From e1329a864c2437b27e4313fd98e7bde962c9f78a Mon Sep 17 00:00:00 2001
From: Jan <jan@example.com>
Date: Thu, 9 Apr 2026 21:40:32 +0200
Subject: [PATCH] feat: add license watermarking and harden restart paths

---
 cmd/fmrtx/main.go                             |  33 +-
 cmd/keygen/main.go                            |  41 ++
 cmd/wmdecode/main.go                          | 314 +++++++++++
 cmd/wmtest/main.go                            | 106 ++++
 internal/app/engine.go                        |  16 +-
 internal/control/control.go                   |   6 +-
 internal/ingest/adapters/aoip/source.go       |   4 +
 internal/ingest/adapters/icecast/source.go    |   5 +
 internal/ingest/adapters/srt/source.go        |   5 +
 internal/ingest/adapters/stdinpcm/source.go   |   3 +
 internal/license/embed.go                     |  13 +
 internal/license/license.go                   | 245 ++++++++
 internal/offline/generator.go                 |  64 ++-
 internal/output/frame_queue.go                |  26 +-
 internal/watermark/watermark.go               | 523 ++++++++++++++++++
 .../watermark/watermark_roundtrip_test.go     | 188 +++++++
 16 files changed, 1571 insertions(+), 21 deletions(-)
 create mode 100644 cmd/keygen/main.go
 create mode 100644 cmd/wmdecode/main.go
 create mode 100644 cmd/wmtest/main.go
 create mode 100644 internal/license/embed.go
 create mode 100644 internal/license/license.go
 create mode 100644 internal/watermark/watermark.go
 create mode 100644 internal/watermark/watermark_roundtrip_test.go

diff --git a/cmd/fmrtx/main.go b/cmd/fmrtx/main.go
index c3e371d..810894c 100644
--- a/cmd/fmrtx/main.go
+++ b/cmd/fmrtx/main.go
@@ -12,6 +12,7 @@ import (
 	"time"
 
 	apppkg "github.com/jan/fm-rds-tx/internal/app"
+	"github.com/jan/fm-rds-tx/internal/license"
 	"github.com/jan/fm-rds-tx/internal/audio"
 	cfgpkg "github.com/jan/fm-rds-tx/internal/config"
 	ctrlpkg "github.com/jan/fm-rds-tx/internal/control"
@@ -34,6 +35,7 @@ func main() {
 	simulateDuration := flag.Duration("simulate-duration", 500*time.Millisecond, "simulated transmit duration")
 	txMode := flag.Bool("tx", false, "start real TX mode (requires hardware + build tags)")
 	txAutoStart := flag.Bool("tx-auto-start", false, "auto-start TX on launch")
+	licenseKey  := flag.String("license", "", "fm-rds-tx license key (omit for evaluation mode with jingle)")
 	listDevices := flag.Bool("list-devices", false, "enumerate SoapySDR devices and exit")
 	audioStdin := flag.Bool("audio-stdin", false, "read S16LE stereo PCM audio from stdin")
 	audioRate := flag.Int("audio-rate", 44100, "sample rate of stdin audio input (Hz)")
@@ -101,12 +103,12 @@ func main() {
 		if driver == nil {
 			log.Fatal("no hardware driver available - build with -tags pluto (or -tags soapy)")
 		}
-		runTXMode(cfg, *configPath, driver, *txAutoStart, *audioStdin, *audioRate, *audioHTTP)
+		runTXMode(cfg, *configPath, driver, *txAutoStart, *audioStdin, *audioRate, *audioHTTP, *licenseKey)
 		return
 	}
 
 	srv := ctrlpkg.NewServer(cfg)
-	configureControlPlanePersistence(srv, *configPath)
+	configureControlPlanePersistence(srv, *configPath, nil)
 	server := ctrlpkg.NewHTTPServer(cfg, srv.Handler())
 	log.Printf("fm-rds-tx listening on %s (TX default: off, use --tx for hardware)", server.Addr)
 	log.Fatal(server.ListenAndServe())
@@ -141,7 +143,7 @@ func selectDriver(cfg cfgpkg.Config) platform.SoapyDriver {
 	return nil
 }
 
-func runTXMode(cfg cfgpkg.Config, configPath string, driver platform.SoapyDriver, autoStart bool, audioStdin bool, audioRate int, audioHTTP bool) {
+func runTXMode(cfg cfgpkg.Config, configPath string, driver platform.SoapyDriver, autoStart bool, audioStdin bool, audioRate int, audioHTTP bool, licenseKey string) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -174,6 +176,16 @@ func runTXMode(cfg cfgpkg.Config, configPath string, driver platform.SoapyDriver
 	}
 
 	engine := apppkg.NewEngine(cfg, driver)
+
+	// License setup.
+	licState := license.NewState(licenseKey)
+	if licState.Licensed() {
+		log.Println("license: valid key — evaluation jingle disabled")
+	} else {
+		log.Printf("license: no valid key — evaluation jingle every %d minutes", license.JingleIntervalMinutes)
+	}
+	engine.SetLicenseState(licState, licenseKey)
+	log.Printf("watermark: embedding key fingerprint into composite signal")
 	cfg = applyLegacyAudioFlags(cfg, audioStdin, audioRate, audioHTTP)
 
 	var streamSrc *audio.StreamSource
@@ -228,7 +240,7 @@ func runTXMode(cfg cfgpkg.Config, configPath string, driver platform.SoapyDriver
 	}
 
 	srv := ctrlpkg.NewServer(cfg)
-	configureControlPlanePersistence(srv, configPath)
+	configureControlPlanePersistence(srv, configPath, cancel)
 	srv.SetDriver(driver)
 	srv.SetTXController(&txBridge{engine: engine})
 	if streamSrc != nil {
@@ -272,7 +284,7 @@ func runTXMode(cfg cfgpkg.Config, configPath string, driver platform.SoapyDriver
 	log.Println("shutdown complete")
 }
 
-func configureControlPlanePersistence(srv *ctrlpkg.Server, configPath string) {
+func configureControlPlanePersistence(srv *ctrlpkg.Server, configPath string, cancel context.CancelFunc) {
 	if strings.TrimSpace(configPath) == "" {
 		return
 	}
@@ -280,8 +292,15 @@ func configureControlPlanePersistence(srv *ctrlpkg.Server, configPath string) {
 		return cfgpkg.Save(configPath, next)
 	})
 	srv.SetHardReload(func() {
-		log.Printf("control: hard reload requested after config save, exiting process")
-		os.Exit(0)
+		// BUG-5 fix: cancel the app context instead of os.Exit(0).
+		// os.Exit skips all defers, Flush/Stop calls, and driver cleanup.
+		// Cancelling ctx lets the normal shutdown sequence run: engine.Stop,
+		// ingestRuntime.Stop, driver.Close — then the process exits naturally.
+		// The supervisor (systemd etc.) will restart the process as intended.
+		log.Printf("control: hard reload — cancelling app context for clean restart")
+		if cancel != nil {
+			cancel()
+		}
 	})
 }
 
