LowpassOnePole: New "frequency" parameter to set the coefficient usin…

…g the Time Dataspace. Coefficients named mathematically so it is easier to figure out what's happening internally.

includes/library/JamomaLowpassOnePole.h

@@ -16,12 +16,13 @@
 namespace Jamoma {
 
     /**	This AudioObject applies a basic <a href="https://en.wikipedia.org/wiki/Low-pass_filter">low-pass filter</a> to a Sample or SampleBundle.
+		Handy for use in smoothing control signals or damping high frequency components.
      */
     class LowpassOnePole : public AudioObject {
 
-        AdaptingSampleBundle		mY1 = {this};           ///< previous output sample for each channel
-        double                      mCoefficientF;			///< filter coefficient
-        double                      mOneMinusCoefficientF;	///< 1 - mCoefficientF
+        AdaptingSampleBundle		mY1 = {this};           ///< previous output sample (for each channel)
+        double                      mA0;					///< gain coefficient
+        double                      mB1;					///< feedback coefficient
 
     public:
         static constexpr Classname classname = { "lowpass.1" };
@@ -34,12 +35,18 @@ namespace Jamoma {
                                                             0.5,
                                                             Range<double>(0.0, 1.0),
                                                             [this]{
-                                                                mCoefficientF = coefficient;
-                                                                mOneMinusCoefficientF = 1 - coefficient;
+                                                                mA0 = coefficient;
+                                                                mB1 = 1 - coefficient;
                                                             }
         };
 
 
+		/** Set filter coefficient using a cutoff frequency.
+			@see http://musicdsp.org/showArchiveComment.php?ArchiveID=237
+		 */
+		Parameter<double, NativeUnit::Hz> frequency = { this, "frequency", 1000.0, [this]{coefficient = 1.0 - exp(-2.0 * 3.1415 * frequency / sampleRate);} };
+
+
 		/**	This algorithm is an IIR filter, meaning that it relies on feedback.  If the filter should
 			not be producing any signal (such as turning audio off and then back on in a host) or if the
 			feedback has become corrupted (such as might happen if a NaN is fed in) then it may be
@@ -61,7 +68,7 @@ namespace Jamoma {
 
         Sample operator()(Sample x, int channel)
         {
-            Sample y = (x * mCoefficientF) + (mY1[channel][0] * mOneMinusCoefficientF); // compute next output sample
+            Sample y = (x * mA0) + (mY1[channel][0] * mB1); // compute next output sample
 
