fix(engine): reset DSP state before restart

Reset generator and upsampler state on engine start, add a full generator Reset path for stateful DSP/runtime components, and cover deterministic first-frame recovery after reset.

internal/app/engine.go

@@ -440,6 +440,10 @@ func (e *Engine) Start(ctx context.Context) error {
 		e.mu.Unlock()
 		return fmt.Errorf("driver start: %w", err)
 	}
+	e.generator.Reset()
+	if e.upsampler != nil {
+		e.upsampler.Reset()
+	}
 
 	runCtx, cancel := context.WithCancel(ctx)
 	e.cancel = cancel

internal/offline/generator.go

@@ -205,6 +205,95 @@ func (g *Generator) RDSEncoder() *rds.Encoder {
 	return g.rdsEnc
 }
 
+func (g *Generator) resetSource() {
+	rawSource, _ := g.sourceFor(g.sampleRate)
+	g.source = NewPreEmphasizedSource(rawSource, g.cfg.FM.PreEmphasisTauUS, g.sampleRate, g.cfg.Audio.Gain)
+}
+
+func (g *Generator) resetRDS2Encoder() {
+	if !g.cfg.RDS.Enabled || !g.cfg.RDS.RDS2Enabled {
+		g.rds2Enc = nil
+		return
+	}
+	g.rds2Enc = rds.NewRDS2Encoder(g.sampleRate)
+	g.rds2Enc.Enable(true)
+	if g.cfg.RDS.StationLogoPath != "" {
+		if err := g.rds2Enc.LoadLogo(g.cfg.RDS.StationLogoPath); err != nil {
+			log.Printf("rds2: failed to load station logo: %v", err)
+		}
+	}
+}
+
+// Reset clears stateful DSP/runtime state so the next run starts from a clean baseline
+// without changing the current live parameters or feature enablement.
+func (g *Generator) Reset() {
+	if !g.initialized {
+		return
+	}
+
+	g.resetSource()
+
+	mode := g.appliedStereoMode
+	if lp := g.liveParams.Load(); lp != nil {
+		mode = canonicalStereoMode(lp.StereoMode)
+	}
+	g.stereoEncoder = stereo.NewStereoEncoder(g.sampleRate)
+	g.stereoEncoder.SetMode(stereo.ParseMode(mode), g.sampleRate)
+	g.appliedStereoMode = mode
+
+	if g.rdsEnc != nil {
+		g.rdsEnc.Reset()
+	}
+	g.resetRDS2Encoder()
+
+	if g.audioLPF_L != nil {
+		g.audioLPF_L.Reset()
+	}
+	if g.audioLPF_R != nil {
+		g.audioLPF_R.Reset()
+	}
+	if g.pilotNotchL != nil {
+		g.pilotNotchL.Reset()
+	}
+	if g.pilotNotchR != nil {
+		g.pilotNotchR.Reset()
+	}
+	if g.limiter != nil {
+		g.limiter.Reset()
+	}
+	if g.cleanupLPF_L != nil {
+		g.cleanupLPF_L.Reset()
+	}
+	if g.cleanupLPF_R != nil {
+		g.cleanupLPF_R.Reset()
+	}
+	if g.mpxNotch19 != nil {
+		g.mpxNotch19.Reset()
+	}
+	if g.mpxNotch57 != nil {
+		g.mpxNotch57.Reset()
+	}
+	if g.bs412 != nil {
+		g.bs412.Reset()
+	}
+	if g.compositeClip != nil {
+		g.compositeClip.Reset()
+	}
+	if g.fmMod != nil {
+		g.fmMod.Reset()
+	}
+
+	if g.watermarkEnabled {
+		g.stftEmbedder = watermark.NewSTFTEmbedder(g.watermarkKey)
+		g.wmDecimLPF = dsp.NewLPF4(5500, g.sampleRate)
+		g.wmInterpLPF = dsp.NewLPF4(5500, g.sampleRate)
+	} else {
+		g.stftEmbedder = nil
+		g.wmDecimLPF = nil
+		g.wmInterpLPF = nil
+	}
+}
+
 func (g *Generator) init() {
 	if g.initialized {
 		return
@@ -268,15 +357,7 @@ func (g *Generator) init() {
 		})
 
 		// RDS2: additional subcarriers (66.5, 71.25, 76 kHz)
-		if g.cfg.RDS.RDS2Enabled {
-			g.rds2Enc = rds.NewRDS2Encoder(g.sampleRate)
-			g.rds2Enc.Enable(true)
-			if g.cfg.RDS.StationLogoPath != "" {
-				if err := g.rds2Enc.LoadLogo(g.cfg.RDS.StationLogoPath); err != nil {
-					log.Printf("rds2: failed to load station logo: %v", err)
-				}
-			}
-		}
+		g.resetRDS2Encoder()
 	}
 	ceiling := g.cfg.FM.LimiterCeiling
 	if ceiling <= 0 {

internal/offline/generator_test.go

@@ -211,3 +211,26 @@ func TestConfigureWatermarkExplicitOptIn(t *testing.T) {
 		t.Fatal("expected watermark embedder after explicit opt-in")
 	}
 }
+
+func TestGeneratorResetRestoresDeterministicFirstFrame(t *testing.T) {
+	cfg := cfgpkg.Default()
+	cfg.RDS.Enabled = false
+	cfg.FM.FMModulationEnabled = true
+	g := NewGenerator(cfg)
+
+	first := g.GenerateFrame(10 * time.Millisecond)
+	_ = g.GenerateFrame(10 * time.Millisecond)
+	g.Reset()
+	afterReset := g.GenerateFrame(10 * time.Millisecond)
+
+	if len(first.Samples) != len(afterReset.Samples) {
+		t.Fatalf("sample length mismatch: %d vs %d", len(first.Samples), len(afterReset.Samples))
+	}
+	for i := range first.Samples {
+		a := first.Samples[i]
+		b := afterReset.Samples[i]
+		if a != b {
+			t.Fatalf("sample %d differs after reset: first=(%v,%v) reset=(%v,%v)", i, a.I, a.Q, b.I, b.Q)
+		}
+	}
+}