diff --git a/cmd/keygen/main.go b/cmd/keygen/main.go
new file mode 100644
index 0000000..b88ae40
--- /dev/null
+++ b/cmd/keygen/main.go
@@ -0,0 +1,41 @@
+// cmd/keygen — fm-rds-tx license key generator.
+// KEEP PRIVATE: this tool contains the HMAC secret. Never distribute.
+//
+// Usage:
+//   go run ./cmd/keygen free           → gratis key (no personal data)
+//   go run ./cmd/keygen studio@wxy.fm  → commercial key for that station
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/jan/fm-rds-tx/internal/license"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintf(os.Stderr, "usage: keygen <payload>\n")
+		fmt.Fprintf(os.Stderr, "  payload: 'free' for gratis key, email for commercial\n")
+		fmt.Fprintf(os.Stderr, "examples:\n")
+		fmt.Fprintf(os.Stderr, "  keygen free\n")
+		fmt.Fprintf(os.Stderr, "  keygen studio@example.fm\n")
+		os.Exit(1)
+	}
+
+	payload := strings.TrimSpace(strings.Join(os.Args[1:], " "))
+	if payload == "" {
+		fmt.Fprintln(os.Stderr, "payload must not be empty")
+		os.Exit(1)
+	}
+
+	key := license.GenerateKey(payload)
+	fmt.Println(key)
+
+	// Self-validate
+	if !license.ValidateKey(key) {
+		fmt.Fprintln(os.Stderr, "ERROR: generated key failed self-validation!")
+		os.Exit(1)
+	}
+}
diff --git a/cmd/wmdecode/main.go b/cmd/wmdecode/main.go
new file mode 100644
index 0000000..e29c2ff
--- /dev/null
+++ b/cmd/wmdecode/main.go
@@ -0,0 +1,314 @@
+// cmd/wmdecode — fm-rds-tx spread-spectrum watermark recovery tool.
+//
+// Records or reads a mono WAV of FM receiver audio output, extracts the
+// embedded key fingerprint using PN correlation with frame synchronisation,
+// applies Reed-Solomon erasure decoding, and checks against known keys.
+//
+// Usage:
+//
+//	wmdecode <file.wav> [key ...]
+//
+// Examples:
+//
+//	wmdecode aufnahme.wav
+//	wmdecode aufnahme.wav free studio@sender.fm
+//
+// Recording hint (Windows, FM receiver line-in):
+//
+//	ffmpeg -f dshow -i audio="Stereo Mix" -ar 48000 -ac 1 -t 30 aufnahme.wav
+package main
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"os"
+	"sort"
+
+	"github.com/jan/fm-rds-tx/internal/watermark"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintln(os.Stderr, "usage: wmdecode <file.wav> [key ...]")
+		os.Exit(1)
+	}
+
+	samples, recRate, err := readMonoWAV(os.Args[1])
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "read WAV: %v\n", err)
+		os.Exit(1)
+	}
+
+	rms := rmsLevel(samples)
+	fmt.Printf("WAV:   %d samples @ %.0f Hz = %.2fs, RMS %.1f dBFS\n",
+		len(samples), recRate, float64(len(samples))/recRate, 20*math.Log10(rms+1e-9))
+
+	samplesPerBit := int(float64(watermark.PnChips) * recRate / float64(watermark.RecordingRate))
+	if samplesPerBit < 1 {
+		samplesPerBit = 1
+	}
+	frameLen := samplesPerBit * watermark.PayloadBits
+	fmt.Printf("Frame: %d samples/bit, %d samples/frame (%.3fs), %d frames in recording\n",
+		samplesPerBit, frameLen, float64(frameLen)/recRate, len(samples)/frameLen)
+
+	if len(samples) < samplesPerBit*2 {
+		fmt.Fprintln(os.Stderr, "recording too short for even 2 bits")
+		os.Exit(1)
+	}
+
+	// ---------------------------------------------------------------
+	// Step 1: Phase search — find sample offset of bit boundaries.
+	//
+	// Coarse pass: test every 8th offset in [0, samplesPerBit).
+	// Fine pass:   refine ±8 around the coarse peak.
+	// For each candidate offset, average |correlation| over several bits.
+	// ---------------------------------------------------------------
+	const coarseStep = 8
+	const syncBits = 64
+
+	bestPhase := 0
+	bestMag := 0.0
+
+	for phase := 0; phase < samplesPerBit; phase += coarseStep {
+		mag := avgCorrMag(samples, phase, samplesPerBit, syncBits, recRate)
+		if mag > bestMag {
+			bestMag = mag
+			bestPhase = phase
+		}
+	}
+
+	fineStart := bestPhase - coarseStep
+	if fineStart < 0 {
+		fineStart = 0
+	}
+	fineEnd := bestPhase + coarseStep
+	if fineEnd > samplesPerBit {
+		fineEnd = samplesPerBit
+	}
+	for phase := fineStart; phase < fineEnd; phase++ {
+		mag := avgCorrMag(samples, phase, samplesPerBit, syncBits, recRate)
+		if mag > bestMag {
+			bestMag = mag
+			bestPhase = phase
+		}
+	}
+
+	fmt.Printf("Phase: offset=%d (%.3fms into recording), avg|corr|=%.4f\n",
+		bestPhase, float64(bestPhase)/recRate*1000, bestMag)
+
+	// ---------------------------------------------------------------
+	// Step 2: Extract bit correlations at found phase, averaged over frames.
+	// ---------------------------------------------------------------
+	nCompleteBits := (len(samples) - bestPhase) / samplesPerBit
+	nFrames := nCompleteBits / watermark.PayloadBits
+	if nFrames == 0 {
+		nFrames = 1
+	}
+
+	fmt.Printf("Sync:  %d complete bits, %d usable frames\n", nCompleteBits, nFrames)
+
+	corrs := make([]float64, watermark.PayloadBits)
+	for i := 0; i < watermark.PayloadBits; i++ {
+		for frame := 0; frame < nFrames; frame++ {
+			bitGlobal := frame*watermark.PayloadBits + i
+			start := bestPhase + bitGlobal*samplesPerBit
+			if start+samplesPerBit > len(samples) {
+				break
+			}
+			corrs[i] += watermark.CorrelateAt(samples, start, recRate)
+		}
+	}
+
+	// ---------------------------------------------------------------
+	// Step 3: Frame sync — try all 128 cyclic rotations.
+	// The correct rotation yields a valid RS codeword.
+	// ---------------------------------------------------------------
+	type decodeResult struct {
+		rotation int
+		payload  [watermark.RsDataBytes]byte
+		erasures int
+	}
+
+	var best *decodeResult
+
+	for rot := 0; rot < watermark.PayloadBits; rot++ {
+		var recv [watermark.RsTotalBytes]byte
+		confs := make([]float64, watermark.PayloadBits)
+
+		for i := 0; i < watermark.PayloadBits; i++ {
+			srcBit := (i + rot) % watermark.PayloadBits
+			c := corrs[srcBit]
+			confs[i] = math.Abs(c)
+			if c < 0 {
+				recv[i/8] |= 1 << uint(7-(i%8))
+			}
+		}
+
+		// Sort by confidence ascending for erasure selection
+		type bitConf struct {
+			idx  int
+			conf float64
+		}
+		ranked := make([]bitConf, watermark.PayloadBits)
+		for i := range ranked {
+			ranked[i] = bitConf{i, confs[i]}
+		}
+		sort.Slice(ranked, func(a, b int) bool {
+			return ranked[a].conf < ranked[b].conf
+		})
+
+		for nErase := 0; nErase <= watermark.RsCheckBytes*8; nErase++ {
+			erasedBytes := map[int]bool{}
+			for _, bc := range ranked[:nErase] {
+				erasedBytes[bc.idx/8] = true
+			}
+			if len(erasedBytes) > watermark.RsCheckBytes {
+				break
+			}
+			erasePos := make([]int, 0, len(erasedBytes))
+			for pos := range erasedBytes {
+				erasePos = append(erasePos, pos)
+			}
+			sort.Ints(erasePos)
+
+			payload, ok := watermark.RSDecode(recv, erasePos)
+			if ok {
+				if best == nil || len(erasePos) < best.erasures {
+					best = &decodeResult{
+						rotation: rot,
+						payload:  payload,
+						erasures: len(erasePos),
+					}
+				}
+				break
+			}
+		}
+
+		if best != nil && best.erasures == 0 {
+			break
+		}
+	}
+
+	if best == nil {
+		fmt.Println("\nRS decode: FAILED — no valid frame alignment found.")
+		fmt.Println("Watermark may not be present, or recording is too noisy/short.")
+		var maxCorr, minCorr float64
+		for _, c := range corrs {
+			ac := math.Abs(c)
+			if ac > maxCorr {
+				maxCorr = ac
+			}
+			if minCorr == 0 || ac < minCorr {
+				minCorr = ac
+			}
+		}
+		fmt.Printf("Correlation range: min |c|=%.4f, max |c|=%.4f\n", minCorr, maxCorr)
+		os.Exit(1)
+	}
+
+	fmt.Printf("\nFrame sync: rotation=%d, RS erasures=%d\n", best.rotation, best.erasures)
+	fmt.Printf("Payload:    %x\n\n", best.payload)
+
+	keys := os.Args[2:]
+	if len(keys) == 0 {
+		fmt.Println("No keys supplied — payload shown above.")
+		fmt.Println("Usage: wmdecode <file.wav> free [other-keys...]")
+		return
+	}
+
+	fmt.Println("Key check:")
+	matched := false
+	for _, key := range keys {
+		if watermark.KeyMatchesPayload(key, best.payload) {
+			fmt.Printf("  ✓ MATCH: %q\n", key)
+			matched = true
+		} else {
+			fmt.Printf("  ✗      : %q\n", key)
+		}
+	}
+	if !matched {
+		fmt.Println("\nNo key matched.")
+	}
+}
+
+func avgCorrMag(samples []float64, phase, samplesPerBit, nBits int, recRate float64) float64 {
+	var total float64
+	var count int
+	for b := 0; b < nBits; b++ {
+		start := phase + b*samplesPerBit
+		if start+samplesPerBit > len(samples) {
+			break
+		}
+		c := watermark.CorrelateAt(samples, start, recRate)
+		total += math.Abs(c)
+		count++
+	}
+	if count == 0 {
+		return 0
+	}
+	return total / float64(count)
+}
+
+func rmsLevel(s []float64) float64 {
+	var acc float64
+	for _, v := range s {
+		acc += v * v
+	}
+	return math.Sqrt(acc / float64(len(s)))
+}
+
+func readMonoWAV(path string) ([]float64, float64, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, 0, err
+	}
+	if len(data) < 44 || string(data[0:4]) != "RIFF" || string(data[8:12]) != "WAVE" {
+		return nil, 0, fmt.Errorf("not a RIFF/WAVE file")
+	}
+	var channels, bitsPerSample uint16
+	var sampleRate uint32
+	var dataStart, dataLen int
+	i := 12
+	for i+8 <= len(data) {
+		id := string(data[i : i+4])
+		sz := int(binary.LittleEndian.Uint32(data[i+4 : i+8]))
+		i += 8
+		switch id {
+		case "fmt ":
+			if sz >= 16 {
+				channels = binary.LittleEndian.Uint16(data[i+2 : i+4])
+				sampleRate = binary.LittleEndian.Uint32(data[i+4 : i+8])
+				bitsPerSample = binary.LittleEndian.Uint16(data[i+14 : i+16])
+			}
+		case "data":
+			dataStart, dataLen = i, sz
+		}
+		i += sz
+		if sz%2 != 0 {
+			i++
+		}
+		if dataStart > 0 && channels > 0 {
+			break
+		}
+	}
+	if dataStart == 0 || bitsPerSample != 16 || channels == 0 {
+		return nil, 0, fmt.Errorf("unsupported WAV (need 16-bit PCM, got bits=%d ch=%d)", bitsPerSample, channels)
+	}
+	if dataStart+dataLen > len(data) {
+		dataLen = len(data) - dataStart
+	}
+	step := int(channels) * 2
+	nFrames := dataLen / step
+	out := make([]float64, nFrames)
+	for j := 0; j < nFrames; j++ {
+		off := dataStart + j*step
+		l := float64(int16(binary.LittleEndian.Uint16(data[off : off+2])))
+		r := l
+		if channels >= 2 {
+			r = float64(int16(binary.LittleEndian.Uint16(data[off+2 : off+4])))
+		}
+		out[j] = (l + r) / 2.0 / 32768.0
+	}
+	return out, float64(sampleRate), nil
+}
diff --git a/cmd/wmtest/main.go b/cmd/wmtest/main.go
new file mode 100644
index 0000000..1020c9c
--- /dev/null
+++ b/cmd/wmtest/main.go
@@ -0,0 +1,106 @@
+// cmd/wmtest — Ferrite watermark self-test tool.
+//
+// Generates a mono WAV file containing only the spread-spectrum watermark
+// signal (silence + watermark, scaled up for visibility). Run wmdecode on
+// the output to verify embedder and decoder work without FM transmission.
+//
+// Usage:
+//
+//	wmtest --key <license-key> --output test.wav --duration 30s
+//	wmdecode test.wav <license-key>
+package main
+
+import (
+	"encoding/binary"
+	"flag"
+	"fmt"
+	"math"
+	"os"
+	"time"
+
+	"github.com/jan/fm-rds-tx/internal/watermark"
+)
+
+func main() {
+	key      := flag.String("key",      "free",       "License key to embed")
+	output   := flag.String("output",   "wmtest.wav", "Output WAV file")
+	duration := flag.Duration("duration", 30*time.Second, "Duration")
+	flag.Parse()
+
+	const compRate = watermark.CompositeRate // 228000
+	const recRate  = watermark.RecordingRate // 48000
+
+	nSamples := int(duration.Seconds() * float64(recRate))
+
+	fmt.Printf("Ferrite watermark self-test\n")
+	fmt.Printf("  Key:      %s\n", *key)
+	fmt.Printf("  Duration: %s (%d samples @ %dHz)\n\n", *duration, nSamples, recRate)
+
+	embedder := watermark.NewEmbedder(*key)
+	samples  := make([]float64, 0, nSamples)
+
+	// Drive embedder at composite rate, collect samples at recording rate.
+	// Bresenham: accumulate recRate each composite step; when >= compRate,
+	// emit one recording sample and subtract compRate.
+	accum := 0
+	var last float64
+	for len(samples) < nSamples {
+		last   = embedder.NextSample()
+		accum += recRate
+		if accum >= compRate {
+			accum -= compRate
+			samples = append(samples, last)
+		}
+	}
+
+	// RMS
+	var rmsAcc float64
+	for _, s := range samples {
+		rmsAcc += s * s
+	}
+	rms := math.Sqrt(rmsAcc / float64(len(samples)))
+	fmt.Printf("Watermark RMS: %.1f dBFS (nominal -48 dBFS)\n", 20*math.Log10(rms+1e-12))
+
+	if err := writeMonoWAV(*output, samples, recRate); err != nil {
+		fmt.Fprintf(os.Stderr, "write WAV: %v\n", err)
+		os.Exit(1)
+	}
+	fmt.Printf("Written: %s\n\n", *output)
+	fmt.Printf("Decode with:\n")
+	fmt.Printf("  .\\wmdecode.exe %s %q\n\n", *output, *key)
+	fmt.Printf("Expected: RS decode clean + MATCH\n")
+}
+
+func writeMonoWAV(path string, samples []float64, rate int) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	le      := binary.LittleEndian
+	dataSz  := uint32(len(samples) * 2)
+
+	f.Write([]byte("RIFF"))
+	binary.Write(f, le, 36+dataSz)
+	f.Write([]byte("WAVE"))
+	f.Write([]byte("fmt "))
+	binary.Write(f, le, uint32(16))
+	binary.Write(f, le, uint16(1))        // PCM
+	binary.Write(f, le, uint16(1))        // mono
+	binary.Write(f, le, uint32(rate))
+	binary.Write(f, le, uint32(rate*2))   // byte rate
+	binary.Write(f, le, uint16(2))        // block align
+	binary.Write(f, le, uint16(16))       // bits/sample
+	f.Write([]byte("data"))
+	binary.Write(f, le, dataSz)
+	for _, s := range samples {
+		// Scale to int16 range — watermark at -48dBFS → ~0.004 amplitude
+		// Multiply by 32767 to get full 16-bit range
+		v := s * 32767.0
+		if v >  32767 { v =  32767 }
+		if v < -32768 { v = -32768 }
+		binary.Write(f, le, int16(v))
+	}
+	return nil
+}
diff --git a/internal/app/engine.go b/internal/app/engine.go
index 12951f1..7722dc8 100644
--- a/internal/app/engine.go
+++ b/internal/app/engine.go
@@ -12,6 +12,7 @@ import (
 
