Improved UI

internal/datasource/synthetic/dsp_bridge.go

@@ -99,6 +99,16 @@ func buildNode(d NodeDef) (dsp.Node, error) {
 	case "expr":
 		return &dsp.ExprNode{Expr: stringParam(p, "expr")}, nil
 
+	case "lowpass":
+		order := int(floatParam(p, "order"))
+		if order < 1 {
+			order = 1
+		}
+		return &dsp.LowPassNode{
+			Freq:  floatParam(p, "freq"),
+			Order: order,
+		}, nil
+
 	case "lua":
 		return &dsp.LuaNode{Script: stringParam(p, "script")}, nil
 

internal/dsp/nodes.go

@@ -535,6 +535,68 @@ func (p *exprParser) parseFactor() (float64, error) {
 	return 0, fmt.Errorf("unexpected character %q at position %d", ch, p.pos)
 }
 
+// ── LowPassNode ───────────────────────────────────────────────────────────────
+
+// LowPassNode implements a cascaded first-order IIR (RC) low-pass filter.
+// The filter coefficient α is recomputed each sample from the elapsed time so
+// it is correct for event-driven (non-fixed-rate) signals.
+// For Order > 1 the single-pole stage is cascaded Order times, approximating
+// a Butterworth response with -20·Order dB/decade roll-off.
+type LowPassNode struct {
+	Freq  float64 // cutoff frequency in Hz (–3 dB point)
+	Order int     // number of cascaded stages (1–8)
+}
+
+func (n *LowPassNode) Type() string { return "lowpass" }
+
+func (n *LowPassNode) Process(inputs []float64, state map[string]any) (float64, error) {
+	if len(inputs) == 0 {
+		return 0, errors.New("lowpass: no inputs")
+	}
+	x := inputs[0]
+	now := time.Now()
+
+	order := n.Order
+	if order < 1 {
+		order = 1
+	}
+	if order > 8 {
+		order = 8
+	}
+	fc := n.Freq
+	if fc <= 0 {
+		fc = 1.0
+	}
+
+	prevT, hasT := state["t"].(time.Time)
+	if !hasT {
+		// First sample: prime all stage states with the input value.
+		for i := range order {
+			state[fmt.Sprintf("y%d", i)] = x
+		}
+		state["t"] = now
+		return x, nil
+	}
+
+	dt := now.Sub(prevT).Seconds()
+	state["t"] = now
+	if dt <= 0 {
+		dt = 1e-9
+	}
+
+	rc := 1.0 / (2.0 * math.Pi * fc)
+	alpha := dt / (rc + dt)
+
+	y := x
+	for i := range order {
+		key := fmt.Sprintf("y%d", i)
+		prev, _ := state[key].(float64)
+		y = alpha*y + (1-alpha)*prev
+		state[key] = y
+	}
+	return y, nil
+}
+
 // ── LuaNode ───────────────────────────────────────────────────────────────────
 
 // LuaNode runs a Lua script in a sandboxed gopher-lua VM.