Clamp PLL phase error to (-M_PI, +M_PI) (#94)

After a period of digital silence, the PLL sometimes suddenly jumps
frequencies and can't recover. This was caused by a large transient
value of phase error (on the order of 10^5). Clamping the phase error
seems to improve this.

CHANGES.md

@@ -20,6 +20,7 @@
   (#77)
 * Fix an off-by-one bug in the RadioText decoder that sometimes caused missing
   characters at the end of messages
+* Fix runaway PLL after digital silence by clamping the phase error (#94)
 
 ## 0.20 (2021-03-08)
 

src/subcarrier.cc

@@ -152,7 +152,10 @@ BitBuffer Subcarrier::processChunk(MPXBuffer<>& chunk) {
       if (symbol.valid) {
         // Modem here is only used to track PLL phase error
         modem_.demodulate(symbol.data);
-        oscillator_.stepPLL(modem_.getPhaseError() * kPLLMultiplier);
+
+        float phase_error = std::min(std::max(modem_.getPhaseError(),
+                            -float(M_PI)), float(M_PI));
+        oscillator_.stepPLL(phase_error * kPLLMultiplier);
 
         auto biphase = biphase_decoder_.push(symbol.data);