1 /***************************************************************************
2
3 sn76496.c
4
5 Routines to emulate the Texas Instruments SN76489 / SN76496 programmable
6 tone /noise generator. Also known as (or at least compatible with) TMS9919.
7
8 Noise emulation is not accurate due to lack of documentation. The noise
9 generator uses a shift register with a XOR-feedback network, but the exact
10 layout is unknown. It can be set for either period or white noise; again,
11 the details are unknown.
12
13 28/03/2005 : Sebastien Chevalier
14 Update th SN76496Write func, according to SN76489 doc found on SMSPower.
15 - On write with 0x80 set to 0, when LastRegister is other then TONE,
16 the function is similar than update with 0x80 set to 1
17 ***************************************************************************/
18
19 #ifndef __GNUC__
20 #pragma warning (disable:4244)
21 #endif
22
23 #include "sn76496.h"
24
25 #define MAX_OUTPUT 0x47ff // was 0x7fff
26
27 #define STEP 0x10000
28
29
30 /* Formulas for noise generator */
31 /* bit0 = output */
32
33 /* noise feedback for white noise mode (verified on real SN76489 by John Kortink) */
34 #define FB_WNOISE 0x14002 /* (16bits) bit16 = bit0(out) ^ bit2 ^ bit15 */
35
36 /* noise feedback for periodic noise mode */
37 //#define FB_PNOISE 0x10000 /* 16bit rorate */
38 #define FB_PNOISE 0x08000 /* JH 981127 - fixes Do Run Run */
39
40 /*
41 0x08000 is definitely wrong. The Master System conversion of Marble Madness
42 uses periodic noise as a baseline. With a 15-bit rotate, the bassline is
43 out of tune.
44 The 16-bit rotate has been confirmed against a real PAL Sega Master System 2.
45 Hope that helps the System E stuff, more news on the PSG as and when!
46 */
47
48 /* noise generator start preset (for periodic noise) */
49 #define NG_PRESET 0x0f35
50
51
52 struct SN76496
53 {
54 //sound_stream * Channel;
55 int SampleRate;
56 unsigned int UpdateStep;
57 int VolTable[16]; /* volume table */
58 int Register[8]; /* registers */
59 int LastRegister; /* last register written */
60 int Volume[4]; /* volume of voice 0-2 and noise */
61 unsigned int RNG; /* noise generator */
62 int NoiseFB; /* noise feedback mask */
63 int Period[4];
64 int Count[4];
65 int Output[4];
66 int pad[1];
67 };
68
69 static struct SN76496 ono_sn; // one and only SN76496
70 int *sn76496_regs;
71
72 //static
SN76496Write(int data)73 void SN76496Write(int data)
74 {
75 struct SN76496 *R = &ono_sn;
76 int n, r, c;
77
78 /* update the output buffer before changing the registers */
79 //stream_update(R->Channel,0);
80
81 r = R->LastRegister;
82 if (data & 0x80)
83 r = R->LastRegister = (data & 0x70) >> 4;
84 c = r / 2;
85
86 if (!(data & 0x80) && (r == 0 || r == 2 || r == 4))
87 // data byte (tone only)
88 R->Register[r] = (R->Register[r] & 0x0f) | ((data & 0x3f) << 4);
89 else
90 R->Register[r] = (R->Register[r] & 0x3f0) | (data & 0x0f);
91
92 data = R->Register[r];
93 switch (r)
94 {
95 case 0: /* tone 0 : frequency */
96 case 2: /* tone 1 : frequency */
97 case 4: /* tone 2 : frequency */
98 R->Period[c] = R->UpdateStep * data;
99 if (R->Period[c] == 0) R->Period[c] = R->UpdateStep;
100 if (r == 4)
101 {
102 /* update noise shift frequency */
103 if ((R->Register[6] & 0x03) == 0x03)
104 R->Period[3] = 2 * R->Period[2];
105 }
106 break;
107 case 1: /* tone 0 : volume */
108 case 3: /* tone 1 : volume */
109 case 5: /* tone 2 : volume */
110 case 7: /* noise : volume */
111 R->Volume[c] = R->VolTable[data & 0x0f];
112 break;
113 case 6: /* noise : frequency, mode */
114 n = data;
115 R->NoiseFB = (n & 4) ? FB_WNOISE : FB_PNOISE;
116 n &= 3;
117 /* N/512,N/1024,N/2048,Tone #3 output */
118 R->Period[3] = (n == 3) ? 2 * R->Period[2] : (R->UpdateStep << (5 + n));
119
120 /* reset noise shifter */
121 R->RNG = NG_PRESET;
122 R->Output[3] = R->RNG & 1;
123 break;
124 }
125 }
126
127 /*
128 WRITE8_HANDLER( SN76496_0_w ) { SN76496Write(0,data); }
129 WRITE8_HANDLER( SN76496_1_w ) { SN76496Write(1,data); }
130 WRITE8_HANDLER( SN76496_2_w ) { SN76496Write(2,data); }
131 WRITE8_HANDLER( SN76496_3_w ) { SN76496Write(3,data); }
132 WRITE8_HANDLER( SN76496_4_w ) { SN76496Write(4,data); }
133 */
134
135 //static
SN76496Update(short * buffer,int length,int stereo)136 void SN76496Update(short *buffer, int length, int stereo)
137 {
138 int i;
139 struct SN76496 *R = &ono_sn;
140
141 /* If the volume is 0, increase the counter */
142 for (i = 0;i < 4;i++)
143 {
