New code for Y-3-8912 emulation

src/spk_ay.c

@@ -16,6 +16,8 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  * 
+ * The AY white noise RNG algorithm is based on info from MAME's ay8910.c -
+ * MAME's licence explicitly permits free use of info (even encourages it).
  */
 
 #include "emulator.h"
@@ -26,20 +28,52 @@
 
 inline void play_ay (unsigned int tstados) {
 	
+	static unsigned int noise = 1; // init value assigned on AY chip reset
+
+	static unsigned int env_period, noise_period, tone_period_a,tone_period_b,tone_period_c;
+
+/* AY output doesn't match the claimed levels; these levels are based
+ * on the measurements posted to comp.sys.sinclair in Dec 2001 by
+ * Matthew Westcott, adjusted as Philip Kendall described in a followup to his post,
+ * then scaled to 0..0xff. Taken from FUSE.
+ */
+ 
+	static const unsigned int levels[16] = {
+    0x00, 0x03, 0x05, 0x07,
+    0x0A, 0x0F, 0x15, 0x23,
+    0x2B, 0x43, 0x5A, 0x73,
+    0x92, 0xAF, 0xD9, 0xFF
+  };
+  
 	if (!ordenador.ay_emul)
 		return;
 
 	ordenador.tst_ay += tstados;
 	ordenador.tst_ay2 += tstados;
 	
-	if (ordenador.tst_ay2 > 255) {
+	// A note about the period of tones, noise and envelope: careful studies of the chip
-		ordenador.tst_ay2 -= 256;
+    // output prove that it counts up from 0 until the counter becomes
-		if ((ordenador.ay_registers[11])
+    // greater or equal to the period. This is an important when the
-		    || (ordenador.ay_registers[12])) {
+    // program is rapidly changing the period to modulate the sound.
-			if (ordenador.aych_envel)
+    // Also, note that period = 0 is the same as period = 1. This is mentioned
-				ordenador.aych_envel--;
+    // in the YM2203 data sheets. However, this does NOT apply to the Envelope
+    // period. In that case, period = 0 is half as period = 1.
+	
+	//The frequency of AY-3-8912 is half the ZX Spectrum frequency
+	
+	//Envelope
+	//Envelope frequency is 1/(256*envelop_period) of AY-3-8912 frequency
+	
+	if (ordenador.tst_ay2 > 127) {
+		ordenador.tst_ay2 -= 128;
+		
+		env_period=2*((unsigned int) ordenador.ay_registers[11]) + 256 * ((unsigned int) (ordenador.ay_registers[12]));
+		if (!env_period) env_period = 1;
+		
+			if (ordenador.aych_envel<env_period) // to check
+				ordenador.aych_envel++;
 			else {
-				ordenador.aych_envel = (((unsigned int) ordenador.ay_registers[11]) + 256 * ((unsigned int) (ordenador.ay_registers[12])));
+				ordenador.aych_envel = 0;
 				if (ordenador.ay_envel_way & 0x02)	// start cycle?
 					switch ((ordenador.
 						 ay_registers[13]) & 0x0F)
@@ -59,8 +93,8 @@ inline void play_ay (unsigned int tstados) {
 						ordenador.ay_envel_way = 5;	// cycle started and incrementing
 						ordenador.ay_envel_value = -1;
 					}
-				if (ordenador.ay_envel_way & 0x04)
+				if (ordenador.ay_envel_way & 0x04) // cycle started?
-				{	// cycle started?
+				{	
 					switch ((ordenador.
 						 ay_registers[13]) & 0x0F)
 					{
@@ -99,16 +133,16 @@ inline void play_ay (unsigned int tstados) {
 
 					case 10:
 					case 14:
-						if (ordenador.ay_envel_way & 0x01)
+						if (ordenador.ay_envel_way & 0x01) //incrementing?
 							ordenador.ay_envel_value++;
 						else
 							ordenador.ay_envel_value--;
 						if (ordenador.ay_envel_value == 16) {
-							ordenador.ay_envel_value = 14;
+							ordenador.ay_envel_value = 15;
 							ordenador.ay_envel_way =4;
 						}
 						if (ordenador.ay_envel_value == -1) {
-							ordenador.ay_envel_value = 1;
+							ordenador.ay_envel_value = 0;
 							ordenador.ay_envel_way = 5;
 						}
 						break;
@@ -135,140 +169,115 @@ inline void play_ay (unsigned int tstados) {
 					}
 				}
 			}
-		}
+		
-		else
-			ordenador.ay_envel_value = 15;
 	}
 