 			ZeroDenormal(y);
 			mY1[channel][0] = y; // update history

tests/LowpassOnePole/LowpassOnePole.cpp

@@ -132,6 +132,84 @@ class LowpassOnePoleTest {
 
 		my_lowpass.coefficient = 1.2;
 		mTest->TEST_ASSERT("high coefficient limiting", my_lowpass.coefficient == 1.0);
+
+
+		// Testing coefficient setting via frequency
+		// The following Max 7 patcher can be convenient as an interactive reference for the
+		// relationship between frequency response and coefficient
+		/*
+		 
+		 <pre><code>
+		 ----------begin_max5_patcher----------
+		 2115.3oc6asrjiZCEcc2eEJplEcRY6fdAlYURVjJUMS9BRkpKYiraxfAG.mz
+		 8L0ju8nm.1FroaaHdQVLF8BoiNb0U2G87k6uCtH6YQAD7dvuAt6tub+c2oaR
+		 0vc152A2vedYBuPOL3FQQAes.NwzWo34Rc6XOOWawQ5VxV7GSCpZrn7kDgtc
+		 WKY6JSDkkurUXVeHD761tR2sINU1odIw1F2xKW9Tb55GyEKKMuBwmMyaB.6o
+		 9kfzkIy7plms4hBQZIuLNKcuWiV+ZteZt3FnoWcjpwud+8pelzSNJU72xM+Q
+		 Tj34s4.zLvTfrzCSwx076.jYHJhIK7tUHv2CnTjm2rusEpze9kSkntoRJSSD
+		 gdFR7Xt7xokSH5H2ysskoCpziOogzCcdekd7I9CpzypjL4jzFcPdMzwD.bAO
+		 c8aVR.gYlGdGQLqxx2v0i1uZZx4aDkh7GEo7EFr4cZZjxLLNB0CdDeEE2Prt
+		 D2PCp3FK3MItwlqE2PgCj3VmzTmmJGVc5Lq.2qkln3A8T41jrx+oE1fE5Zju
+		 KJNak5bPQ7m06ejGlNwMW4qiSs2zp2TyqgKqITuBTH0vcgZ4MeGSx1mO1lEm
+		 ZlIFxMW4REFhD9BQhd+8q70owk6hDfGh9op6jhx1vkvnZT+bt3O2IRW9B3ge
+		 4yUi5.Fuw5JIx81.E6VT0lxJjpO.x9Vlkjka5vaFch6m.l7.Q81QI19T7xOk
+		 JLerTZTbcn2lOVKollk5j0k8lDmJZzopJOutaIbWH0o00auJNQpxS2AuLKZQ
+		 cOFNR9h7b82DuFHx1mHUKDgwdrV5sAlxVez7Jkl9jH2xXtY.XtGUqqvUXtqP
+		 fqfuq.yUf5JPbEvtBHWA2L6lX275lV2r5lT2b5lRbCj4.lCWNX4PkCTNL4fj
+		 CQN.YwiENVzXAiEKVnXQhEHVbXggEEVPXwfEBVDz3ZP45aW9ZiklaWayR6NO
+		 OwdJjzPaF5P8R0eQ0GmbeP0CSZVDrBuFyqqnQH9CvFeuk89AXi4TeLtV3aJh
+		 faHgY50J94cTGceZvz+dxd5keJRq5cJVaU8Ti4QSMptmZXjoAlgL2LjPyPPd
+		 ysuucdT37n8QSpwrcUTgcQgxmvp0FJeBqf.T9DVgDn7IrBPP80XNbAkOgUvC
+		 JeBqPIT9DVCVnp.rFzJ.XvCwAHBt4Gir7Xol+Ge9P1119K11MMatOpSMfAJo
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+		 dyBaiiXCJGgoM4Hb3LV+3H7bbMoNpjDqMRZXiiGx3n6qmjPFIO+QmjZ8z1vd
+		 oqyqlNIoAKr2emzLq1t+j9ZtkQdWk5pfW84Gjc6Z7EiL.2h10sFRyjAayRDf
+		 jr+dqbv.qoxGSDjvK+1CpKv8l8q01g46caZZYUfolOyavuUAAdWax4jfgUio
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+		 cEroBQWg.fibHg6RyCE+ehlGeS3+5slmNSY81m3EBvC47nXdZQm4rNUonOQ5
+		 MTDnJ5d2RZiFnTz7V0EgtTUQVSEQUmBIUrj9Ln+DW1szI25nbacFsQUGpf1o
+		 UVlBsSrrrOzN0xxpj2gLuK5jIMcnxLjKcoFUnSsoI0jyTSdTOVabmYBUq4wl
+		 3QiZHaJLo5J1LX5qqXye4b.rZ0UYyDre1TGcUQcXyuQrMRTxiSZyhwpvHnBr
+		 QpzHMcW+XdLO4UZL4Q+6sZXIkpeD1VpLZD7ERGAew+5QdQwKKmsU4SYwxmDa
+		 3uG3nyh30o.45Jx4kY4uGDIRyjaRSkenT9ccsTREf.31Hc+qBoqPmzwRd9Kc
+		 P0jyS017hgCeabcS2zzuo9j6A+kPqWdU66+AnHaW9R2dw8mlKnF.RRtTwnxs
+		 XyAQ1aPOEGIo9ltSDEWn7vHpamq5KdB75AdXdiFdX8AOGPhCId7o2V7yAhFs
+		 iG73AGzMF8biINqt5473Ib7NdguwNtStsvC0+l53kJkw2RvgdaAmaKcgz9n6
+		 gLd3AG1C7PGuyVj9nKTY1H.ON3g1W7fFG7zGcyGPhCp7SefC8lBN3w6lTbut
+		 oHXbszfdtOW9mAOahipibl5uoHlJ7Kz.cjBBo5+TPz0pWnq6N.cN8U3W2N.E
+		 FpBVDMP67muD5LasytCL95w2t8ujd4ZQgF7R2p+CSP7BlnqFmZppc6GlK9qX
+		 230nBxyktlVJ8KcWtwc2ms+2UDtIKRjmtK15moj1t25V7A9WV4.cIeqwYYIA
+		 shuKobeVsoy1RJ6iRm0AeLd8SprYZ1aSreTxERR5.GvgajSXriHzAo4q2+u.
+		 80vVA
+		 -----------end_max5_patcher-----------
+		 </code></pre>
+		 
+		 */
+
+		my_lowpass.sampleRate = 96000;
+
+		my_lowpass.frequency = 1000.0; // hz
+		mTest->TEST_ASSERT("coefficient for 1K cutoff @ 96K fs", mTest->compare((double)my_lowpass.coefficient, 0.06335217076965427));
+
+		my_lowpass.frequency = 4000.0; // hz
+		mTest->TEST_ASSERT("coefficient for 1K cutoff @ 96K fs", mTest->compare((double)my_lowpass.coefficient, 0.23032864479520065));
+
+		my_lowpass.sampleRate = 44100;
+
+		my_lowpass.frequency = 1000.0; // hz
+		mTest->TEST_ASSERT("coefficient for 1K cutoff @ 44.1K fs", mTest->compare((double)my_lowpass.coefficient, 0.132787865213287));
+
+		my_lowpass.frequency = 4000.0; // hz
+		mTest->TEST_ASSERT("coefficient for 1K cutoff @ 44.1K fs", mTest->compare((double)my_lowpass.coefficient, 0.43441043913502342));
+
 	}
 
 };