144 if (R->Volume[i] == 0)
145 {
146 /* note that I do count += length, NOT count = length + 1. You might think */
147 /* it's the same since the volume is 0, but doing the latter could cause */
148 /* interferencies when the program is rapidly modulating the volume. */
149 if (R->Count[i] <= length*STEP) R->Count[i] += length*STEP;
150 }
151 }
152
153 while (length > 0)
154 {
155 int vol[4];
156 unsigned int out;
157 int left;
158
159
160 /* vol[] keeps track of how long each square wave stays */
161 /* in the 1 position during the sample period. */
162 vol[0] = vol[1] = vol[2] = vol[3] = 0;
163
164 for (i = 0;i < 3;i++)
165 {
166 if (R->Output[i]) vol[i] += R->Count[i];
167 R->Count[i] -= STEP;
168 /* Period[i] is the half period of the square wave. Here, in each */
169 /* loop I add Period[i] twice, so that at the end of the loop the */
170 /* square wave is in the same status (0 or 1) it was at the start. */
171 /* vol[i] is also incremented by Period[i], since the wave has been 1 */
172 /* exactly half of the time, regardless of the initial position. */
173 /* If we exit the loop in the middle, Output[i] has to be inverted */
174 /* and vol[i] incremented only if the exit status of the square */
175 /* wave is 1. */
176 while (R->Count[i] <= 0)
177 {
178 R->Count[i] += R->Period[i];
179 if (R->Count[i] > 0)
180 {
181 R->Output[i] ^= 1;
182 if (R->Output[i]) vol[i] += R->Period[i];
183 break;
184 }
185 R->Count[i] += R->Period[i];
186 vol[i] += R->Period[i];
187 }
188 if (R->Output[i]) vol[i] -= R->Count[i];
189 }
190
191 left = STEP;
192 do
193 {
194 int nextevent;
195
196 if (R->Count[3] < left) nextevent = R->Count[3];
197 else nextevent = left;
198
199 if (R->Output[3]) vol[3] += R->Count[3];
200 R->Count[3] -= nextevent;
201 if (R->Count[3] <= 0)
202 {
203 if (R->RNG & 1) R->RNG ^= R->NoiseFB;
204 R->RNG >>= 1;
205 R->Output[3] = R->RNG & 1;
206 R->Count[3] += R->Period[3];
207 if (R->Output[3]) vol[3] += R->Period[3];
208 }
209 if (R->Output[3]) vol[3] -= R->Count[3];
210
211 left -= nextevent;
212 } while (left > 0);
213
214 out = vol[0] * R->Volume[0] + vol[1] * R->Volume[1] +
215 vol[2] * R->Volume[2] + vol[3] * R->Volume[3];
216
217 if (out > MAX_OUTPUT * STEP) out = MAX_OUTPUT * STEP;
218
219 if ((out /= STEP)) // will be optimized to shift; max 0x47ff = 18431
220 *buffer += out;
221 if(stereo) buffer+=2; // only left for stereo, to be mixed to right later
222 else buffer++;
223
224 length--;
225 }
226 }
227
228
SN76496_set_clock(struct SN76496 * R,int clock)229 static void SN76496_set_clock(struct SN76496 *R,int clock)
230 {
231
232 /* the base clock for the tone generators is the chip clock divided by 16; */
233 /* for the noise generator, it is clock / 256. */
234 /* Here we calculate the number of steps which happen during one sample */
235 /* at the given sample rate. No. of events = sample rate / (clock/16). */
236 /* STEP is a multiplier used to turn the fraction into a fixed point */
237 /* number. */
238 R->UpdateStep = ((double)STEP * R->SampleRate * 16) / clock;
239 }
240
241
SN76496_set_gain(struct SN76496 * R,int gain)242 static void SN76496_set_gain(struct SN76496 *R,int gain)
243 {
244 int i;
245 double out;
246
247
248 gain &= 0xff;
249
250 /* increase max output basing on gain (0.2 dB per step) */
251 out = MAX_OUTPUT / 3;
252 while (gain-- > 0)
253 out *= 1.023292992; /* = (10 ^ (0.2/20)) */
254
255 /* build volume table (2dB per step) */
256 for (i = 0;i < 15;i++)
257 {
258 /* limit volume to avoid clipping */
259 if (out > MAX_OUTPUT / 3) R->VolTable[i] = MAX_OUTPUT / 3;
260 else R->VolTable[i] = out;
261
262 out /= 1.258925412; /* = 10 ^ (2/20) = 2dB */
263 }
264 R->VolTable[15] = 0;
265 }
266
267
268 //static
SN76496_init(int clock,int sample_rate)269 int SN76496_init(int clock,int sample_rate)
270 {
271 struct SN76496 *R = &ono_sn;
272 int i;
273
274 //R->Channel = stream_create(0,1, sample_rate,R,SN76496Update);
275 sn76496_regs = R->Register;
276
277 R->SampleRate = sample_rate;
278 SN76496_set_clock(R,clock);
279
280 for (i = 0;i < 4;i++) R->Volume[i] = 0;
281
282 R->LastRegister = 0;
283 for (i = 0;i < 8;i+=2)
284 {
285 R->Register[i] = 0;
286 R->Register[i + 1] = 0x0f; /* volume = 0 */
287 }
288
289 for (i = 0;i < 4;i++)
290 {
291 R->Output[i] = 0;
292 R->Period[i] = R->Count[i] = R->UpdateStep;
293 }
294 R->RNG = NG_PRESET;
295 R->Output[3] = R->RNG & 1;
296
297 // added
298 SN76496_set_gain(R, 0);
299
300 return 0;
301 }
302
303