 	"github.com/jan/fm-rds-tx/internal/audio"
 	cfgpkg "github.com/jan/fm-rds-tx/internal/config"
+	"github.com/jan/fm-rds-tx/internal/license"
 	"github.com/jan/fm-rds-tx/internal/dsp"
 	offpkg "github.com/jan/fm-rds-tx/internal/offline"
 	"github.com/jan/fm-rds-tx/internal/output"
@@ -196,11 +197,21 @@ func (e *Engine) SetStreamSource(src *audio.StreamSource) {
 		compositeRate = 228000
 	}
 	resampler := audio.NewStreamResampler(src, compositeRate)
-	e.generator.SetExternalSource(resampler)
+	if err := e.generator.SetExternalSource(resampler); err != nil {
+		// Should never happen: SetStreamSource must be called before Start().
+		log.Printf("engine: SetExternalSource failed (called too late): %v", err)
+		return
+	}
 	log.Printf("engine: live audio stream wired — initial %d Hz → %.0f Hz composite (buffer %d frames); actual decoded rate auto-corrects on first chunk",
 		src.SampleRate, compositeRate, src.Stats().Capacity)
 }
 
+// SetLicenseState passes the license/jingle state and raw key to the generator.
+// Must be called before Start(). key is used to derive the watermark payload.
+func (e *Engine) SetLicenseState(s *license.State, key string) {
+	e.generator.SetLicense(s, key)
+}
+
 // StreamSource returns the live audio stream source, or nil.
 // Used by the control server for stats and HTTP audio ingest.
 func (e *Engine) StreamSource() *audio.StreamSource {
@@ -407,6 +418,9 @@ func (e *Engine) Start(ctx context.Context) error {
 	e.state = EngineRunning
 	e.setRuntimeState(RuntimeStateArming)
 	e.startedAt = time.Now()
+	// BUG-A fix: discard any frames left from a previous run so writerLoop
+	// does not send stale data with expired timestamps on restart.
+	e.frameQueue.Drain()
 	e.wg.Add(1)
 	e.mu.Unlock()
 
diff --git a/internal/control/control.go b/internal/control/control.go
index 4f021c8..81b012e 100644
--- a/internal/control/control.go
+++ b/internal/control/control.go
@@ -515,7 +515,11 @@ func (s *Server) handleConfig(w http.ResponseWriter, r *http.Request) {
 		}
 		r.Body = http.MaxBytesReader(w, r.Body, maxConfigBodyBytes)
 		var patch ConfigPatch
-		if err := json.NewDecoder(r.Body).Decode(&patch); err != nil {
+		// BUG-4 fix: reject unknown JSON fields (typos) with 400 rather than
+		// silently ignoring them (e.g. "outputDrvie" would succeed and do nothing).
+		dec := json.NewDecoder(r.Body)
+		dec.DisallowUnknownFields()
+		if err := dec.Decode(&patch); err != nil {
 			statusCode := http.StatusBadRequest
 			if strings.Contains(err.Error(), "http: request body too large") {
 				statusCode = http.StatusRequestEntityTooLarge
diff --git a/internal/ingest/adapters/aoip/source.go b/internal/ingest/adapters/aoip/source.go
index 24e30f1..813dc3b 100644
--- a/internal/ingest/adapters/aoip/source.go
+++ b/internal/ingest/adapters/aoip/source.go
@@ -136,6 +136,10 @@ func (s *Source) Start(ctx context.Context) error {
 	if !s.started.CompareAndSwap(false, true) {
 		return nil
 	}
+	// BUG-2 fix: recreate channels and reset closeOnce so Stop+Start works.
+	s.chunks    = make(chan ingest.PCMChunk, 64)
+	s.errs      = make(chan error, 8)
+	s.closeOnce = sync.Once{}
 
 	rx, err := s.factory(s.cfg, s.handleFrame)
 	if err != nil {
diff --git a/internal/ingest/adapters/icecast/source.go b/internal/ingest/adapters/icecast/source.go
index 52c825f..6c08356 100644
--- a/internal/ingest/adapters/icecast/source.go
+++ b/internal/ingest/adapters/icecast/source.go
@@ -129,6 +129,11 @@ func (s *Source) Start(ctx context.Context) error {
 	if s.url == "" {
 		return fmt.Errorf("icecast url is required")
 	}
+	// BUG-2 fix: recreate channels on every Start() so that Stop+Start works.
+	// loop() closes chunks/errs/title when it exits; reusing closed channels panics.
+	s.chunks = make(chan ingest.PCMChunk, 64)
+	s.errs   = make(chan error, 8)
+	s.title  = make(chan string, 16)
 	runCtx, cancel := context.WithCancel(ctx)
 	s.cancel = cancel
 	s.lastError.Store("")
diff --git a/internal/ingest/adapters/srt/source.go b/internal/ingest/adapters/srt/source.go
index af3685d..fdff063 100644
--- a/internal/ingest/adapters/srt/source.go
+++ b/internal/ingest/adapters/srt/source.go
@@ -110,6 +110,11 @@ func (s *Source) Start(ctx context.Context) error {
 	if !s.started.CompareAndSwap(false, true) {
 		return nil
 	}
+	// BUG-2 fix: recreate channels and reset closeOnce so Stop+Start works.
+	// closeChannels() uses sync.Once — reset it so the new channels can be closed.
+	s.chunks    = make(chan ingest.PCMChunk, 64)
+	s.errs      = make(chan error, 8)
+	s.closeOnce = sync.Once{}
 
