diff --git a/internal/classifier/classifier.go b/internal/classifier/classifier.go
new file mode 100644
index 0000000..9bb54b0
--- /dev/null
+++ b/internal/classifier/classifier.go
@@ -0,0 +1,11 @@
+package classifier
+
+// Classify builds features and applies the rule-based classifier.
+func Classify(input SignalInput, spectrum []float64, sampleRate int, fftSize int) *Classification {
+	if len(spectrum) == 0 || input.FirstBin < 0 || input.LastBin < 0 {
+		return nil
+	}
+	feat := ExtractFeatures(input, spectrum, sampleRate, fftSize)
+	cls := RuleClassify(feat)
+	return &cls
+}
diff --git a/internal/classifier/classifier_test.go b/internal/classifier/classifier_test.go
new file mode 100644
index 0000000..dc9625b
--- /dev/null
+++ b/internal/classifier/classifier_test.go
@@ -0,0 +1,21 @@
+package classifier
+
+import "testing"
+
+func TestRuleClassifyWFM(t *testing.T) {
+	sampleRate := 1_000_000
+	fftSize := 1024
+	spectrum := make([]float64, fftSize)
+	for i := range spectrum {
+		spectrum[i] = -100
+	}
+	start := 100
+	end := 350 // ~244 bins -> ~238 kHz
+	for i := start; i <= end; i++ {
+		spectrum[i] = -10
+	}
+	cls := Classify(SignalInput{FirstBin: start, LastBin: end}, spectrum, sampleRate, fftSize)
+	if cls == nil || cls.ModType != ClassWFM {
+		t.Fatalf("expected WFM, got %+v", cls)
+	}
+}
diff --git a/internal/classifier/features.go b/internal/classifier/features.go
new file mode 100644
index 0000000..a30159a
--- /dev/null
+++ b/internal/classifier/features.go
@@ -0,0 +1,167 @@
+package classifier
+
+import (
+	"math"
+)
+
+// ExtractFeatures computes spectral features for a signal slice.
+// spectrum is full-band power in dB (length fftSize).
+func ExtractFeatures(s SignalInput, spectrum []float64, sampleRate int, fftSize int) Features {
+	if fftSize <= 0 {
+		fftSize = len(spectrum)
+	}
+	if len(spectrum) == 0 || s.FirstBin < 0 || s.LastBin < s.FirstBin || s.FirstBin >= len(spectrum) {
+		return Features{}
+	}
+	if s.LastBin >= len(spectrum) {
+		s.LastBin = len(spectrum) - 1
+	}
+	binHz := float64(sampleRate) / float64(max(1, fftSize))
+	// slice
+	start := s.FirstBin
+	end := s.LastBin
+	peakDb := -1e9
+	peakIdx := start
+	sumLin := 0.0
+	geoSum := 0.0
+	count := 0
+	for i := start; i <= end; i++ {
+		db := spectrum[i]
+		if db > peakDb {
+			peakDb = db
+			peakIdx = i
+		}
+		p := math.Pow(10, db/10.0)
+		sumLin += p
+		if p > 0 {
+			geoSum += math.Log(p)
+		}
+		count++
+	}
+	avgLin := 0.0
+	if count > 0 {
+		avgLin = sumLin / float64(count)
+	}
+	// Peak-to-avg in dB
+	peakToAvg := 0.0
+	if avgLin > 0 {
+		peakToAvg = 10 * math.Log10(math.Pow(10, peakDb/10.0)/avgLin)
+	}
+	// Spectral flatness
+	flat := 0.0
+	if count > 0 && avgLin > 0 {
+		geoMean := math.Exp(geoSum / float64(count))
+		flat = geoMean / avgLin
+	}
+	// BW3dB
+	bw3 := bwAtThreshold(spectrum, start, end, peakDb-3.0) * binHz
+	// BW90 (90% energy)
+	bw90 := bwEnergy(spectrum, start, end, 0.90) * binHz
+	// Symmetry (power left/right of peak)
+	leftSum, rightSum := 0.0, 0.0
+	for i := start; i <= end; i++ {
+		p := math.Pow(10, spectrum[i]/10.0)
+		if i <= peakIdx {
+			leftSum += p
+		} else {
+			rightSum += p
+		}
+	}
+	sym := 0.0
+	if leftSum+rightSum > 0 {
+		sym = (rightSum - leftSum) / (rightSum + leftSum)
+	}
+	// Rolloff (dB/kHz) at edges
