fix: reduce extractor resets and filter WFM audio

internal/demod/gpudemod/streaming_types.go

@@ -3,7 +3,6 @@ package gpudemod
 import (
 	"fmt"
 	"hash/fnv"
-	"math"
 )
 
 type StreamingExtractJob struct {
@@ -49,18 +48,17 @@ func ResetExtractStreamState(state *ExtractStreamState, cfgHash uint64) {
 }
 
 func StreamingConfigHash(signalID int64, offsetHz float64, bandwidth float64, outRate int, numTaps int, sampleRate int) uint64 {
-	// Quantize offset and bandwidth to 1 kHz resolution before hashing.
-	// The detector's exponential smoothing causes CenterHz (and therefore offsetHz)
-	// to jitter by fractions of a Hz every frame. With %.9f formatting, this
-	// produced a new hash every frame → full state reset (NCOPhase=0, History=[],
-	// PhaseCount=0) → FIR settling + phase discontinuity → audible clicks.
+	// Hash only structural parameters that change the FIR/decimation geometry.
+	// Offset is NOT included because the NCO phase_inc tracks it smoothly each frame.
+	// Bandwidth is NOT included because taps are rebuilt every frame in getOrInitExtractState.
+	// A state reset (zeroing NCO phase, history, phase count) is only needed when
+	// decimation factor, tap count, or sample rate changes — all of which affect
+	// buffer sizes and polyphase structure.
 	//
-	// The NCO phase_inc is computed from the exact offset each frame, so small
-	// frequency changes are tracked smoothly without a reset. Only structural
-	// changes (bandwidth affecting FIR taps, decimation, tap count) need a reset.
-	qOff := math.Round(offsetHz / 1000) * 1000
-	qBW := math.Round(bandwidth / 1000) * 1000
+	// Previous bug: offset and bandwidth were formatted at %.9f precision, causing
+	// a new hash (and full state reset) every single frame because the detector's
+	// exponential smoothing changes CenterHz by sub-Hz fractions each frame.
 	h := fnv.New64a()
-	_, _ = h.Write([]byte(fmt.Sprintf("sig=%d|off=%.0f|bw=%.0f|out=%d|taps=%d|sr=%d", signalID, qOff, qBW, outRate, numTaps, sampleRate)))
+	_, _ = h.Write([]byte(fmt.Sprintf("sig=%d|out=%d|taps=%d|sr=%d", signalID, outRate, numTaps, sampleRate)))
 	return h.Sum64()
 }

internal/recorder/streamer.go

@@ -127,6 +127,13 @@ type streamSession struct {
 	pilotLPFHi       *dsp.StatefulFIRReal // ~21kHz LP for pilot bandpass high
 	pilotLPFLo       *dsp.StatefulFIRReal // ~17kHz LP for pilot bandpass low
 
+	// WFM 15kHz audio LPF — removes pilot (19kHz), L-R subcarrier (23-53kHz),
+	// and RDS (57kHz) from the FM discriminator output before resampling.
+	// Without this, the pilot leaks into the audio as a 19kHz tone (+55dB above
+	// noise floor) and L-R subcarrier energy causes audible click-like artifacts.
+	wfmAudioLPF     *dsp.StatefulFIRReal
+	wfmAudioLPFRate int
+
 	// Stateful pre-demod anti-alias FIR (eliminates cold-start transients
 	// and avoids per-frame FIR recomputation)
 	preDemodFIR       *dsp.StatefulFIRComplex
@@ -1348,6 +1355,11 @@ func (sess *streamSession) processSnippet(snippet []complex64, snipRate int, col
 			audio = stereoAudio
 		} else {
 			sess.stereoState = "mono-fallback"
+			// Apply 15kHz LPF before output: the raw discriminator contains
+			// the 19kHz pilot (+55dB), L-R subcarrier (23-53kHz), and RDS (57kHz).
+			// Without filtering, the pilot leaks into audio and subcarrier
+			// energy produces audible click-like artifacts.
+			audio = sess.wfmAudioFilter(audio, actualDemodRate)
 			dual := make([]float32, len(audio)*2)
 			for i, s := range audio {
 				dual[i*2] = s
@@ -1358,6 +1370,9 @@ func (sess *streamSession) processSnippet(snippet []complex64, snipRate int, col
 		if (prevPlayback != sess.playbackMode || prevStereo != sess.stereoState) && len(sess.audioSubs) > 0 {
 			sendAudioInfo(sess.audioSubs, sess.audioInfo())
 		}
+	} else if isWFM {
+		// Plain WFM (not stereo): also needs 15kHz LPF on discriminator output
+		audio = sess.wfmAudioFilter(audio, actualDemodRate)
 	}
 
 	// --- Polyphase resample to exact 48kHz ---
@@ -1460,6 +1475,20 @@ func pllCoefficients(loopBW, damping float64, sampleRate int) (float64, float64)
 	return alpha, beta
 }
 
+// wfmAudioFilter applies a stateful 15kHz lowpass to WFM discriminator output.
+// Removes the 19kHz stereo pilot, L-R DSB-SC subcarrier (23-53kHz), and RDS (57kHz)
+// that would otherwise leak into the audio output as clicks and tonal artifacts.
+func (sess *streamSession) wfmAudioFilter(audio []float32, sampleRate int) []float32 {
+	if len(audio) == 0 || sampleRate <= 0 {
+		return audio
+	}
+	if sess.wfmAudioLPF == nil || sess.wfmAudioLPFRate != sampleRate {
+		sess.wfmAudioLPF = dsp.NewStatefulFIRReal(dsp.LowpassFIR(15000, sampleRate, 101))
+		sess.wfmAudioLPFRate = sampleRate
+	}
+	return sess.wfmAudioLPF.Process(audio)
+}
+
 // stereoDecodeStateful: pilot-locked 38kHz oscillator for L-R extraction.
 // Uses persistent FIR filter state across frames for click-free stereo.
 // Reuses session scratch buffers to minimize allocations.
@@ -1612,6 +1641,8 @@ type dspStateSnapshot struct {
 	preDemodRate        int
 	preDemodCutoff      float64
 	preDemodDecimPhase  int
+	wfmAudioLPF         *dsp.StatefulFIRReal
+	wfmAudioLPFRate     int
 }
 
 func (sess *streamSession) captureDSPState() dspStateSnapshot {
@@ -1645,6 +1676,8 @@ func (sess *streamSession) captureDSPState() dspStateSnapshot {
 		preDemodRate:        sess.preDemodRate,
 		preDemodCutoff:      sess.preDemodCutoff,
 		preDemodDecimPhase:  sess.preDemodDecimPhase,
+		wfmAudioLPF:         sess.wfmAudioLPF,
+		wfmAudioLPFRate:     sess.wfmAudioLPFRate,
 	}
 }
 
@@ -1678,6 +1711,8 @@ func (sess *streamSession) restoreDSPState(s dspStateSnapshot) {
 	sess.preDemodRate = s.preDemodRate
 	sess.preDemodCutoff = s.preDemodCutoff
 	sess.preDemodDecimPhase = s.preDemodDecimPhase
+	sess.wfmAudioLPF = s.wfmAudioLPF
+	sess.wfmAudioLPFRate = s.wfmAudioLPFRate
 }
 
 // ---------------------------------------------------------------------------
@@ -2071,5 +2106,6 @@ func (st *Streamer) ResetStreams() {
 		sess.preDemodDecimPhase = 0
 		sess.stereoResampler = nil
 		sess.monoResampler = nil
+		sess.wfmAudioLPF = nil
 	}
 }