-	while (ordenador.tst_ay >= 16)
+	//Tone and noise
+	//Tone frequency is 1/(16*tone_period) of AY-3-8912 frequency
+	//Noise frequency is 1/(16*noise_period) of AY-3-8912 frequency
+	
+	while (ordenador.tst_ay > 15)
 	{
 		ordenador.tst_ay -= 16;
 		
-		if ((ordenador.ay_registers[0])
+		tone_period_a= ((unsigned int) ordenador.ay_registers[0]) + 256 * ((unsigned int) ((ordenador.ay_registers[1]) & 0x0F));
-		    || (ordenador.ay_registers[1]))
+		tone_period_b= ((unsigned int) ordenador.ay_registers[2]) + 256 * ((unsigned int) ((ordenador.ay_registers[3]) & 0x0F));
-		{
+		tone_period_c= ((unsigned int) ordenador.ay_registers[4]) + 256 * ((unsigned int) ((ordenador.ay_registers[5]) & 0x0F));
-			if (ordenador.aych_a)
+		noise_period= ((unsigned int) ordenador.ay_registers[6]) & 0x1F;
-				ordenador.aych_a--;
-			else
-			{
-				ordenador.ayval_a = 1 - ordenador.ayval_a;
-				ordenador.aych_a =
-					(((unsigned int) ordenador.
-					  ay_registers[0]) +
-					 256 *
-					 ((unsigned
-					   int) ((ordenador.
-						  ay_registers[1]) & 0x0F))) /
-					2;
-			}
-		}
-		else
-			ordenador.ayval_a = 0;
 	
-		if ((ordenador.ay_registers[2])
-		    || (ordenador.ay_registers[3]))
-		{
-			if (ordenador.aych_b)
-				ordenador.aych_b--;
-			else
-			{
-				ordenador.ayval_b = 1 - ordenador.ayval_b;
-				ordenador.aych_b =
-					(((unsigned int) ordenador.
-					  ay_registers[2]) +
-					 256 *
-					 ((unsigned
-					   int) ((ordenador.
-						  ay_registers[3]) & 0x0F))) /
-					2;
-			}
-		}
-		else
-			ordenador.ayval_b = 0;
 		
-		if ((ordenador.ay_registers[4])
+		if (!tone_period_a) tone_period_a = 1;
-		    || (ordenador.ay_registers[5]))
+		if (!tone_period_b) tone_period_b = 1;
-		{
+		if (!tone_period_c) tone_period_c = 1;
-			if (ordenador.aych_c)
+		if (!noise_period) noise_period = 1;
-				ordenador.aych_c--;
-			else
-			{
-				ordenador.ayval_c = 1 - ordenador.ayval_c;
-				ordenador.aych_c =
-					(((unsigned int) ordenador.
-					  ay_registers[4]) +
-					 256 *
-					 ((unsigned
-					   int) ((ordenador.
-						  ay_registers[5]) & 0x0F))) /
-					2;
-			}
-		}
-		else
-			ordenador.ayval_c = 0;
 	
