diff --git a/internal/offline/generator.go b/internal/offline/generator.go
index d8fa6eb..cd4bb9e 100644
--- a/internal/offline/generator.go
+++ b/internal/offline/generator.go
@@ -85,15 +85,16 @@ type Generator struct {
 	cfg cfgpkg.Config
 
 	// Persistent DSP state across GenerateFrame calls
-	source        *PreEmphasizedSource
-	stereoEncoder stereo.StereoEncoder
-	rdsEnc        *rds.Encoder
-	rds2Enc       *rds.RDS2Encoder
-	combiner      mpx.DefaultCombiner
-	fmMod         *dsp.FMModulator
-	sampleRate    float64
-	initialized   bool
-	frameSeq      uint64
+	source            *PreEmphasizedSource
+	stereoEncoder     stereo.StereoEncoder
+	appliedStereoMode string // canonical mode currently applied to stereoEncoder; DSP goroutine only
+	rdsEnc            *rds.Encoder
+	rds2Enc           *rds.RDS2Encoder
+	combiner          mpx.DefaultCombiner
+	fmMod             *dsp.FMModulator
+	sampleRate        float64
+	initialized       bool
+	frameSeq          uint64
 
 	// Broadcast-standard clip-filter-clip chain (per channel L/R):
 	//
@@ -178,13 +179,10 @@ func (g *Generator) SetExternalSource(src frameSource) error {
 	return nil
 }
 
-// UpdateLive hot-swaps DSP parameters. Thread-safe — called from control API,
-// applied at the next chunk boundary by the DSP goroutine.
+// UpdateLive hot-swaps DSP parameters. Thread-safe — called from control API.
+// The DSP goroutine applies mode changes at the next chunk boundary.
 func (g *Generator) UpdateLive(p LiveParams) {
-	// Detect stereo mode change: requires reconfiguring the encoder's Hilbert filter.
-	if old := g.liveParams.Load(); old != nil && old.StereoMode != p.StereoMode {
-		g.stereoEncoder.SetMode(stereo.ParseMode(p.StereoMode), g.sampleRate)
-	}
+	p.StereoMode = canonicalStereoMode(p.StereoMode)
 	g.liveParams.Store(&p)
 }
 
@@ -197,6 +195,10 @@ func (g *Generator) CurrentLiveParams() LiveParams {
 	return LiveParams{OutputDrive: 1.0, LimiterCeiling: 1.0, MpxGain: 1.0}
 }
 
+func canonicalStereoMode(mode string) string {
+	return stereo.ParseMode(mode).String()
+}
+
 // RDSEncoder returns the live RDS encoder, or nil if RDS is disabled.
 // Used by the Engine to forward text updates.
 func (g *Generator) RDSEncoder() *rds.Encoder {
@@ -223,7 +225,8 @@ func (g *Generator) init() {
 	rawSource, _ := g.sourceFor(g.sampleRate)
 	g.source = NewPreEmphasizedSource(rawSource, g.cfg.FM.PreEmphasisTauUS, g.sampleRate, g.cfg.Audio.Gain)
 	g.stereoEncoder = stereo.NewStereoEncoder(g.sampleRate)
-	g.stereoEncoder.SetMode(stereo.ParseMode(g.cfg.FM.StereoMode), g.sampleRate)
+	g.appliedStereoMode = canonicalStereoMode(g.cfg.FM.StereoMode)
+	g.stereoEncoder.SetMode(stereo.ParseMode(g.appliedStereoMode), g.sampleRate)
 	g.combiner = mpx.DefaultCombiner{
 		MonoGain: 1.0, StereoGain: 1.0,
 		PilotGain: g.cfg.FM.PilotLevel, RDSGain: g.cfg.FM.RDSInjection,
@@ -335,7 +338,7 @@ func (g *Generator) init() {
 	g.liveParams.Store(&LiveParams{
 		OutputDrive:             g.cfg.FM.OutputDrive,
 		StereoEnabled:           g.cfg.FM.StereoEnabled,
-		StereoMode:              g.cfg.FM.StereoMode,
+		StereoMode:              g.appliedStereoMode,
 		PilotLevel:              g.cfg.FM.PilotLevel,
 		RDSInjection:            g.cfg.FM.RDSInjection,
 		RDSEnabled:              g.cfg.RDS.Enabled,
@@ -418,6 +421,11 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		lp = &LiveParams{OutputDrive: 1.0, LimiterCeiling: 1.0, MpxGain: 1.0}
 	}
 
+	if mode := canonicalStereoMode(lp.StereoMode); mode != g.appliedStereoMode {
+		g.stereoEncoder.SetMode(stereo.ParseMode(mode), g.sampleRate)
+		g.appliedStereoMode = mode
+	}
+
 	// Apply live tone and gain updates each chunk. GenerateFrame runs on a
 	// single goroutine so these field writes are safe without additional locking.
 	if g.toneSource != nil {