 	var (
 		rx  *aoiprxkit.SRTReceiver
diff --git a/internal/ingest/adapters/stdinpcm/source.go b/internal/ingest/adapters/stdinpcm/source.go
index 104b66b..159df9b 100644
--- a/internal/ingest/adapters/stdinpcm/source.go
+++ b/internal/ingest/adapters/stdinpcm/source.go
@@ -77,6 +77,9 @@ func (s *Source) Start(ctx context.Context) error {
 	if s.reader == nil {
 		return fmt.Errorf("stdin source reader is nil")
 	}
+	// BUG-2 fix: recreate channels — readLoop() closes them on exit.
+	s.chunks = make(chan ingest.PCMChunk, 8)
+	s.errs   = make(chan error, 4)
 	runCtx, cancel := context.WithCancel(ctx)
 	s.cancel = cancel
 	s.state.Store("running")
diff --git a/internal/license/embed.go b/internal/license/embed.go
new file mode 100644
index 0000000..3ec8e82
--- /dev/null
+++ b/internal/license/embed.go
@@ -0,0 +1,13 @@
+package license
+
+import _ "embed"
+
+// jingleWAV holds the fm-rds-tx station identification jingle.
+// Replace jingle.wav with your actual jingle before shipping.
+// Requirements: 16-bit PCM WAV, stereo, 44100 Hz, max ~30 seconds.
+//
+//go:embed jingle.wav
+var jingleWAV []byte
+
+// JingleWAV returns the raw embedded jingle WAV bytes.
+func JingleWAV() []byte { return jingleWAV }
diff --git a/internal/license/license.go b/internal/license/license.go
new file mode 100644
index 0000000..5beca35
--- /dev/null
+++ b/internal/license/license.go
@@ -0,0 +1,245 @@
+// Package license handles fm-rds-tx key validation and jingle injection.
+//
+// Key format:  FMRTX-<BASE32(HMAC-SHA256(payload, secret)[:10])>
+// payload:     "free" for gratis keys, email for paid keys.
+//
+// Without a valid key the jingle WAV is mixed into the composite output
+// every JingleIntervalMinutes minutes. With a valid key the jingle is silent.
+package license
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base32"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"strings"
+	"time"
+)
+
+// hmacSecret is the shared secret used to sign and verify keys.
+// Change this value in your private fork — keys signed with the old
+// secret stop working, forcing a re-issue. Never commit the real secret.
+const hmacSecret = "Q7m!xP2#rL9$vN4@tK8%hD3&yF6*zC1+uB5"
+
+// JingleIntervalMinutes is how often the jingle fires when unlicensed.
+const JingleIntervalMinutes = 20
+
+// jingleSampleRate and jingleChannels must match the embedded jingle.wav.
+// The mixer resamples on-the-fly if the composite rate differs.
+const (
+	jingleWAVRate     = 44100
+	jingleWAVChannels = 2
+)
+
+// State holds the runtime license + jingle state for a running generator.
+type State struct {
+	licensed bool
+
+	active      bool    // jingle currently playing
+	pos         int     // playback position in jingleFrames
+	nextFire    time.Time
+	jingleLevel float64 // composite injection amplitude (0..1)
+}
+
+// NewState validates the provided key and returns a ready State.
+// If key is empty or invalid, the jingle fires every JingleIntervalMinutes.
+func NewState(key string) *State {
+	s := &State{
+		licensed:    ValidateKey(key),
+		jingleLevel: 0.25, // 25% composite injection — loud but not clipping
+	}
+	if !s.licensed {
+		s.nextFire = time.Now().Add(time.Duration(JingleIntervalMinutes) * time.Minute)
+	}
+	return s
+}
+
+// Licensed reports whether a valid key was supplied.
+func (s *State) Licensed() bool { return s.licensed }
+
+// NextSample returns the jingle contribution for one composite sample.
+// Call once per sample from the DSP loop — it is not thread-safe and must
+// be called from the single GenerateFrame goroutine only.
+// Returns 0 when licensed, not active, or no jingle loaded.
+func (s *State) NextSample(frames []JingleFrame) float64 {
+	if s.licensed || len(frames) == 0 {
+		return 0
+	}
+	if !s.active {
+		return 0
+	}
+	f := frames[s.pos%len(frames)]
+	jingleMono := float64(f.L+f.R) / 2.0
+	s.pos++
+	if s.pos >= len(frames) {
+		s.active = false
+		s.pos = 0
+		s.nextFire = time.Now().Add(time.Duration(JingleIntervalMinutes) * time.Minute)
+	}
+	return s.jingleLevel * jingleMono
+}
+
+// Tick checks whether a new jingle playback should start.
+// Call once per chunk (not per sample) from GenerateFrame.
+// Safe to call from the single DSP goroutine — no locking needed after init.
+func (s *State) Tick() {
+	if s.licensed || s.active {
+		return
+	}
+	if time.Now().After(s.nextFire) {
+		s.active = true
+		s.pos = 0
+	}
+}
+
+// MixComposite is kept for compatibility; prefer Tick()+NextSample() per sample.
+func (s *State) MixComposite(composite float64, frames []JingleFrame, _ time.Time) float64 {
+	return composite + s.NextSample(frames)
+}
+
+// jingleFrame is a normalised stereo frame from the embedded WAV.
+type JingleFrame struct{ L, R float32 }
+
+// LoadJingleFrames decodes the embedded WAV bytes into normalised frames
+// and resamples them from jingleWAVRate to targetRate using linear interpolation.
+func LoadJingleFrames(wavBytes []byte, targetRate float64) ([]JingleFrame, error) {
+	raw, err := decodeWAV(wavBytes)
+	if err != nil {
+		return nil, fmt.Errorf("license: decode jingle WAV: %w", err)
+	}
+	if targetRate <= 0 || math.Abs(targetRate-float64(jingleWAVRate)) < 1 {
+		return raw, nil
+	}
+	// Linear resample to composite rate.
+	ratio := float64(jingleWAVRate) / targetRate
+	dstLen := int(float64(len(raw)) / ratio)
+	out := make([]JingleFrame, dstLen)
+	for i := range out {
+		pos := float64(i) * ratio
+		idx := int(pos)
+		frac := float32(pos - float64(idx))
+		if idx+1 < len(raw) {
+			a, b := raw[idx], raw[idx+1]
+			out[i] = JingleFrame{
+				L: a.L*(1-frac) + b.L*frac,
+				R: a.R*(1-frac) + b.R*frac,
+			}
+		} else if idx < len(raw) {
+			out[i] = raw[idx]
+		}
+	}
+	return out, nil
+}
+
+// decodeWAV parses a minimal PCM WAV (16-bit stereo) into normalised frames.
+func decodeWAV(data []byte) ([]JingleFrame, error) {
+	if len(data) < 44 {
+		return nil, fmt.Errorf("WAV too short")
+	}
+	if string(data[0:4]) != "RIFF" || string(data[8:12]) != "WAVE" {
+		return nil, fmt.Errorf("not a RIFF/WAVE file")
+	}
+	// Find fmt and data chunks.
+	var (
+		channels      uint16
+		bitsPerSample uint16
+		dataStart     int
+		dataLen       int
+	)
+	i := 12
+	for i+8 <= len(data) {
+		id := string(data[i : i+4])
+		chunkSize := int(binary.LittleEndian.Uint32(data[i+4 : i+8]))
+		i += 8
+		switch id {
+		case "fmt ":
+			if chunkSize < 16 || i+16 > len(data) {
+				return nil, fmt.Errorf("fmt chunk too small")
+			}
+			if binary.LittleEndian.Uint16(data[i:i+2]) != 1 {
+				return nil, fmt.Errorf("only PCM WAV supported")
+			}
+			channels = binary.LittleEndian.Uint16(data[i+2 : i+4])
+			bitsPerSample = binary.LittleEndian.Uint16(data[i+14 : i+16])
+		case "data":
+			dataStart = i
+			dataLen = chunkSize
+		}
+		i += chunkSize
+		if chunkSize%2 != 0 {
+			i++
+		}
+		if dataStart > 0 && channels > 0 {
+			break
+		}
+	}
+	if dataStart == 0 || channels == 0 || bitsPerSample != 16 {
+		return nil, fmt.Errorf("unsupported WAV format (need 16-bit PCM, got bits=%d ch=%d)", bitsPerSample, channels)
+	}
+	if dataStart+dataLen > len(data) {
+		dataLen = len(data) - dataStart
+	}
+
+	step := int(channels) * 2
+	frames := make([]JingleFrame, 0, dataLen/step)
+	for j := dataStart; j+step <= dataStart+dataLen; j += step {
+		l := float32(int16(binary.LittleEndian.Uint16(data[j:j+2]))) / 32768.0
+		r := l
+		if channels >= 2 {
+			r = float32(int16(binary.LittleEndian.Uint16(data[j+2:j+4]))) / 32768.0
+		}
+		frames = append(frames, JingleFrame{L: l, R: r})
+	}
+	return frames, nil
+}
+
+// --- Key validation ---
+
+// ValidateKey returns true if key is a valid fm-rds-tx license key.
+func ValidateKey(key string) bool {
+	key = strings.TrimSpace(key)
+	if !strings.HasPrefix(key, "FMRTX-") {
+		return false
+	}
+	body := strings.TrimPrefix(key, "FMRTX-")
+	// Decode the base32 payload.
+	// Format: BASE32(payload_len_byte || payload_bytes || mac_10_bytes)
+	padded := body
+	if pad := len(padded) % 8; pad != 0 {
+		padded += strings.Repeat("=", 8-pad)
+	}
+	raw, err := base32.StdEncoding.DecodeString(strings.ToUpper(padded))
+	if err != nil || len(raw) < 11 {
+		return false
+	}
+	payloadLen := int(raw[0])
+	if payloadLen+1+10 > len(raw) {
+		return false
+	}
+	payload := raw[1 : 1+payloadLen]
+	mac := raw[1+payloadLen : 1+payloadLen+10]
+	expected := computeMAC(payload)
+	return hmac.Equal(mac, expected[:10])
+}
+
+// GenerateKey generates a signed license key for the given payload string.
+// Call this from cmd/keygen — not from the main binary.
+func GenerateKey(payload string) string {
+	p := []byte(payload)
+	raw := make([]byte, 1+len(p)+10)
+	raw[0] = byte(len(p))
+	copy(raw[1:], p)
+	mac := computeMAC(p)
+	copy(raw[1+len(p):], mac[:10])
+	encoded := base32.StdEncoding.EncodeToString(raw)
+	encoded = strings.TrimRight(encoded, "=")
+	return "FMRTX-" + encoded
+}
+
+func computeMAC(payload []byte) []byte {
+	h := hmac.New(sha256.New, []byte(hmacSecret))
+	h.Write(payload)
+	return h.Sum(nil)
+}
diff --git a/internal/offline/generator.go b/internal/offline/generator.go
index b55c63a..54850f2 100644
--- a/internal/offline/generator.go
+++ b/internal/offline/generator.go
@@ -4,12 +4,16 @@ import (
 	"context"
 	"encoding/binary"
 	"fmt"
+	"log"
+	"math"
 	"path/filepath"
 	"sync/atomic"
 	"time"
 
 	"github.com/jan/fm-rds-tx/internal/audio"
 	cfgpkg "github.com/jan/fm-rds-tx/internal/config"
+	"github.com/jan/fm-rds-tx/internal/license"
+	"github.com/jan/fm-rds-tx/internal/watermark"
 	"github.com/jan/fm-rds-tx/internal/dsp"
 	"github.com/jan/fm-rds-tx/internal/mpx"
 	"github.com/jan/fm-rds-tx/internal/output"
@@ -121,24 +125,40 @@ type Generator struct {
 	// Tone source reference — non-nil when a ToneSource is the active audio input.
 	// Allows live-updating tone parameters via LiveParams each chunk.
 	toneSource *audio.ToneSource
+
+	// License: jingle injection when unlicensed.
+	licenseState *license.State
+	jingleFrames []license.JingleFrame
+
+	// Watermark: spread-spectrum key fingerprint, always active.
+	watermark *watermark.Embedder
 }
 
 func NewGenerator(cfg cfgpkg.Config) *Generator {
 	return &Generator{cfg: cfg}
 }
 
+// SetLicense configures license state (jingle) and creates the watermark
+// embedder. Must be called before the first GenerateFrame.
+func (g *Generator) SetLicense(state *license.State, key string) {
+	g.licenseState = state
+	g.watermark = watermark.NewEmbedder(key)
+	// Gate threshold: -40 dBFS ≈ 0.01 linear amplitude.
+	// Watermark is muted during silence to prevent audibility.
+	// Composite rate will be set in init(); use 228000 as default.
+	g.watermark.EnableGate(0.01, 228000)
+}
+
 // SetExternalSource sets a live audio source (e.g. StreamResampler) that
 // takes priority over WAV/tone sources. Must be called before the first
 // GenerateFrame() call; calling it after init() has no effect because
 // g.source is already wired to the old source.
-func (g *Generator) SetExternalSource(src frameSource) {
+func (g *Generator) SetExternalSource(src frameSource) error {
 	if g.initialized {
-		// init() already called sourceFor() and wired g.source. Updating
-		// g.externalSource here would have no effect on the live DSP chain.
-		// This is a programming error — log loudly rather than silently break.
-		panic("generator: SetExternalSource called after GenerateFrame; call it before the engine starts")
+		return fmt.Errorf("generator: SetExternalSource called after GenerateFrame; call it before the engine starts")
 	}
 	g.externalSource = src
+	return nil
 }
 
 // UpdateLive hot-swaps DSP parameters. Thread-safe — called from control API,
@@ -242,6 +262,21 @@ func (g *Generator) init() {
 		AudioGain:      g.cfg.Audio.Gain,
 	})
 
+	if g.licenseState != nil {
+		frames, err := license.LoadJingleFrames(license.JingleWAV(), g.sampleRate)
+		if err != nil {
+			log.Printf("license: jingle load failed: %v", err)
+		} else {
+			g.jingleFrames = frames
+		}
+	}
+
+	// Update watermark gate ramp rate with actual composite rate (may differ
+	// from the 228000 default used in SetLicense).
+	if g.watermark != nil {
+		g.watermark.EnableGate(0.01, g.sampleRate)
+	}
+
 	g.initialized = true
 }
 
@@ -335,6 +370,10 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		bs412Gain = g.bs412.CurrentGain()
 	}
 
+	if g.licenseState != nil {
+		g.licenseState.Tick()
+	}
+
 	for i := 0; i < samples; i++ {
 		in := g.source.NextFrame()
 
@@ -344,6 +383,17 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		r := g.audioLPF_R.Process(float64(in.R))
 		r = g.pilotNotchR.Process(r)
 
+		// Watermark injection — AFTER 14kHz LPF, before Drive/Clip.
+		// Audio-level gate: measure level and smooth-ramp watermark to
+		// prevent audibility during silence/fades.
+		if g.watermark != nil {
+			audioLevel := (math.Abs(l) + math.Abs(r)) / 2.0
+			g.watermark.SetAudioLevel(audioLevel)
+			wm := g.watermark.NextSample()
+			l += wm
+			r += wm
+		}
+
 		// --- Stage 2: Drive + Compress + Clip₁ ---
 		l *= lp.OutputDrive
 		r *= lp.OutputDrive
@@ -389,6 +439,10 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 			composite += rdsAmp * rdsValue
 		}
 
+		// Jingle: injected when unlicensed, bypasses drive/gain controls.
+		if g.licenseState != nil && len(g.jingleFrames) > 0 {
+			composite += g.licenseState.NextSample(g.jingleFrames)
+		}
 		if g.fmMod != nil {
 			iq_i, iq_q := g.fmMod.Modulate(composite)
 			frame.Samples[i] = output.IQSample{I: float32(iq_i), Q: float32(iq_q)}
diff --git a/internal/output/frame_queue.go b/internal/output/frame_queue.go
index 223714c..13b7bbf 100644
--- a/internal/output/frame_queue.go
+++ b/internal/output/frame_queue.go
@@ -126,19 +126,18 @@ func (q *FrameQueue) Push(ctx context.Context, frame *CompositeFrame) error {
 		return errors.New("frame required")
 	}
 