-		if (ordenador.ay_registers[6])
+		if (ordenador.aych_a<tone_period_a)
-		{
+			ordenador.aych_a++;
-			if (ordenador.aych_n)
-				ordenador.aych_n--;
-			else
-			{
-				ordenador.ayval_n = 1 - ordenador.ayval_n;
-				ordenador.aych_n =
-					((((unsigned int) ordenador.
-					   ay_registers[6]) & 0x1F) +
-					 (rand () % 3)) / 2;
-				if (ordenador.aych_n > 16)
-					ordenador.aych_n = 16;
-			}
-		}
 		else
-			ordenador.ayval_n = 0;
+		{
+			ordenador.ayval_a = !ordenador.ayval_a;
+			ordenador.aych_a =0;		
+		}
+
+		if (ordenador.aych_b<tone_period_b)
+			ordenador.aych_b++;
+		else
+		{
+			ordenador.ayval_b = !ordenador.ayval_b;
+			ordenador.aych_b =0;			
+		}
+		
+		if (ordenador.aych_c<tone_period_c)
+			ordenador.aych_c++;
+		else
+		{
+			ordenador.ayval_c = !ordenador.ayval_c;
+			ordenador.aych_c =0;			
+		}
+		
+	
+		if (ordenador.aych_n<noise_period)
+			ordenador.aych_n++;
+		else
+		{
+			
+			//from FUSE
+			
+			//rng is 17-bit shift reg, bit 0 is output.
+			//input is bit 0 xor bit 2.
+			
+			/*
+			if( ( noise & 1 ) ^ ( ( noise & 2 ) ? 1 : 0 ) ) ordenador.ayval_n = !ordenador.ayval_n;
+	
+			noise |= ( ( noise & 1 ) ^ ( ( noise & 4 ) ? 1 : 0 ) ) ? 0x20000 : 0;
+			noise >>= 1;
+			*/
+			
+			//From MAME AY
+			/* The Random Number Generator of the 8910 is a 17-bit shift */
+			/* register. The input to the shift register is bit0 XOR bit3 */
+			/* (bit0 is the output). This was verified on AY-3-8910 and YM2149 chips. */
+
+			/* The following is a fast way to compute bit17 = bit0^bit3. */
+			/* Instead of doing all the logic operations, we only check */
+			/* bit0, relying on the fact that after three shifts of the */
+			/* register, what now is bit3 will become bit0, and will */
+			/* invert, if necessary, bit14, which previously was bit17. */
+			
+			if ((noise+1)&2) ordenador.ayval_n = !ordenador.ayval_n; //xor bit 1 and 2
+			
+			if( noise & 1 ) {          
+			noise ^= 0x24000 ;
+			}
+			noise >>= 1 ; 
+			
+			ordenador.aych_n =0;
+		}	
+
+		// Volume
+		//Each channel max 51
 		
 		if (ordenador.ay_registers[8] & 0x10)
 			ordenador.vol_a =
-				(unsigned
+				(unsigned char) (levels[ordenador.ay_envel_value]*(unsigned int) ordenador.volume/80);
-				 char) ((((unsigned int) ordenador.
-					  ay_envel_value)) *
-					(unsigned int) ordenador.volume) / 10;
 		else
 			ordenador.vol_a =
-				(unsigned
+				(unsigned char) (levels[ordenador.ay_registers[8] &0x0F]*(unsigned int) ordenador.volume/80);
-				 char) ((((unsigned int) (ordenador.
-							  ay_registers[8] &
-							  0x0F)) *
-					 (unsigned int) ordenador.volume) /
-					10);
 
 		if (ordenador.ay_registers[10] & 0x10)
 			ordenador.vol_c =
-				(unsigned
+				(unsigned char) (levels[ordenador.ay_envel_value] *(unsigned int) ordenador.volume/80);
-				 char) ((((unsigned int) ordenador.
-					  ay_envel_value)) *
-					(unsigned int) ordenador.volume) / 10;
 		else
 			ordenador.vol_c =
-				(unsigned
+				(unsigned char) (levels[ordenador.ay_registers[10] & 0x0F] *(unsigned int) ordenador.volume/80);		
-				 char) ((((unsigned int) (ordenador.
-							  ay_registers[10] &
-							  0x0F)) *
-					 (unsigned int) ordenador.volume) /
-					10);
 
 		if (ordenador.ay_registers[9] & 0x10)
 			ordenador.vol_b =
-				(unsigned
+				(unsigned char) (levels[ordenador.ay_envel_value] *(unsigned int) ordenador.volume/80);
-				 char) ((((unsigned int) ordenador.
-					  ay_envel_value)) *
-					(unsigned int) ordenador.volume) / 10;
 		else
 			ordenador.vol_b =
-				(unsigned
+				(unsigned char)(levels[ordenador.ay_registers[9] &0x0F] *(unsigned int) ordenador.volume/80);
-				 char) ((((unsigned int) (ordenador.
+					
-							  ay_registers[9] &
-							  0x0F)) *
-					 (unsigned int) ordenador.volume) /
-					10);
 
