--- /dev/null
+//
+// Virtual AY-3-8910 Emulator
+//
+// by James Hammons
+// (C) 2018 Underground Software
+//
+// This was written mainly from the General Instruments datasheet for the 8910
+// part. I would have used the one from MAME, but it was so poorly written and
+// so utterly incomprehensible that I decided to start from scratch to see if I
+// could do any better; and so here we are. I *did* use a bit of code from
+// MAME's AY-3-8910 RNG, as it was just too neat not to use. :-)
+//
+
+#include "vay8910.h"
+
+#include <string.h> // for memset()
+#include "log.h"
+#include "sound.h"
+
+
+// AY-3-8910 register IDs
+enum { AY_AFINE = 0, AY_ACOARSE, AY_BFINE, AY_BCOARSE, AY_CFINE, AY_CCOARSE,
+ AY_NOISEPER, AY_ENABLE, AY_AVOL, AY_BVOL, AY_CVOL, AY_EFINE, AY_ECOARSE,
+ AY_ESHAPE, AY_PORTA, AY_PORTB };
+
+// Class variable instantiation/initialization
+float VAY_3_8910::maxVolume = 8192.0f;
+float VAY_3_8910::normalizedVolume[16];// = {};
+
+
+VAY_3_8910::VAY_3_8910()
+{
+ // Our normalized volume levels are from 0 to -48 dB, in 3 dB steps.
+ // N.B.: It's 3dB steps because those sound the best. Dunno what it really
+ // is, as nothing in the documentation tells you (it only says that
+ // each channel's volume is normalized from 0 to 1.0V).
+ float level = 1.0f;
+
+ for(int i=15; i>=0; i--)
+ {
+ normalizedVolume[i] = level;
+ level /= 1.4125375446228; // 10.0 ^ (3.0 / 20.0) = 3 dB
+ }
+
+ // In order to get a scale that goes from 0 to 1 smoothly, we renormalize
+ // our volumes so that volume[0] is actually 0, and volume[15] is 1.
+ // Basically, we're sliding the curve down the Y-axis so that volume[0]
+ // touches the X-axis, then stretching the result so that it fits into the
+ // interval (0, 1).
+ float vol0 = normalizedVolume[0];
+ float vol15 = normalizedVolume[15] - vol0;
+
+ for(int i=0; i<16; i++)
+ normalizedVolume[i] = (normalizedVolume[i] - vol0) / vol15;
+
+#if 0
+ WriteLog("\nRenormalized volume, level (max=%d):\n", (int)maxVolume);
+ for(int i=0; i<16; i++)
+ WriteLog("%lf, %d\n", normalizedVolume[i], (int)(normalizedVolume[i] * maxVolume));
+ WriteLog("\n");
+#endif
+}
+
+
+void VAY_3_8910::Reset(void)
+{
+ memset(this, 0, sizeof(struct VAY_3_8910));
+ prng = 1; // Set correct PRNG seed
+}
+
+
+void VAY_3_8910::WriteControl(uint8_t value)
+{
+ if ((value & 0x04) == 0)
+ Reset();
+ else if ((value & 0x03) == 0x03)
+ regLatch = data;
+ else if ((value & 0x03) == 0x02)
+ SetRegister();
+}
+
+
+void VAY_3_8910::WriteData(uint8_t value)
+{
+ data = value;
+}
+
+
+void VAY_3_8910::SetRegister(void)
+{
+#if 0
+static char regname[16][32] = {
+ "AY_AFINE ",
+ "AY_ACOARSE ",
+ "AY_BFINE ",
+ "AY_BCOARSE ",
+ "AY_CFINE ",
+ "AY_CCOARSE ",
+ "AY_NOISEPER",
+ "AY_ENABLE ",
+ "AY_AVOL ",
+ "AY_BVOL ",
+ "AY_CVOL ",
+ "AY_EFINE ",
+ "AY_ECOARSE ",
+ "AY_ESHAPE ",
+ "AY_PORTA ",
+ "AY_PORTB "
+};
+WriteLog("*** AY(%d) Reg: %s = $%02X\n", chipNum, regname[reg], value);
+#endif
+ uint16_t value = (uint16_t)data;
+
+ switch (regLatch)
+ {
+ case AY_AFINE:
+ // The square wave period is the passed in value times 16, so we handle
+ // that here.
