working on todo.md features

src/math_channel.cpp

@@ -61,6 +61,13 @@ void biquad(MathOp op, double fc, double q, double dt, double& b0, double& b1, d
 /// that actually need carrying between samples.
 double biquad_step(double x, double b0, double b1, double b2, double a1, double a2, double& z1,
                    double& z2) {
+  // A reserved hole is NaN in the source, and a NaN integrated into the state words is
+  // permanent: every later sample would come out NaN, so a filtered trace died at the first
+  // gap and never came back -- which is what a dropped packet did to every low pass in
+  // relative mode, where a counter gap reserves NaN slots by construction. A gap in the input
+  // is a gap in the output (the hole stays visible), and the filter's history is the last
+  // *real* samples it saw, so the state is left alone and the trace resumes after the hole.
+  if (!std::isfinite(x)) return kNaN;
   const double y = b0 * x + z1;
   z1 = b1 * x - a1 * y + z2;
   z2 = b2 * x - a2 * y;