 	}
 }
@@ -291,34 +300,40 @@ inline void play_sound (unsigned int tstados) {
 		if (sound_type!=1) //!SOUND_OSS
 			for (bucle = 0; bucle < ordenador.increment; bucle++) {
 				sample_v = ordenador.sample1b[bucle];
-				if ((ordenador.sound_bit) && (sample_v)) 
+				if (ordenador.sound_bit && sample_v) 
+					//Sound bit volume max 96
 					ordenador.sound_current_value=ordenador.volume*6;
 					else ordenador.sound_current_value=0;
 				value = ordenador.sound_current_value;
 				
+				//Mixer
+				
+				// The 8912 has three outputs, each output is the mix of one of the three
+				// tone generators and of the (single) noise generator. The two are mixed
+				// BEFORE going into the DAC. The formula to mix each channel is:
+				// (ToneOn | ToneDisable) & (NoiseOn | NoiseDisable).
+				// Note that this means that if both tone and noise are disabled, the output
+				// is 1, not 0, and can be modulated changing the volume.
+
+				// If the channels are disabled, set their output to 1, and increase the
+				// counter, if necessary, so they will not be inverted during this update.
+				// Setting the output to 1 is necessary because a disabled channel is locked
+				// into the ON state (see above); and it has no effect if the volume is 0.
+				// If the volume is 0, increase the counter, but don't touch the output.
+				
 				if (ordenador.ay_emul) {	// if emulation is ON, emulate it
-					if ((ordenador.ayval_a) && (sample_v)
+				//ordenador.ayval_n = 1;
-						&& (!(ordenador.ay_registers[7] & 0x01)))
+					if (sample_v &&((ordenador.ayval_a || (ordenador.ay_registers[7] & 0x01))&&(ordenador.ayval_n || (ordenador.ay_registers[7] & 0x08))))
 						value += (int) ordenador.vol_a;
-					if ((ordenador.ayval_b) && (sample_v)
+					if (sample_v &&((ordenador.ayval_b || (ordenador.ay_registers[7] & 0x02))&&(ordenador.ayval_n || (ordenador.ay_registers[7] & 0x10))))
-						&& (!(ordenador.ay_registers[7] & 0x02)))
 						value += (int) ordenador.vol_b;
-					if ((ordenador.ayval_c) && (sample_v)
+					if (sample_v &&((ordenador.ayval_c || (ordenador.ay_registers[7] & 0x04))&&(ordenador.ayval_n || (ordenador.ay_registers[7] & 0x20))))
-						&& (!(ordenador.ay_registers[7] & 0x04)))
-						value += (int) ordenador.vol_c;
-					if ((ordenador.ayval_n) && (sample_v)
-						&& (!(ordenador.ay_registers[7] & 0x08)))
-						value += (int) ordenador.vol_a;
-					if ((ordenador.ayval_n) && (sample_v)
-						&& (!(ordenador.ay_registers[7] & 0x10)))
-						value += (int) ordenador.vol_b;
-					if ((ordenador.ayval_n) && (sample_v)
-						&& (!(ordenador.ay_registers[7] & 0x20)))
 						value += (int) ordenador.vol_c;
+
 				}
 				if (value > 255)
 					value = 255;
-				sample_v = (char)(value - (unsigned int)ordenador.sign);
+				sample_v = (unsigned char)(value - (unsigned int)ordenador.sign);
 				*ordenador.current_buffer =	sample_v;	
 				ordenador.current_buffer++;
 			}