Make 6581 caps configurable

src/Filter6581.cpp

@@ -72,4 +72,9 @@ void Filter6581::setFilterRange(double adjustment)
     FilterModelConfig6581::getInstance()->setFilterRange(adjustment);
 }
 
+void Filter6581::enableOldCaps(bool enable)
+{
+    FilterModelConfig6581::getInstance()->enableOldCaps(enable);
+}
+
 } // namespace reSIDfp

src/Filter6581.h

@@ -361,6 +361,13 @@ class Filter6581 final : public Filter
      *                   This also affects the range. Default is 0.5
      */
     void setFilterRange(double adjustment);
+
+    /**
+     * Set filter offset and range based on single parameter.
+     *
+     * @param enable true to enable old 2200pF caps, false to use new 470pF caps.
+     */
+    void enableOldCaps(bool enable);
 };
 
 } // namespace reSIDfp

src/FilterModelConfig.cpp

@@ -41,6 +41,7 @@ FilterModelConfig::FilterModelConfig(
     Vdd(vdd),
     Vth(vth),
     Vddt(Vdd - Vth),
+    uCox(ucox),
     vmin(opamp_voltage[0].x),
     vmax(std::max(Vddt, opamp_voltage[0].y)),
     denorm(vmax - vmin),
@@ -52,7 +53,7 @@ FilterModelConfig::FilterModelConfig(
     volume(new unsigned short[16 * (1 << 16)]),
     resonance(new unsigned short[16 * (1 << 16)])
 {
-    setUCox(ucox);
+    calcCurrFactorCoeff();
 
     // Convert op-amp voltage transfer to 16 bit values.
 
@@ -87,9 +88,8 @@ FilterModelConfig::~FilterModelConfig()
     delete [] resonance;
 }
 
-void FilterModelConfig::setUCox(double new_uCox)
+void FilterModelConfig::calcCurrFactorCoeff()
 {
-    uCox = new_uCox;
     currFactorCoeff = denorm * (uCox / 2. * 1.0e-6 / C);
 }
 

src/FilterModelConfig.h

@@ -168,7 +168,7 @@ class FilterModelConfig
 
     ~FilterModelConfig();
 
-    void setUCox(double new_uCox);
+    void calcCurrFactorCoeff();
 
     virtual double getVoiceDC(unsigned int env) const = 0;
 

src/FilterModelConfig6581.cpp

@@ -1,7 +1,7 @@
 /*
  * This file is part of libsidplayfp, a SID player engine.
  *
- * Copyright 2011-2024 Leandro Nini <drfiemost@users.sourceforge.net>
+ * Copyright 2011-2026 Leandro Nini <drfiemost@users.sourceforge.net>
  * Copyright 2007-2010 Antti Lankila
  * Copyright 2010 Dag Lem
  *
@@ -83,6 +83,9 @@ constexpr Spline::Point opamp_voltage[OPAMP_SIZE] =
   { 10.31,  0.81 },  // Approximate end of actual range
 };
 
+constexpr double CAPS_OLD = 2200e-12; // ASSY 326298 uses 2200pF caps
+constexpr double CAPS_NEW =  470e-12; // Standard caps used on other ASSY
+
 std::unique_ptr<FilterModelConfig6581> FilterModelConfig6581::instance(nullptr);
 
 std::once_flag flag6581;
@@ -110,13 +113,32 @@ void FilterModelConfig6581::setFilterRange(double adjustment)
     if (std::abs(uCox - new_uCox) < 1e-12)
         return;
 
-    setUCox(new_uCox);
+    uCox = new_uCox;
+    updateParams();
+}
+
+void FilterModelConfig6581::enableOldCaps(bool enable)
+{
+    if (m_oldCaps == enable)
+        return;
+
+    m_oldCaps = enable;
+    updateParams();
+}
+
+void FilterModelConfig6581::updateParams()
+{
+    calcCurrFactorCoeff();
+
+    double caps_mult = m_oldCaps ? CAPS_NEW/CAPS_OLD : 1.0;
+    currFactorCoeff *= caps_mult;
+    vcr_mult = uCox * caps_mult;
 }
 
