Update sound filter to MAME91 needed by the Namco sound changes

src/sound/filter.h

@@ -53,4 +53,64 @@ static INLINE void filter_insert(filter* f, filter_state* s, filter_real x) {
 /* Compute the filter output */
 filter_real filter_compute(filter* f, filter_state* s);
 
+
+/* Filter types */
+#define FILTER_LOWPASS		0
+#define FILTER_HIGHPASS		1
+#define FILTER_BANDPASS		2
+
+#define Q_TO_DAMP(q)	(1.0/q)
+
+
+struct filter2_context
+{
+	double x0, x1, x2;	/* x[k], x[k-1], x[k-2], current and previous 2 input values */
+	double y0, y1, y2;	/* y[k], y[k-1], y[k-2], current and previous 2 output values */
+	double a1, a2;		/* digital filter coefficients, denominator */
+	double b0, b1, b2;	/* digital filter coefficients, numerator */
+};
+
+
+/* Setup the filter context based on the passed filter type info.
+ * type - 1 of the 3 defined filter types
+ * fc   - center frequency
+ * d    - damp = 1/Q
+ * gain - overall filter gain. Set to 1 if not needed.
+ */
+void filter2_setup(int type, double fc, double d, double gain,
+					struct filter2_context *filter);
+
+
+/* Reset the input/output voltages to 0. */
+void filter2_reset(struct filter2_context *filter);
+
+
+/* Step the filter.
+ * x0 is the new input, which needs to be set before stepping.
+ * y0 is the new filter output.
+ */
+void filter2_step(struct filter2_context *filter);
+
+
+/* Setup a filter2 structure based on an op-amp multipole bandpass circuit.
+ * NOTE: If r2 is not used then set to 0.
+ *       vRef is not needed to setup filter.
+ *
+ *                             .--------+---------.
+ *                             |        |         |
+ *                            --- c1    Z         |
+ *                            ---       Z r3      |
+ *                             |        Z         |
+ *            r1               |  c2    |  |\     |
+ *   In >----ZZZZ----+---------+--||----+  | \    |
+ *                   Z                  '--|- \   |
+ *                   Z r2                  |   >--+------> out
+ *                   Z                  .--|+ /
+ *                   |                  |  | /
+ *                  gnd        vRef >---'  |/
+ *
+ */
+void filter_opamp_m_bandpass_setup(double r1, double r2, double r3, double c1, double c2,
+					struct filter2_context *filter);
+
 #endif