Add cutoff param smoothing in multimode_filter example

Signed-off-by: Christopher Arndt <chris@chrisarndt.de>

examples/multimode_filter.nim

@@ -2,6 +2,7 @@
 
 import nymph
 
+import paramsmooth
 import svf
 
 const PluginUri = "urn:nymph:examples:multimode-filter"
@@ -19,6 +20,7 @@ type
         q: ptr cfloat
         mode: ptr cfloat
         svf: FilterSV
+        smoothCutoff: ParamSmooth
 
 
 proc instantiate(descriptor: ptr Lv2Descriptor; sampleRate: cdouble;
@@ -26,6 +28,7 @@ proc instantiate(descriptor: ptr Lv2Descriptor; sampleRate: cdouble;
                  Lv2Handle {.cdecl.} =
      let plug = createShared(SVFPlugin)
      plug.svf = initFilterSV(fmLowPass, sampleRate)
+     plug.smoothCutoff = initParamSmooth(20.0, sampleRate)
      return plug
 
 
@@ -56,12 +59,13 @@ proc activate(instance: Lv2Handle) {.cdecl.} =
 
 proc run(instance: Lv2Handle; nSamples: cuint) {.cdecl.} =
     let plug = cast[ptr SVFPlugin](instance)
+    let cutoff = plug.cutoff[].clamp(16.0, 7_000.0)
     plug.svf.setMode(plug.mode[].int.clamp(0, 3).FilterMode)
-    plug.svf.setCutoff(plug.cutoff[].clamp(16.0, 7_000.0))
     plug.svf.setQ(plug.q[].clamp(0.8, 10.0))
-    plug.svf.calcCoef()
 
     for pos in 0 ..< nSamples:
+        plug.svf.setCutoff(plug.smoothCutoff.process(cutoff))
+        plug.svf.calcCoef()
         plug.output[pos] = plug.svf.process(plug.input[pos])
 
 

examples/paramsmooth.nim

@@ -0,0 +1,35 @@
+## One-pole LPF for smooth parameter changes
+##
+## https://www.musicdsp.org/en/latest/Filters/257-1-pole-lpf-for-smooth-parameter-changes.html
+
+import math
+
+const TwoPi = PI * 2
+
+type
+    ParamSmooth* = object
+        a, b, t, z: float
+        fs: float64
+
+
+proc reset*(self: var ParamSmooth) =
+    self.z = 0.0
+
+
+proc setSampleRate*(self: var ParamSmooth, sampleRate: float64) =
+    if sampleRate != self.fs:
+        self.fs = sampleRate
+        self.a = exp(-TwoPi / (self.t * 0.001 * sampleRate))
+        self.b = 1.0 - self.a
+        self.z = 0.0
+
+
+proc process*(self: var ParamSmooth, sample: float): float =
+    self.z = (sample * self.b) + (self.z * self.a)
+    return self.z
+
+
+proc initParamSmooth*(smoothingTimeMs: float = 20.0, sampleRate: float64 = 48_000.0): ParamSmooth =
+    result.t = smoothingTimeMs
+    result.setSampleRate(sampleRate)
+

examples/svf.nim

@@ -11,15 +11,7 @@ type
 
     FilterSV* = object
         mode: FilterMode
-        cutoff: float
-        q: float
-        lowPass: float
-        hiPass: float
-        bandPass: float
-        bandReject: float
-        a: float
-        b: float
-        maxCutoff: float
+        cutoff, q, lowPass, hiPass, bandPass, bandReject, a, b, maxCutoff: float
         sampleRate: float64
         needs_update: bool