 FilterModelConfig6581::FilterModelConfig6581() :
     FilterModelConfig(
         1.5,                    // voice voltage range FIXME should theoretically be ~3,571V
-        470e-12,                // capacitor value - FIXME ASSY 326298 uses 2200pF caps
+        CAPS_NEW,               // capacitor value
         12. * VOLTAGE_SKEW,     // Vdd
         1.31,                   // Vth
         20e-6,                  // uCox
@@ -127,8 +149,11 @@ FilterModelConfig6581::FilterModelConfig6581() :
     WL_snake(1.0 / 115.0),
     dac_zero(6.65),
     dac_scale(2.63),
-    dac(DAC_BITS)
+    dac(DAC_BITS),
+    m_oldCaps(false)
 {
+    updateParams();
+
     dac.kinkedDac(MOS6581);
 
     {

src/FilterModelConfig6581.h

@@ -1,7 +1,7 @@
 /*
  * This file is part of libsidplayfp, a SID player engine.
  *
- * Copyright 2011-2025 Leandro Nini <drfiemost@users.sourceforge.net>
+ * Copyright 2011-2026 Leandro Nini <drfiemost@users.sourceforge.net>
  * Copyright 2007-2010 Antti Lankila
  * Copyright 2004,2010 Dag Lem
  *
@@ -75,12 +75,17 @@ class FilterModelConfig6581 final : public FilterModelConfig
     double vcr_n_Ids_term[1 << 16];
     //@}
 
+    double vcr_mult;
+    bool m_oldCaps;
+
     // Voice DC offset LUT
     double voiceDC[256];
 
 private:
     double getDacZero(double adjustment) const { return dac_zero + 3. * adjustment - 1.; }
 
+    void updateParams();
+
     FilterModelConfig6581();
     ~FilterModelConfig6581() = default;
 
@@ -99,6 +104,8 @@ class FilterModelConfig6581 final : public FilterModelConfig
 
     void setFilterRange(double adjustment);
 
+    void enableOldCaps(bool enable);
+
     /**
      * Construct an 11 bit cutoff frequency DAC output voltage table.
      * Ownership is transferred to the requester which becomes responsible
@@ -114,7 +121,7 @@ class FilterModelConfig6581 final : public FilterModelConfig
     inline unsigned short getVcr_nVg(int i) const { return vcr_nVg[i]; }
     inline unsigned short getVcr_n_Ids_term(int i) const
     {
-        return to_ushort(vcr_n_Ids_term[i] * uCox);
+        return to_ushort(vcr_n_Ids_term[i] * vcr_mult);
     }
     // only used if SLOPE_FACTOR is defined
     static inline constexpr double getUt() { return Ut; }

src/SID.cpp

@@ -178,6 +178,11 @@ void SID::enableFilter(bool enable)
     filter8580->enable(enable);
 }
 
+void SID::enableOld6581caps(bool enable)
+{
+    filter6581->enableOldCaps(enable);
+}
+
 void SID::voiceSync(bool sync)
 {
     if (sync)

src/SID.h

@@ -264,7 +264,7 @@ class SID
     *
     * @see Filter6581::setFilterRange(double)
     */
-    void setFilter6581Range ( double adjustment );
+    void setFilter6581Range(double adjustment);
 
     /**
      * Set filter curve parameter for 8580 model.
@@ -279,6 +279,11 @@ class SID
      * @param enable false to turn off filter emulation
      */
     void enableFilter(bool enable);
+
+    /**
+     * Enable/disable old caps (2200pF) for 6581 model.
+     */
+    void enableOld6581caps(bool enable);
 };
 
 } // namespace reSIDfp

src/residfp/residfp.cpp

@@ -131,3 +131,8 @@ void residfp::enableFilter(bool enable)
 {
     sid.enableFilter(enable);
 }
+
+void residfp::enableOld6581caps(bool enable)
+{
+    sid.enableOld6581caps(enable);
+}

src/residfp/residfp.h

@@ -175,7 +175,7 @@ class RESIDFP_EXTERN residfp
     void setFilter6581Curve(double filterCurve);
 
     /**
-    * Set filter range parameter for 6581 model
+    * Set filter range parameter for 6581 model.
     *
     * @see Filter6581::setFilterRange(double)
     */
@@ -194,6 +194,14 @@ class RESIDFP_EXTERN residfp
      * @param enable false to turn off filter emulation
      */
     void enableFilter(bool enable);
+
+    /**
+     * Enable/disable old caps for 6581 model.
+     *
+     * @param enable true to enable old 2200pF caps used on ASSY 326298
+     *               false to use the standard 470pF caps.
+     */
+    void enableOld6581caps(bool enable);
 };
 
 }