From 354cacc1c79e04373a3dfcd61c2f69db92632e34 Mon Sep 17 00:00:00 2001
From: Couriersud <couriersud@users.noreply.github.com>
Date: Sun, 5 Jan 2014 16:18:26 +0000
Subject: [PATCH] Fixed noise period on AY8910. [Dr. Venom, Couriersud]

---
 src/emu/sound/ay8910.c | 49 ++++++++++++++++++++++--------------------
 1 file changed, 26 insertions(+), 23 deletions(-)

diff --git a/src/emu/sound/ay8910.c b/src/emu/sound/ay8910.c
index 707533c5f17..d8407ccb2cc 100644
--- a/src/emu/sound/ay8910.c
+++ b/src/emu/sound/ay8910.c
@@ -9,6 +9,13 @@
 
   Mostly rewritten by couriersud in 2008
 
+  Public documentation:
+
+  - http://privatfrickler.de/blick-auf-den-chip-soundchip-general-instruments-ay-3-8910/
+    Die pictures of the AY8910
+
+  - US Patent 4933980
+
   Games using ADSR: gyruss
 
   A list with more games using ADSR can be found here:
@@ -144,6 +151,7 @@ has twice the steps, happening twice as fast.
 #define TONE_VOLUME(_psg, _chan)    ( (_psg)->regs[AY_AVOL + (_chan)] & 0x0f)
 #define TONE_ENVELOPE(_psg, _chan)  (((_psg)->regs[AY_AVOL + (_chan)] >> 4) & (((_psg)->device->type() == AY8914) ? 3 : 1))
 #define ENVELOPE_PERIOD(_psg)       (((_psg)->regs[AY_EFINE] | ((_psg)->regs[AY_ECOARSE]<<8)))
+#define NOISE_OUTPUT(_psg)          ((_psg)->rng & 1)
 
 /*************************************
  *
@@ -171,7 +179,7 @@ struct ay8910_context
 	INT32 last_enable;
 	INT32 count[NUM_CHANNELS];
 	UINT8 output[NUM_CHANNELS];
-	UINT8 output_noise;
+	UINT8 prescale_noise;
 	INT32 count_noise;
 	INT32 count_env;
 	INT8 env_step;
@@ -485,7 +493,6 @@ static void ay8910_write_reg(ay8910_context *psg, int r, int v)
 				/* write out 0xff if port set to input */
 				psg->portBwrite(0, (psg->regs[AY_ENABLE] & 0x80) ? psg->regs[AY_PORTB] : 0xff);
 			}
-
 			psg->last_enable = psg->regs[AY_ENABLE];
 			break;
 		case AY_ESHAPE:
@@ -577,37 +584,33 @@ static STREAM_UPDATE( ay8910_update )
 			if (psg->count[chan] >= TONE_PERIOD(psg, chan))
 			{
 				psg->output[chan] ^= 1;
-				psg->count[chan] = 0;;
+				psg->count[chan] = 0;
 			}
 		}
 
 		psg->count_noise++;
 		if (psg->count_noise >= NOISE_PERIOD(psg))
 		{
-			/* Is noise output going to change? */
-			if ((psg->rng + 1) & 2) /* (bit0^bit1)? */
-			{
-				psg->output_noise ^= 1;
-			}
+		    /* toggle the prescaler output. Noise is no different to
+		     * channels.
+		     */
+            psg->count_noise = 0;
+		    psg->prescale_noise ^= 1;
 
-			/* 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. */
+		    if ( psg->prescale_noise)
+		    {
+	            /* 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 (psg->rng & 1)
-				psg->rng ^= 0x24000; /* This version is called the "Galois configuration". */
-			psg->rng >>= 1;
-			psg->count_noise = 0;
+	            psg->rng ^= (((psg->rng & 1) ^ ((psg->rng >> 3) & 1)) << 17);
+	            psg->rng >>= 1;
+		    }
 		}
 
 		for (chan = 0; chan < NUM_CHANNELS; chan++)
 		{
-			psg->vol_enabled[chan] = (psg->output[chan] | TONE_ENABLEQ(psg, chan)) & (psg->output_noise | NOISE_ENABLEQ(psg, chan));
+			psg->vol_enabled[chan] = (psg->output[chan] | TONE_ENABLEQ(psg, chan)) & (NOISE_OUTPUT(psg) | NOISE_ENABLEQ(psg, chan));
 		}
 
 		/* update envelope */
@@ -712,7 +715,7 @@ static void ay8910_statesave(ay8910_context *psg, device_t *device)
 	device->save_item(NAME(psg->env_volume));
 
 	device->save_item(NAME(psg->output));
-	device->save_item(NAME(psg->output_noise));
+	device->save_item(NAME(psg->prescale_noise));
 
 	device->save_item(NAME(psg->env_step));
 	device->save_item(NAME(psg->hold));
@@ -804,7 +807,7 @@ void ay8910_reset_ym(void *chip)
 	psg->count[2] = 0;
 	psg->count_noise = 0;
 	psg->count_env = 0;
-	psg->output_noise = 0x01;
+	psg->prescale_noise = 0;
 	psg->last_enable = -1;  /* force a write */
 	for (i = 0;i < AY_PORTA;i++)
 		ay8910_write_reg(psg,i,0);