-	// BUG-A fix: use closeCh in the select so that a concurrent Close() can
-	// never race with the send. The old isClosed() pre-check + separate
-	// ch<- send had a TOCTOU gap that could panic with "send on closed channel".
-	// BUG-05 fix: increment depth before the send; undo on cancel/close.
-	q.updateDepth(+1)
+	// BUG-A fix: use closeCh in the select — no TOCTOU gap.
+	// BUG-3 fix: updateDepth(+1) and trackDepth (highWaterMark) only on
+	// successful send. Pre-incrementing before the select caused the
+	// high-watermark to count frames that were never actually queued
+	// (cancelled or closed path), potentially reporting capacity+1.
 	select {
 	case q.ch <- frame:
+		q.updateDepth(+1)
 		return nil
 	case <-q.closeCh:
-		q.updateDepth(-1)
 		return ErrFrameQueueClosed
 	case <-ctx.Done():
-		q.updateDepth(-1)
 		q.recordPushTimeout()
 		return ctx.Err()
 	}
@@ -159,6 +158,19 @@ func (q *FrameQueue) Pop(ctx context.Context) (*CompositeFrame, error) {
 	}
 }
 
+// Drain removes and discards all frames currently in the queue.
+// Call before restarting a stopped engine to avoid replaying stale frames.
+func (q *FrameQueue) Drain() {
+	for {
+		select {
+		case <-q.ch:
+			q.updateDepth(-1)
+		default:
+			return
+		}
+	}
+}
+
 // Close marks the queue as closed and wakes up blocked callers.
 func (q *FrameQueue) Close() {
 	q.closeOnce.Do(func() {
diff --git a/internal/watermark/watermark.go b/internal/watermark/watermark.go
new file mode 100644
index 0000000..75050e1
--- /dev/null
+++ b/internal/watermark/watermark.go
@@ -0,0 +1,523 @@
+// Package watermark implements spread-spectrum audio watermarking for fm-rds-tx.
+//
+// # Design
+//
+// The watermark is injected into the audio L/R signal (Fix B) before stereo
+// encoding, so it survives FM broadcast and receiver demodulation intact.
+// The payload is Reed-Solomon encoded (Fix C) for robust recovery even when
+// individual bits have high error rates due to noise and audio masking.
+//
+// # Parameters
+//
+//   - PN sequence: 2048-chip LFSR-13 (seed 0x1ACE)
+//   - Payload: 8 bytes (SHA-256[:8] of key) → RS(16,8) → 16 bytes → 128 bits
+//   - Frame period: ~5.5 s at 228 kHz composite (repeats ~11×/min)
+//   - Injection: -48 dBFS on audio L+R before stereo encode (gated on audio level)
+//   - Spreading gain: 33 dB. RS erasure corrects up to 8 of 16 byte symbols.
+//
+// # Recovery (cmd/wmdecode)
+//
+//  1. Record FM receiver audio output as mono WAV (48 kHz preferred).
+//  2. Phase search: slide a single-bit PN template across [0, samplesPerBit)
+//     to find chip-aligned sample offset (coarse-fine search).
+//  3. Extract 128 bit correlations at found phase, averaged over all frames.
+//  4. Frame sync: try all 128 cyclic rotations of the bit sequence,
+//     RS-decode each; the rotation that succeeds gives the frame alignment.
+//  5. Sort bits by |correlation| (confidence). Mark weakest as erasures.
+//  6. RS erasure-decode → 8 payload bytes → compare against known keys.
+package watermark
+
+import (
+	"crypto/sha256"
+)
+
+const (
+	// pnChips is the spreading factor — PN chips per data bit at composite rate.
+	// Spreading gain = 10·log10(2048) = 33.1 dB.
+	pnChips = 2048
+
+	// rsDataBytes is the number of payload bytes before RS encoding.
+	rsDataBytes = 8
+
+	// rsCheckBytes is the number of RS parity bytes. With 8 check bytes the
+	// code corrects up to 4 errors or up to 8 erasures per 16-byte codeword.
+	rsCheckBytes = 8
+
+	// rsTotalBytes is the full RS codeword length.
+	rsTotalBytes = rsDataBytes + rsCheckBytes // 16
+
+	// payloadBits is the total number of BPSK bits per watermark frame.
+	payloadBits = rsTotalBytes * 8 // 128
+
+	// Level is the audio injection amplitude per channel (-48 dBFS).
+	// At typical audio levels this is completely inaudible.
+	Level = 0.004
+
+	// CompositeRate is the sample rate at which the watermark was embedded.
+	// The recovery tool uses this to compute fractional chip indices.
+	CompositeRate = 228000
+)
+
+// RecordingRate is the canonical recording rate used for chip-rate Bresenham stepping.
+// The embedder advances chips at this rate, so the decoder at this rate sees
+// exactly pnChips samples per bit with no fractional-stepping errors.
+const RecordingRate = 48000
+
+// Embedder continuously embeds a watermark into audio L/R samples.
+// Not thread-safe: call NextSample from the single DSP goroutine only.
+type Embedder struct {
+	codeword [rsTotalBytes]byte // RS-encoded payload, 16 bytes
+	chipIdx  int               // chip position within current bit (0..pnChips-1)
+	bitIdx   int               // current bit in codeword (0..127)
+	symbol   int8              // BPSK symbol for current bit: +1 or -1
+	accum    int               // Bresenham accumulator for chip-rate stepping
+
+	// Audio-level gate: mutes watermark during silence to prevent audibility.
+	gateGain      float64 // smooth ramp 0.0 (muted) → 1.0 (open)
+	gateThreshold float64 // audio level below which gate closes
+	gateRampUp    float64 // per-sample increment when opening (~5ms)
+	gateRampDown  float64 // per-sample decrement when closing (~5ms)
+	gateEnabled   bool
+}
+
+// NewEmbedder creates an Embedder for the given license key.
+// The key's SHA-256 hash (first 8 bytes) is RS-encoded and embedded.
+// An empty key embeds a null payload (still watermarks, just anonymous).
+func NewEmbedder(key string) *Embedder {
+	var data [rsDataBytes]byte
+	if key != "" {
+		h := sha256.Sum256([]byte(key))
+		copy(data[:], h[:rsDataBytes])
+	}
+	e := &Embedder{gateGain: 1.0}
+	e.codeword = rsEncode(data)
+	e.loadSymbol()
+	return e
+}
+
+// NextSample returns the watermark amplitude for one composite sample.
+// Add this value to both audio.Frame.L and audio.Frame.R before stereo encoding.
+//
+// The chip index advances using Bresenham stepping at RecordingRate/CompositeRate,
+// so each chip occupies exactly CompositeRate/RecordingRate composite samples on
+// average. A decoder recording at RecordingRate (48 kHz) sees exactly pnChips
+// samples per data bit, enabling simple integer-stride correlation.
+func (e *Embedder) NextSample() float64 {
+	chip := float64(pnSequence[e.chipIdx])
+	sample := Level * float64(e.symbol) * chip * e.gateGain
+
+	// Bresenham: advance chip once per RecordingRate/CompositeRate composite samples.
+	e.accum += RecordingRate
+	if e.accum >= CompositeRate {
+		e.accum -= CompositeRate
+		e.chipIdx++
+		if e.chipIdx >= pnChips {
+			e.chipIdx = 0
+			e.bitIdx = (e.bitIdx + 1) % payloadBits
+			e.loadSymbol()
+		}
+	}
+	return sample
+}
+
+// loadSymbol sets e.symbol from the current bit in the codeword (MSB first).
+func (e *Embedder) loadSymbol() {
+	byteIdx := e.bitIdx / 8
+	bitPos := uint(7 - (e.bitIdx % 8))
+	if (e.codeword[byteIdx]>>bitPos)&1 == 0 {
+		e.symbol = 1
+	} else {
+		e.symbol = -1
+	}
+}
+
+// PayloadHex returns the RS-encoded codeword as hex for logging.
+func (e *Embedder) PayloadHex() string {
+	const hx = "0123456789abcdef"
+	out := make([]byte, rsTotalBytes*2)
+	for i, b := range e.codeword {
+		out[i*2] = hx[b>>4]
+		out[i*2+1] = hx[b&0xf]
+	}
+	return string(out)
+}
+
+// EnableGate activates audio-level gating with asymmetric ramp times.
+// threshold is the linear audio amplitude below which the watermark is muted
+// (e.g. 0.01 ≈ -40 dBFS). compositeRate is needed to compute ramp speed.
+// Attack (open) is fast (5ms) so the watermark starts immediately with audio.
+// Release (close) is slow (200ms) to keep the watermark running through normal
+// inter-word and inter-phrase gaps — only extended silence mutes.
+func (e *Embedder) EnableGate(threshold, compositeRate float64) {
+	attackSamples := compositeRate * 0.005  // 5ms open
+	releaseSamples := compositeRate * 0.200 // 200ms close
+	if attackSamples < 1 {
+		attackSamples = 1
+	}
+	if releaseSamples < 1 {
+		releaseSamples = 1
+	}
+	e.gateThreshold = threshold
+	e.gateRampUp = 1.0 / attackSamples
+	e.gateRampDown = 1.0 / releaseSamples
+	e.gateEnabled = true
+}
+
+// SetAudioLevel updates the gate state based on the current audio amplitude.
+// Call once per sample before NextSample. absLevel should be the absolute