+ period[0] = (period[0] & 0xF000) | (value << 4);
+ break;
+ case AY_ACOARSE:
+ period[0] = ((value & 0x0F) << 12) | (period[0] & 0xFF0);
+ break;
+ case AY_BFINE:
+ period[1] = (period[1] & 0xF000) | (value << 4);
+ break;
+ case AY_BCOARSE:
+ period[1] = ((value & 0x0F) << 12) | (period[1] & 0xFF0);
+ break;
+ case AY_CFINE:
+ period[2] = (period[2] & 0xF000) | (value << 4);
+ break;
+ case AY_CCOARSE:
+ period[2] = ((value & 0x0F) << 12) | (period[2] & 0xFF0);
+ break;
+ case AY_NOISEPER:
+ // Like the square wave period, the value is the what's passed * 16.
+ noisePeriod = (value & 0x1F) << 4;
+ break;
+ case AY_ENABLE:
+ toneEnable[0] = (value & 0x01 ? false : true);
+ toneEnable[1] = (value & 0x02 ? false : true);
+ toneEnable[2] = (value & 0x04 ? false : true);
+ noiseEnable[0] = (value & 0x08 ? false : true);
+ noiseEnable[1] = (value & 0x10 ? false : true);
+ noiseEnable[2] = (value & 0x20 ? false : true);
+ break;
+ case AY_AVOL:
+ volume[0] = value & 0x0F;
+ envEnable[0] = (value & 0x10 ? true : false);
+
+ if (envEnable[0])
+ {
+ envCount[0] = 0;
+ volume[0] = (envAttack ? 0 : 15);
+ envDirection[0] = (envAttack ? 1 : -1);
+ }
+ break;
+ case AY_BVOL:
+ volume[1] = value & 0x0F;
+ envEnable[1] = (value & 0x10 ? true : false);
+
+ if (envEnable[1])
+ {
+ envCount[1] = 0;
+ volume[1] = (envAttack ? 0 : 15);
+ envDirection[1] = (envAttack ? 1 : -1);
+ }
+ break;
+ case AY_CVOL:
+ volume[2] = value & 0x0F;
+ envEnable[2] = (value & 0x10 ? true : false);
+
+ if (envEnable[2])
+ {
+ envCount[2] = 0;
+ volume[2] = (envAttack ? 0 : 15);
+ envDirection[2] = (envAttack ? 1 : -1);
+ }
+ break;
+ case AY_EFINE:
+ // The envelope period is 256 times the passed in value
+ envPeriod = (envPeriod & 0xFF0000) | (value << 8);
+ break;
+ case AY_ECOARSE:
+ envPeriod = (value << 16) | (envPeriod & 0xFF00);
+ break;
+ case AY_ESHAPE:
+ envAttack = (value & 0x04 ? true : false);
+ envAlternate = (value & 0x02 ? true : false);
+ envHold = (value & 0x01 ? true : false);
+
+ // If the Continue bit is *not* set, the Alternate bit is forced to the
+ // Attack bit, and Hold is forced on.
+ if (!(value & 0x08))
+ {
+ envAlternate = envAttack;
+ envHold = true;
+ }
+
+ // Reset all voice envelope counts...
+ for(int i=0; i<3; i++)
+ {
+ envCount[i] = 0;
+ envDirection[i] = (envAttack ? 1 : -1);
+
+ // Only reset the volume if the envelope is enabled!
+ if (envEnable[i])
+ volume[i] = (envAttack ? 0 : 15);
+ }
+ break;
+ }
+}
+
+
+//
+// Generate one sample and quit
+//
+bool logAYInternal = false;
+uint16_t VAY_3_8910::GetSample(void)
+{
+ uint16_t sample = 0;
+
+ // Number of cycles per second to run the PSG is the 6502 clock rate
+ // divided by the host sample rate
+ const static double exactCycles = 1020484.32 / (double)SAMPLE_RATE;
+ static double overflow = 0;
+
+ int fullCycles = (int)exactCycles;
+ overflow += exactCycles - (double)fullCycles;
+
+ if (overflow >= 1.0)
+ {
+ fullCycles++;
+ overflow -= 1.0;
+ }
+
+ for(int i=0; i<fullCycles; i++)
+ {
+ for(int j=0; j<3; j++)
+ {
+ // Tone generators only run if the corresponding voice is enabled.
