp_Add(wavePtr+2,Rout);
+			WordOp_Add(wavePtr  ,lerp_int(state.Lout_prev,Lout,time));
+			WordOp_Add(wavePtr+2,lerp_int(state.Rout_prev,Rout,time));
+			time+=time_add;
 			state.timeBalance-=SAMPLING_RATE;
 			wavePtr+=4;
 			++nFilled;
@@ -469,6 +491,9 @@ unsigned int RF5C68::AddWaveForNumSamples(unsigned char waveBuf[],unsigned int n
 			}
 		}
 		state.timeBalance+=outSamplingRate;
+
+		state.Lout_prev=Lout;
+		state.Rout_prev=Rout;
 	}
 
 	for(unsigned int chNum=0; chNum<NUM_CHANNELS; ++chNum)

diff --git a/src/rf5c68/rf5c68.h b/src/rf5c68/rf5c68.h
@@ -26,10 +26,10 @@ class RF5C68
 public:
 	enum
 	{
-		SAMPLING_RATE=20725,  // Supposed to be 19200.  Where is this 20725 come from?
+		SAMPLING_RATE=20833,  // Supposed to be 19200.  Where is this 20833 come from? The master clock of the RF5C68 in FM TOWNS is 8 MHz, and dividing it by 384 gives 20.833 Hz.
 		WAVERAM_SIZE=65536,
 		NUM_CHANNELS=8,
-		FREQ=20725,
+		FREQ=20833,
 		FD_BIT_SHIFT=11,
 		BANK_SHIFT=12, // 2^12 bytes per bank.
 		LOOP_STOP_CODE=0xFF,
@@ -78,19 +78,23 @@ class RF5C68
 		std::vector <unsigned char> waveRAM;
 		Channel ch[NUM_CHANNELS];
 
+		int timeBalance;       // For WAV generation.
+		int volume=WAVE_OUTPUT_AMPLITUDE_MAX_DEFAULT;
+
+		// For Linear interpolation
+		int Lout_prev;
+		int Rout_prev;
+
 		bool playing;          // Bit 7 of I/O 04F7H
 		unsigned short Bank;   // Bank x000H
 		unsigned char CB;      // Channel
 		unsigned char chOnOff; // I/O 04F8H
-		int timeBalance;       // For WAV generation.
 
 		inline bool IRQ() const
 		{
 			return 0!=IRQBank;
 		}
 		unsigned char IRQBank,IRQBankMask;
-
-		int volume=WAVE_OUTPUT_AMPLITUDE_MAX_DEFAULT;
 	};
 	State state;
 	bool chMute[NUM_CHANNELS]={false,false,false,false,false,false,false,false};