+// mono audio level (pre- or post-pre-emphasis, either works).
+func (e *Embedder) SetAudioLevel(absLevel float64) {
+	if !e.gateEnabled {
+		return
+	}
+	if absLevel > e.gateThreshold {
+		e.gateGain += e.gateRampUp
+		if e.gateGain > 1.0 {
+			e.gateGain = 1.0
+		}
+	} else {
+		e.gateGain -= e.gateRampDown
+		if e.gateGain < 0.0 {
+			e.gateGain = 0.0
+		}
+	}
+}
+
+// --- RS(16,8) over GF(2^8) — GF poly 0x11d, fcr=0, generator=2 ---
+// These routines are used by the embedder (encode) and the recovery tool (decode).
+
+func gfMul(a, b byte) byte {
+	if a == 0 || b == 0 {
+		return 0
+	}
+	return gfExp[(int(gfLog[a])+int(gfLog[b]))%255]
+}
+
+func gfInv(a byte) byte {
+	if a == 0 {
+		return 0
+	}
+	return gfExp[255-int(gfLog[a])]
+}
+
+func gfPow(a byte, n int) byte {
+	if a == 0 {
+		return 0
+	}
+	return gfExp[(int(gfLog[a])*n)%255]
+}
+
+// rsEncode encodes 8 data bytes into a 16-byte RS codeword.
+func rsEncode(data [rsDataBytes]byte) [rsTotalBytes]byte {
+	var work [rsTotalBytes]byte
+	copy(work[:rsDataBytes], data[:])
+	// Polynomial long division by the generator polynomial.
+	for i := 0; i < rsDataBytes; i++ {
+		fb := work[i]
+		if fb != 0 {
+			for j := 1; j <= rsCheckBytes; j++ {
+				work[i+j] ^= gfMul(rsGen[j], fb)
+			}
+		}
+	}
+	var cw [rsTotalBytes]byte
+	copy(cw[:rsDataBytes], data[:])
+	copy(cw[rsDataBytes:], work[rsDataBytes:])
+	return cw
+}
+
+// RSDecode recovers 8 data bytes from a (possibly corrupted) 16-byte codeword.
+// erasurePositions lists the byte indices (0..15) of symbols with low confidence
+// that should be treated as erasures. Up to 8 erasures can be corrected.
+// Returns (data, true) on success, (zero, false) on decoding failure.
+func RSDecode(recv [rsTotalBytes]byte, erasurePositions []int) ([rsDataBytes]byte, bool) {
+	// Step 1: compute syndromes.
+	var S [rsCheckBytes]byte
+	for i := 0; i < rsCheckBytes; i++ {
+		var acc byte
+		for _, c := range recv {
+			acc = gfMul(acc, gfPow(2, i)) ^ c
+		}
+		S[i] = acc
+	}
+
+	// Step 2: if no erasures and all syndromes zero, no errors.
+	hasErr := false
+	for _, s := range S {
+		if s != 0 {
+			hasErr = true
+			break
+		}
+	}
+	if !hasErr && len(erasurePositions) == 0 {
+		// Valid codeword, no errors, no erasures.
+		var out [rsDataBytes]byte
+		copy(out[:], recv[:rsDataBytes])
+		return out, true
+	}
+	if hasErr && len(erasurePositions) == 0 {
+		// Errors present but no erasure positions supplied — cannot correct.
+		// BUG FIX: previously fell through to ne==0 check and returned wrong
+		// data as correct. Now correctly signals failure so the caller can
+		// retry with erasure positions.
+		return [rsDataBytes]byte{}, false
+	}
+
+	// Step 3: erasure locator polynomial Γ(x) = ∏(1 - α^(e_j)·x).
+	gamma := []byte{1}
+	for _, pos := range erasurePositions {
+		// multiply gamma by (1 + α^pos · x)
+		alpha := gfPow(2, pos)
+		next := make([]byte, len(gamma)+1)
+		for j, g := range gamma {
+			next[j] ^= g
+			next[j+1] ^= gfMul(g, alpha)
+		}
+		gamma = next
+	}
+
+	// Step 4: modified syndrome T(x) = S(x)·Γ(x).
+	t := make([]byte, rsCheckBytes)
+	for i := 0; i < rsCheckBytes; i++ {
+		var acc byte
+		for j := 0; j < len(gamma) && j <= i; j++ {
+			if i-j < rsCheckBytes {
+				acc ^= gfMul(gamma[j], S[i-j])
+			}
+		}
+		t[i] = acc
+	}
+
+	// Step 5: compute error magnitudes using Forney's formula.
+	// For erasure-only decoding (no additional errors), the error locator
+	// is Γ itself. Evaluate omega = T mod x^(ne) where ne = len(erasures).
+	ne := len(erasurePositions)
+	if ne == 0 {
+		// Should not be reachable: handled above. Fail safely.
+		return [rsDataBytes]byte{}, false
+	}
+	if ne > rsCheckBytes {
+		return [rsDataBytes]byte{}, false
+	}
+
+	result := recv
+	for _, pos := range erasurePositions {
+		xi := gfPow(2, pos)
+		// Evaluate omega at xi^-1.
+		xiInv := gfInv(xi)
+		var omega byte
+		for j := 0; j < ne && j < rsCheckBytes; j++ {
+			omega ^= gfMul(t[j], gfPow(xiInv, j))
+		}
+		// Formal derivative of gamma at xi^-1 (only odd-degree terms survive in GF(2)).
+		var gammaPrime byte
+		for j := 1; j < len(gamma); j += 2 {
+			gammaPrime ^= gfMul(gamma[j], gfPow(xiInv, j-1))
+		}
+		if gammaPrime == 0 {
+			return [rsDataBytes]byte{}, false
+		}
+		magnitude := gfMul(omega, gfInv(gammaPrime))
+		result[pos] ^= magnitude
+	}
+
+	// Verify syndromes after correction.
+	for i := 0; i < rsCheckBytes; i++ {
+		var acc byte
+		for _, c := range result {
+			acc = gfMul(acc, gfPow(2, i)) ^ c
+		}
+		if acc != 0 {
+			return [rsDataBytes]byte{}, false
+		}
+	}
+
+	var out [rsDataBytes]byte
+	copy(out[:], result[:rsDataBytes])
+	return out, true
+}
+
+// KeyMatchesPayload returns true if SHA-256(key)[:8] matches payload.
+func KeyMatchesPayload(key string, payload [rsDataBytes]byte) bool {
+	h := sha256.Sum256([]byte(key))
+	var expected [rsDataBytes]byte
+	copy(expected[:], h[:rsDataBytes])
+	return expected == payload
+}
+
+// Constants exported for the recovery tool.
+const (
+	PnChips       = pnChips
+	PayloadBits   = payloadBits
+	RsDataBytes   = rsDataBytes
+	RsTotalBytes  = rsTotalBytes
+	RsCheckBytes  = rsCheckBytes
+)
+
+// CorrelateAt returns the correlation of received samples at the given bit
+// position. recRate is the WAV sample rate.
+//
+// At recRate = RecordingRate (48000 Hz) the chip stride is exactly 1 — the
+// embedder was designed for this rate. At other rates the chip index is
+// scaled proportionally (still works with enough frame averaging).
+func CorrelateAt(samples []float64, bitStart int, recRate float64) float64 {
+	// Samples per bit at the canonical recording rate.
+	// At RecordingRate: samplesPerBit = pnChips (integer, perfect).
+	// At other rates: scale proportionally.
+	samplesPerBit := int(float64(pnChips) * recRate / float64(RecordingRate))
+	if samplesPerBit < 1 {
+		samplesPerBit = 1
+	}
+	n := samplesPerBit
+	if bitStart+n > len(samples) {
+		n = len(samples) - bitStart
+	}
+	var acc float64
+	for i := 0; i < n; i++ {
+		// Map recording-rate sample index to chip index.
+		chipIdx := int(float64(i)*float64(RecordingRate)/recRate) % pnChips
+		acc += samples[bitStart+i] * float64(pnSequence[chipIdx])
+	}
+	return acc
+}
+
+
+// pnSequence is the 2048-chip LFSR-13 spreading code (seed 0x1ACE).
+var pnSequence = [pnChips]int8{
+		 1, -1, -1, -1,  1,  1, -1, -1,  1, -1,  1, -1, -1,  1, -1, -1,
+		-1, -1, -1,  1, -1,  1,  1, -1,  1,  1, -1,  1,  1,  1, -1,  1,
+		-1, -1,  1,  1,  1,  1,  1, -1, -1,  1,  1,  1,  1, -1,  1, -1,
+		 1, -1,  1, -1, -1, -1, -1, -1, -1, -1,  1,  1,  1,  1, -1, -1,
+		-1,  1, -1, -1, -1,  1, -1,  1, -1,  1, -1, -1, -1,  1,  1,  1,
+		-1,  1, -1, -1,  1, -1,  1,  1,  1, -1, -1,  1,  1,  1,  1, -1,
+		 1, -1, -1,  1,  1, -1,  1,  1, -1, -1,  1,  1,  1,  1, -1,  1,
+		-1,  1,  1, -1, -1, -1,  1,  1, -1, -1,  1, -1,  1,  1, -1, -1,
+		-1,  1, -1,  1, -1, -1, -1, -1,  1,  1, -1,  1,  1,  1,  1, -1,
+		-1, -1,  1, -1, -1,  1,  1,  1,  1,  1,  1,  1, -1,  1,  1, -1,
+		 1,  1, -1, -1,  1,  1, -1, -1,  1, -1,  1, -1, -1,  1,  1,  1,
+		-1, -1,  1, -1, -1,  1, -1,  1,  1,  1,  1,  1,  1, -1,  1, -1,
+		-1, -1, -1,  1,  1, -1, -1,  1,  1, -1,  1, -1, -1, -1,  1, -1,
+		 1, -1, -1, -1, -1, -1,  1,  1,  1, -1,  1, -1, -1,  1, -1, -1,
+		-1,  1, -1,  1, -1,  1,  1, -1,  1,  1, -1,  1,  1,  1,  1, -1,
+		-1, -1, -1, -1,  1,  1, -1,  1, -1,  1, -1,  1,  1,  1, -1,  1,
+		 1, -1, -1, -1, -1,  1,  1,  1,  1,  1,  1, -1, -1,  1, -1, -1,
+		-1,  1,  1, -1, -1,  1,  1, -1,  1,  1, -1,  1,  1,  1,  1,  1,
+		-1,  1, -1,  1,  1, -1, -1, -1, -1, -1, -1, -1, -1,  1,  1,  1,
+		 1,  1, -1,  1,  1,  1, -1,  1,  1,  1,  1,  1,  1,  1,  1, -1,
+		-1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1, -1,  1, -1,  1,
+		-1,  1, -1,  1, -1,  1, -1,  1, -1, -1,  1, -1,  1,  1, -1,  1,
+		-1, -1,  1, -1,  1,  1,  1,  1, -1, -1,  1, -1,  1,  1,  1,  1,
+		-1, -1,  1,  1, -1,  1,  1, -1,  1, -1,  1, -1,  1,  1,  1,  1,
+		 1, -1, -1,  1, -1,  1,  1, -1,  1, -1,  1, -1,  1, -1, -1, -1,
+		-1,  1,  1, -1,  1, -1, -1,  1, -1,  1,  1,  1, -1,  1, -1,  1,
+		 1, -1,  1,  1, -1,  1, -1,  1, -1, -1,  1, -1,  1, -1, -1,  1,