+ // N.B.: We also reject any period set that is less than 2.
+ if (toneEnable[j] && (period[j] > 16))
+ {
+ count[j]++;
+
+ // It's (period / 2) because one full period of a square wave
+ // is zero for half of its period and one for the other half!
+ if (count[j] > (period[j] / 2))
+ {
+ count[j] = 0;
+ state[j] = !state[j];
+ }
+ }
+
+ // Envelope generator only runs if the corresponding voice flag is
+ // enabled.
+ if (envEnable[j])
+ {
+ envCount[j]++;
+
+ // It's (EP / 16) because there are 16 volume steps in each EP.
+ if (envCount[j] > (envPeriod / 16))
+ {
+ // Attack 0 = \, 1 = / (attack lasts one EP)
+ // Alternate = mirror envelope's last attack
+ // Hold = run 1 EP, hold at level (Alternate XOR Attack)
+ envCount[j] = 0;
+
+ // We've hit a point where we need to make a change to the
+ // envelope's volume, so do it:
+ volume[j] += envDirection[j];
+
+ // If we hit the end of the EP, change the state of the
+ // envelope according to the envelope's variables.
+ if ((volume[j] > 15) || (volume[j] < 0))
+ {
+ // Hold means we set the volume to (Alternate XOR
+ // Attack) and stay there after the Attack EP.
+ if (envHold)
+ {
+ volume[j] = (envAttack != envAlternate ? 15: 0);
+ envDirection[j] = 0;
+ }
+ else
+ {
+ // If the Alternate bit is set, we mirror the
+ // Attack pattern; otherwise we reset it to the
+ // whatever level was set by the Attack bit.
+ if (envAlternate)
+ {
+ envDirection[j] = -envDirection[j];
+ volume[j] += envDirection[j];
+ }
+ else
+ volume[j] = (envAttack ? 0 : 15);
+ }
+ }
+ }
+ }
+ }
+
+ // Noise generator (the PRNG) runs all the time:
+ noiseCount++;
+
+ if (noiseCount > noisePeriod)
+ {
+ noiseCount = 0;
+
+ // The following is from MAME's AY-3-8910 code:
+ // The Pseudo 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 (prng & 0x00001)
+ {
+ // This version is called the "Galois configuration".
+ prng ^= 0x24000;
+ // The noise wave *toggles* when a one shows up in bit0...
+ noiseState = !noiseState;
+ }
+
+ prng >>= 1;
+ }
+ }
+
+ // We mix channels A-C here into one sample, because the Mockingboard just
+ // sums the output of the AY-3-8910 by tying their lines together.
+ // We also handle the various cases (of which there are four) of mixing
+ // pure tones and "noise" tones together.
+ for(int i=0; i<3; i++)
+ {
+ // Set the volume level scaled by the maximum volume (which can be
+ // altered outside of this module).
+ int level = (int)(normalizedVolume[volume[i]] * maxVolume);
+
+ if (toneEnable[i] && !noiseEnable[i])
+ sample += (state[i] ? level : 0);
+ else if (!toneEnable[i] && noiseEnable[i])
+ sample += (noiseState ? level : 0);
+ else if (toneEnable[i] && noiseEnable[i])
+ sample += (state[i] & noiseState ? level : 0);
+ else if (!toneEnable[i] && !noiseEnable[i])
+ sample += level;
+ }
+
+ if (logAYInternal)
+ {
+ WriteLog(" (%d) State A,B,C: %s %s %s, Sample: $%04X, P: $%X, $%X, $%X\n", id, (state[0] ? "1" : "0"), (state[1] ? "1" : "0"), (state[2] ? "1" : "0"), sample, period[0], period[1], period[2]);
+ }
+
+ return sample;
+}
+
projects / apple2 / blobdiff