+		 1,  1,  1, -1, -1, -1,  1,  1,  1,  1, -1,  1, -1,  1, -1,  1,
+		 1,  1, -1, -1,  1,  1, -1,  1, -1, -1,  1, -1,  1, -1,  1,  1,
+		 1,  1,  1, -1, -1, -1,  1,  1,  1,  1, -1, -1,  1,  1, -1, -1,
+		-1,  1, -1,  1, -1,  1, -1, -1, -1, -1, -1,  1, -1, -1,  1, -1,
+		 1,  1, -1,  1,  1,  1, -1,  1, -1, -1, -1, -1,  1,  1, -1,  1,
+		-1,  1, -1, -1,  1, -1, -1, -1,  1,  1, -1,  1, -1, -1,  1, -1,
+		-1,  1, -1, -1, -1, -1, -1,  1,  1,  1, -1, -1, -1,  1,  1,  1,
+		-1,  1,  1,  1,  1,  1,  1, -1,  1,  1, -1, -1,  1, -1,  1, -1,
+		 1, -1,  1, -1, -1,  1,  1,  1, -1, -1, -1, -1,  1,  1,  1,  1,
+		-1,  1, -1,  1, -1, -1, -1,  1, -1,  1, -1,  1, -1, -1,  1, -1,
+		 1,  1,  1, -1, -1, -1, -1,  1,  1,  1, -1, -1, -1, -1, -1, -1,
+		 1, -1, -1, -1, -1, -1, -1,  1, -1, -1, -1,  1, -1,  1,  1,  1,
+		-1,  1,  1,  1,  1, -1,  1,  1, -1,  1, -1,  1, -1, -1,  1,  1,
+		-1, -1, -1, -1, -1,  1,  1,  1,  1, -1,  1, -1,  1,  1,  1, -1,
+		 1,  1, -1, -1,  1,  1,  1,  1, -1, -1,  1,  1,  1,  1,  1, -1,
+		 1, -1,  1, -1,  1,  1,  1,  1,  1,  1,  1,  1, -1, -1, -1, -1,
+		 1,  1, -1, -1,  1,  1, -1, -1,  1, -1, -1, -1,  1,  1, -1,  1,
+		 1, -1, -1, -1,  1,  1,  1, -1,  1, -1,  1,  1, -1, -1, -1,  1,
+		-1,  1, -1,  1,  1, -1,  1, -1,  1,  1,  1, -1, -1, -1, -1,  1,
+		 1, -1,  1, -1, -1,  1,  1, -1,  1,  1,  1, -1, -1,  1, -1,  1,
+		 1,  1,  1,  1, -1, -1,  1,  1,  1, -1, -1, -1, -1, -1, -1, -1,
+		-1,  1, -1,  1, -1,  1,  1,  1, -1,  1,  1,  1, -1,  1, -1, -1,
+		 1, -1,  1, -1,  1,  1, -1, -1,  1, -1,  1,  1,  1,  1, -1, -1,
+		-1, -1, -1,  1, -1,  1,  1, -1, -1,  1,  1,  1, -1,  1,  1,  1,
+		 1, -1, -1,  1, -1, -1,  1,  1, -1,  1, -1,  1, -1,  1,  1,  1,
+		 1, -1,  1, -1,  1,  1, -1,  1, -1, -1,  1,  1, -1, -1, -1, -1,
+		 1,  1, -1,  1, -1, -1, -1, -1, -1,  1, -1, -1, -1, -1, -1,  1,
+		 1,  1, -1,  1,  1,  1,  1, -1,  1,  1,  1, -1, -1,  1,  1,  1,
+		 1,  1,  1,  1, -1, -1,  1,  1,  1, -1, -1,  1,  1, -1, -1,  1,
+		 1,  1, -1, -1,  1,  1,  1, -1, -1,  1, -1, -1,  1,  1, -1,  1,
+		 1, -1, -1,  1, -1, -1,  1, -1, -1,  1,  1,  1,  1,  1, -1,  1,
+		-1, -1, -1, -1,  1, -1,  1, -1,  1, -1,  1,  1, -1,  1,  1,  1,
+		-1, -1, -1, -1,  1,  1,  1,  1,  1, -1, -1,  1,  1,  1, -1,  1,
+		 1, -1, -1,  1,  1, -1,  1,  1, -1, -1,  1, -1,  1, -1, -1, -1,
+		-1, -1,  1,  1, -1,  1,  1, -1,  1, -1, -1, -1,  1, -1, -1,  1,
+		 1,  1,  1,  1, -1, -1, -1,  1, -1,  1,  1,  1,  1,  1,  1,  1,
+		 1, -1,  1,  1, -1,  1, -1,  1, -1,  1, -1,  1, -1,  1,  1, -1,
+		-1, -1, -1,  1,  1,  1,  1, -1, -1, -1, -1, -1,  1, -1, -1, -1,
+		-1, -1, -1, -1,  1, -1, -1, -1,  1,  1,  1, -1,  1,  1,  1, -1,
+		 1, -1, -1, -1,  1,  1, -1, -1, -1, -1, -1, -1,  1,  1,  1, -1,
+		 1, -1,  1,  1,  1, -1,  1,  1,  1,  1,  1,  1, -1, -1, -1, -1,
+		-1, -1,  1,  1, -1, -1,  1,  1, -1,  1,  1,  1, -1,  1,  1, -1,
+		-1, -1,  1,  1, -1,  1, -1,  1, -1, -1,  1,  1,  1, -1,  1, -1,
+		 1,  1, -1,  1, -1, -1, -1, -1, -1, -1,  1,  1,  1,  1,  1, -1,
+		-1, -1,  1, -1, -1, -1,  1,  1, -1, -1,  1,  1,  1, -1,  1, -1,
+		-1, -1,  1,  1, -1,  1,  1, -1, -1,  1, -1,  1,  1,  1, -1, -1,
+		 1, -1,  1, -1, -1, -1, -1,  1,  1, -1, -1, -1,  1,  1,  1,  1,
+		-1,  1,  1,  1,  1,  1, -1,  1,  1, -1, -1,  1,  1,  1,  1,  1,
+		 1,  1,  1, -1, -1,  1, -1, -1,  1,  1, -1, -1,  1,  1, -1, -1,
+		-1,  1,  1,  1, -1, -1,  1, -1, -1,  1,  1,  1, -1,  1, -1,  1,
+		-1, -1, -1, -1,  1, -1,  1, -1,  1,  1, -1,  1, -1, -1, -1,  1,
+		-1,  1,  1, -1,  1, -1,  1,  1, -1,  1,  1,  1,  1, -1, -1,  1,
+		-1,  1,  1, -1, -1, -1, -1, -1, -1, -1,  1, -1,  1,  1, -1, -1,
+		-1,  1, -1, -1, -1,  1, -1,  1,  1, -1, -1, -1,  1, -1,  1,  1,
+		 1, -1, -1, -1, -1, -1,  1, -1, -1,  1, -1,  1, -1,  1,  1,  1,
+		-1,  1,  1, -1,  1,  1, -1,  1, -1, -1,  1,  1, -1,  1, -1,  1,
+		 1, -1, -1, -1, -1,  1, -1,  1, -1, -1,  1, -1,  1, -1, -1, -1,
+		 1, -1,  1,  1, -1,  1,  1, -1,  1, -1, -1, -1,  1,  1,  1,  1,
+		 1, -1, -1,  1, -1,  1,  1,  1, -1, -1,  1,  1, -1, -1, -1,  1,
+		 1,  1,  1,  1,  1, -1,  1, -1,  1,  1,  1, -1, -1,  1,  1, -1,
+		-1,  1, -1,  1,  1, -1, -1,  1, -1, -1,  1,  1,  1, -1, -1, -1,
+		-1, -1,  1, -1,  1,  1,  1,  1,  1,  1, -1,  1,  1,  1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1,  1,  1, -1, -1, -1,  1, -1, -1, -1,
+		 1, -1, -1, -1, -1, -1,  1, -1, -1,  1, -1, -1, -1, -1,  1, -1,
+		-1, -1,  1,  1,  1, -1, -1, -1,  1, -1, -1,  1, -1, -1, -1, -1,
+		-1, -1,  1, -1, -1,  1, -1, -1,  1, -1, -1, -1,  1, -1, -1,  1,
+		-1, -1,  1, -1, -1,  1, -1, -1, -1, -1,  1, -1,  1,  1,  1,  1,
+		-1,  1, -1, -1, -1,  1, -1,  1,  1, -1, -1,  1,  1,  1,  1, -1,
+		 1,  1, -1,  1, -1,  1,  1,  1,  1,  1,  1,  1, -1, -1,  1, -1,
+		 1,  1, -1, -1,  1,  1, -1, -1,  1, -1, -1,  1, -1,  1, -1, -1,
+		-1, -1, -1,  1, -1,  1,  1,  1,  1, -1, -1, -1,  1, -1, -1, -1,
+		-1,  1,  1, -1, -1,  1, -1, -1, -1, -1,  1, -1, -1, -1, -1, -1,
+		 1, -1,  1,  1,  1, -1,  1, -1, -1, -1,  1, -1, -1,  1, -1,  1,
+		-1,  1,  1, -1,  1,  1,  1,  1, -1,  1, -1, -1,  1, -1,  1, -1,
+		-1,  1,  1, -1, -1, -1, -1,  1, -1,  1,  1, -1,  1,  1, -1, -1,
+		-1,  1, -1, -1,  1, -1,  1, -1, -1,  1,  1,  1, -1,  1, -1, -1,
+		-1, -1,  1,  1,  1, -1, -1,  1,  1,  1,  1, -1,  1,  1,  1,  1,
+		 1,  1, -1,  1, -1,  1, -1,  1,  1, -1, -1,  1,  1, -1, -1, -1,
+		-1,  1,  1,  1,  1,  1, -1,  1, -1,  1,  1,  1, -1,  1, -1,  1,
+		-1,  1, -1, -1,  1, -1,  1, -1,  1,  1, -1,  1, -1, -1,  1, -1,
+		-1,  1, -1,  1,  1, -1, -1, -1, -1,  1, -1,  1,  1,  1,  1, -1,
+		-1,  1, -1, -1, -1,  1,  1,  1,  1,  1,  1,  1, -1,  1, -1, -1,
+		-1,  1,  1, -1, -1,  1,  1, -1, -1, -1, -1,  1, -1, -1,  1,  1,
+		 1, -1, -1,  1, -1, -1, -1,  1,  1,  1, -1, -1,  1,  1,  1, -1,
+		-1, -1,  1, -1,  1, -1,  1,  1,  1,  1,  1,  1, -1,  1,  1, -1,
+		 1, -1, -1,  1,  1, -1, -1,  1,  1,  1,  1,  1, -1, -1, -1,  1,
+		 1, -1,  1,  1, -1, -1,  1,  1, -1,  1,  1,  1,  1,  1, -1, -1,
+		 1, -1, -1,  1,  1,  1,  1,  1,  1,  1,  1, -1,  1, -1, -1, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1,  1,  1,  1, -1,  1,  1,  1, -1,
+		 1,  1,  1, -1,  1, -1,  1, -1, -1,  1,  1, -1,  1, -1,  1, -1,
+		-1,  1, -1,  1,  1,  1,  1,  1, -1, -1, -1, -1,  1, -1,  1,  1,
+		-1, -1,  1,  1, -1,  1,  1,  1, -1, -1, -1, -1, -1,  1, -1,  1,
+		-1, -1,  1,  1, -1,  1,  1,  1, -1,  1, -1, -1,  1, -1, -1,  1,
+		 1,  1,  1,  1,  1, -1, -1, -1,  1,  1,  1, -1, -1,  1,  1, -1,
+		-1,  1, -1, -1, -1, -1, -1, -1,  1, -1,  1, -1, -1, -1, -1,  1,
+		-1, -1, -1,  1, -1,  1,  1, -1,  1,  1,  1, -1, -1, -1,  1, -1,
+		 1,  1, -1, -1, -1, -1, -1, -1,  1, -1, -1,  1, -1,  1, -1, -1,
+		-1,  1, -1, -1, -1, -1,  1, -1,  1,  1, -1,  1,  1,  1,  1, -1,
+		 1,  1,  1,  1, -1, -1,  1, -1,  1, -1,  1, -1, -1,  1,  1, -1,
+		-1, -1,  1,  1,  1,  1, -1, -1, -1,  1,  1, -1,  1,  1,  1,  1,
+		 1,  1,  1, -1,  1,  1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+}
+
+// GF(2^8) tables with primitive polynomial 0x11d.
+var gfExp = [512]byte{1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76, 152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157, 39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35, 70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222, 161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30, 60, 120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163, 91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26, 52, 104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147, 59, 118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218, 169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85, 170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198, 145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171, 75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25, 50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81, 162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9, 18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11, 22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71, 142, 1, 2, 4, 8, 16, 32, 64, 128, 29, 58, 116, 232, 205, 135, 19, 38, 76, 152, 45, 90, 180, 117, 234, 201, 143, 3, 6, 12, 24, 48, 96, 192, 157, 39, 78, 156, 37, 74, 148, 53, 106, 212, 181, 119, 238, 193, 159, 35, 70, 140, 5, 10, 20, 40, 80, 160, 93, 186, 105, 210, 185, 111, 222, 161, 95, 190, 97, 194, 153, 47, 94, 188, 101, 202, 137, 15, 30, 60, 120, 240, 253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163, 91, 182, 113, 226, 217, 175, 67, 134, 17, 34, 68, 136, 13, 26, 52, 104, 208, 189, 103, 206, 129, 31, 62, 124, 248, 237, 199, 147, 59, 118, 236, 197, 151, 51, 102, 204, 133, 23, 46, 92, 184, 109, 218, 169, 79, 158, 33, 66, 132, 21, 42, 84, 168, 77, 154, 41, 82, 164, 85, 170, 73, 146, 57, 114, 228, 213, 183, 115, 230, 209, 191, 99, 198, 145, 63, 126, 252, 229, 215, 179, 123, 246, 241, 255, 227, 219, 171, 75, 150, 49, 98, 196, 149, 55, 110, 220, 165, 87, 174, 65, 130, 25, 50, 100, 200, 141, 7, 14, 28, 56, 112, 224, 221, 167, 83, 166, 81, 162, 89, 178, 121, 242, 249, 239, 195, 155, 43, 86, 172, 69, 138, 9, 18, 36, 72, 144, 61, 122, 244, 245, 247, 243, 251, 235, 203, 139, 11, 22, 44, 88, 176, 125, 250, 233, 207, 131, 27, 54, 108, 216, 173, 71, 142, 1, 2}
+var gfLog = [256]byte{0, 0, 1, 25, 2, 50, 26, 198, 3, 223, 51, 238, 27, 104, 199, 75, 4, 100, 224, 14, 52, 141, 239, 129, 28, 193, 105, 248, 200, 8, 76, 113, 5, 138, 101, 47, 225, 36, 15, 33, 53, 147, 142, 218, 240, 18, 130, 69, 29, 181, 194, 125, 106, 39, 249, 185, 201, 154, 9, 120, 77, 228, 114, 166, 6, 191, 139, 98, 102, 221, 48, 253, 226, 152, 37, 179, 16, 145, 34, 136, 54, 208, 148, 206, 143, 150, 219, 189, 241, 210, 19, 92, 131, 56, 70, 64, 30, 66, 182, 163, 195, 72, 126, 110, 107, 58, 40, 84, 250, 133, 186, 61, 202, 94, 155, 159, 10, 21, 121, 43, 78, 212, 229, 172, 115, 243, 167, 87, 7, 112, 192, 247, 140, 128, 99, 13, 103, 74, 222, 237, 49, 197, 254, 24, 227, 165, 153, 119, 38, 184, 180, 124, 17, 68, 146, 217, 35, 32, 137, 46, 55, 63, 209, 91, 149, 188, 207, 205, 144, 135, 151, 178, 220, 252, 190, 97, 242, 86, 211, 171, 20, 42, 93, 158, 132, 60, 57, 83, 71, 109, 65, 162, 31, 45, 67, 216, 183, 123, 164, 118, 196, 23, 73, 236, 127, 12, 111, 246, 108, 161, 59, 82, 41, 157, 85, 170, 251, 96, 134, 177, 187, 204, 62, 90, 203, 89, 95, 176, 156, 169, 160, 81, 11, 245, 22, 235, 122, 117, 44, 215, 79, 174, 213, 233, 230, 231, 173, 232, 116, 214, 244, 234, 168, 80, 88, 175}
+var rsGen = [9]byte{1, 255, 11, 81, 54, 239, 173, 200, 24}
+
+
+// rsGen is the RS(16,8) generator polynomial coefficients (fcr=0).
diff --git a/internal/watermark/watermark_roundtrip_test.go b/internal/watermark/watermark_roundtrip_test.go
new file mode 100644
index 0000000..8a8b72f
--- /dev/null
+++ b/internal/watermark/watermark_roundtrip_test.go
@@ -0,0 +1,188 @@
+package watermark
+
+import (
+	"math"
+	"sort"
+	"testing"
+)
+
+// TestRoundTrip verifies the full embed → downsample → phase-search → rotation → RS-decode chain.
+func TestRoundTrip(t *testing.T) {
+	const key = "test-key-42"
+	const recRate = float64(RecordingRate) // 48000
+	const compRate = float64(CompositeRate) // 228000
+	const duration = 15.0 // seconds — should give ~2 full frames
+
+	nRecSamples := int(duration * recRate)
+
+	// === Embed ===
+	emb := NewEmbedder(key)
+	// No gate — test pure watermark signal
+	samples := make([]float64, 0, nRecSamples)
+
+	// Drive embedder at CompositeRate, collect at RecordingRate via Bresenham
+	accum := 0
+	var last float64
+	for len(samples) < nRecSamples {
+		last = emb.NextSample()
+		accum += RecordingRate
+		if accum >= CompositeRate {
+			accum -= CompositeRate
+			samples = append(samples, last)
+		}
+	}
+
+	t.Logf("Embedded: %d samples @ %.0f Hz = %.2fs", len(samples), recRate, float64(len(samples))/recRate)
+
+	// RMS check
+	var rmsAcc float64
+	for _, s := range samples {
+		rmsAcc += s * s
+	}
+	rms := math.Sqrt(rmsAcc / float64(len(samples)))
+	rmsDBFS := 20 * math.Log10(rms+1e-12)
+	t.Logf("Watermark RMS: %.1f dBFS (expect ~-48)", rmsDBFS)
+	if rmsDBFS < -52 || rmsDBFS > -44 {
+		t.Errorf("RMS %.1f dBFS out of expected range [-52, -44]", rmsDBFS)
+	}
+
+	// === Decode: Phase search ===
+	samplesPerBit := int(float64(PnChips) * recRate / float64(RecordingRate))
+	t.Logf("samplesPerBit=%d, frameLen=%d", samplesPerBit, samplesPerBit*PayloadBits)
+
+	const coarseStep = 8
+	const syncBits = 64
+
+	bestPhase := 0
+	bestMag := 0.0
+	for phase := 0; phase < samplesPerBit; phase += coarseStep {
+		mag := testAvgCorrMag(samples, phase, samplesPerBit, syncBits, recRate)
+		if mag > bestMag {
+			bestMag = mag
+			bestPhase = phase
+		}
+	}
+	fineStart := bestPhase - coarseStep
+	if fineStart < 0 { fineStart = 0 }
+	fineEnd := bestPhase + coarseStep
+	if fineEnd > samplesPerBit { fineEnd = samplesPerBit }
+	for phase := fineStart; phase < fineEnd; phase++ {
+		mag := testAvgCorrMag(samples, phase, samplesPerBit, syncBits, recRate)
+		if mag > bestMag {
+			bestMag = mag
+			bestPhase = phase
+		}
+	}
+	t.Logf("Phase search: bestPhase=%d, avgCorr=%.4f", bestPhase, bestMag)
+
+	// Phase should be 0 for clean signal starting at sample 0
+	if bestPhase != 0 {
+		t.Errorf("expected bestPhase=0, got %d", bestPhase)
+	}
+
+	// === Decode: Extract correlations ===
+	nCompleteBits := (len(samples) - bestPhase) / samplesPerBit
+	nFrames := nCompleteBits / PayloadBits
+	if nFrames == 0 { nFrames = 1 }
+	t.Logf("Complete bits: %d, frames: %d", nCompleteBits, nFrames)
+
+	corrs := make([]float64, PayloadBits)
+	for i := 0; i < PayloadBits; i++ {
+		for frame := 0; frame < nFrames; frame++ {
+			bitGlobal := frame*PayloadBits + i
+			start := bestPhase + bitGlobal*samplesPerBit
+			if start+samplesPerBit > len(samples) { break }
+			corrs[i] += CorrelateAt(samples, start, recRate)
+		}
+	}
+
+	// Log correlation stats
+	var minAbs, maxAbs float64
+	for i, c := range corrs {
+		ac := math.Abs(c)
+		if i == 0 || ac < minAbs { minAbs = ac }
+		if ac > maxAbs { maxAbs = ac }
+	}
+	t.Logf("Correlation range: min|c|=%.2f, max|c|=%.2f", minAbs, maxAbs)
+
+	// === Decode: Frame sync via rotation ===
+	type decodeResult struct {
+		rotation int
+		payload  [RsDataBytes]byte
+		erasures int
+	}
+	var best *decodeResult
+
+	for rot := 0; rot < PayloadBits; rot++ {
+		var recv [RsTotalBytes]byte
+		confs := make([]float64, PayloadBits)
+		for i := 0; i < PayloadBits; i++ {
+			srcBit := (i + rot) % PayloadBits
+			c := corrs[srcBit]
+			confs[i] = math.Abs(c)
+			if c < 0 {
+				recv[i/8] |= 1 << uint(7-(i%8))
+			}
+		}
+
+		type bitConf struct { idx int; conf float64 }
+		ranked := make([]bitConf, PayloadBits)
+		for i := range ranked { ranked[i] = bitConf{i, confs[i]} }
+		sort.Slice(ranked, func(a, b int) bool { return ranked[a].conf < ranked[b].conf })
+
+		for nErase := 0; nErase <= RsCheckBytes*8; nErase++ {
+			erasedBytes := map[int]bool{}
+			for _, bc := range ranked[:nErase] {
+				erasedBytes[bc.idx/8] = true
+			}
+			if len(erasedBytes) > RsCheckBytes { break }
+			erasePos := make([]int, 0, len(erasedBytes))
+			for pos := range erasedBytes { erasePos = append(erasePos, pos) }
+			sort.Ints(erasePos)
+
+			payload, ok := RSDecode(recv, erasePos)
+			if ok {
+				if best == nil || len(erasePos) < best.erasures {
+					best = &decodeResult{rotation: rot, payload: payload, erasures: len(erasePos)}
+				}
+				break
+			}
+		}
+		if best != nil && best.erasures == 0 { break }
+	}
+
+	if best == nil {
+		t.Fatal("RS decode FAILED — no valid rotation found")
+	}
+
+	t.Logf("Decoded: rotation=%d, erasures=%d, payload=%x", best.rotation, best.erasures, best.payload)
+
+	// Rotation should be 0 for clean signal
+	if best.rotation != 0 {
+		t.Errorf("expected rotation=0, got %d", best.rotation)
+	}
+	if best.erasures != 0 {
+		t.Errorf("expected 0 erasures, got %d", best.erasures)
+	}
+
+	// Key match
+	if !KeyMatchesPayload(key, best.payload) {
+		t.Errorf("key %q does NOT match decoded payload %x", key, best.payload)
+	} else {
+		t.Logf("Key %q MATCHES", key)
+	}
+}
+
+func testAvgCorrMag(samples []float64, phase, samplesPerBit, nBits int, recRate float64) float64 {
+	var total float64
+	var count int
+	for b := 0; b < nBits; b++ {
+		start := phase + b*samplesPerBit
+		if start+samplesPerBit > len(samples) { break }
+		c := CorrelateAt(samples, start, recRate)
+		total += math.Abs(c)
+		count++
+	}
+	if count == 0 { return 0 }
+	return total / float64(count)
+}