+	leftDb := spectrum[start]
+	rightDb := spectrum[end]
+	leftHz := math.Max(binHz, float64(peakIdx-start)*binHz)
+	rightHz := math.Max(binHz, float64(end-peakIdx)*binHz)
+	rollL := (peakDb - leftDb) / (leftHz / 1e3)
+	rollR := (peakDb - rightDb) / (rightHz / 1e3)
+
+	return Features{
+		BW3dB:        bw3,
+		BW90:         bw90,
+		SpectralFlat: clamp01(flat),
+		PeakToAvg:    peakToAvg,
+		Symmetry:     sym,
+		RolloffLeft:  rollL,
+		RolloffRight: rollR,
+	}
+}
+
+func bwAtThreshold(spectrum []float64, start, end int, threshDb float64) float64 {
+	left := start
+	right := end
+	for i := start; i <= end; i++ {
+		if spectrum[i] >= threshDb {
+			left = i
+			break
+		}
+	}
+	for i := end; i >= start; i-- {
+		if spectrum[i] >= threshDb {
+			right = i
+			break
+		}
+	}
+	if right < left {
+		return float64(end - start + 1)
+	}
+	return float64(right - left + 1)
+}
+
+func bwEnergy(spectrum []float64, start, end int, frac float64) float64 {
+	if frac <= 0 {
+		return 0
+	}
+	if frac > 1 {
+		frac = 1
+	}
+	powers := make([]float64, 0, end-start+1)
+	sum := 0.0
+	for i := start; i <= end; i++ {
+		p := math.Pow(10, spectrum[i]/10.0)
+		sum += p
+		powers = append(powers, p)
+	}
+	if sum == 0 {
+		return float64(end - start + 1)
+	}
+	// accumulate from center outward
+	center := (start + end) / 2
+	l := center
+	r := center
+	acc := powers[center-start]
+	for acc/sum < frac && (l > start || r < end) {
+		if l > start {
+			l--
+			acc += powers[l-start]
+		}
+		if acc/sum >= frac {
+			break
+		}
+		if r < end {
+			r++
+			acc += powers[r-start]
+		}
+	}
+	return float64(r - l + 1)
+}
+
+func clamp01(v float64) float64 {
+	if v < 0 {
+		return 0
+	}
+	if v > 1 {
+		return 1
+	}
+	return v
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
diff --git a/internal/classifier/rules.go b/internal/classifier/rules.go
new file mode 100644
index 0000000..85b9377
--- /dev/null
+++ b/internal/classifier/rules.go
@@ -0,0 +1,74 @@
+package classifier
+
+import "math"
+
+func RuleClassify(feat Features) Classification {
+	bw := feat.BW3dB
+	flat := feat.SpectralFlat
+	sym := feat.Symmetry
+	p2a := feat.PeakToAvg
+
+	best := ClassUnknown
+	second := ClassUnknown
+	conf := 0.3
+
+	switch {
+	case bw > 100e3:
+		best = ClassWFM
+		conf = 0.9
+	case bw >= 6e3 && bw <= 16e3:
+		best = ClassNFM
+		conf = 0.8
+		if flat > 0.7 {
+			second = ClassNoise
+		}
+	case bw >= 500 && bw < 3e3:
+		if sym > 0.2 {
+			best = ClassSSBUSB
+			conf = 0.7
+		} else if sym < -0.2 {
+			best = ClassSSBLSB
+			conf = 0.7
+		} else if p2a > 3 && flat < 0.4 {
+			best = ClassAM
+			conf = 0.6
+		}
+	case bw < 500:
+		best = ClassCW
+		conf = 0.7
+	}
+
+	// noise hint
+	if best == ClassUnknown && flat > 0.85 && bw > 2e3 {
+		best = ClassNoise
+		conf = 0.6
+	}
+
+	// edge-case: if symmetry is strong, second best opposite side
+	if (best == ClassSSBUSB || best == ClassSSBLSB) && second == ClassUnknown {
+		if best == ClassSSBUSB {
+			second = ClassSSBLSB
+		} else {
+			second = ClassSSBUSB
+		}
+	}
+
+	// slightly scale confidence by feature strength
+	if best == ClassNFM || best == ClassWFM {
+		conf = conf * (0.8 + 0.2*clamp01(1-flat))
+	}
+	if best == ClassAM {
+		conf = conf * (0.7 + 0.3*clamp01(p2a/6.0))
+	}
+	if math.IsNaN(conf) || conf <= 0 {
+		conf = 0.3
+	}
+
+	return Classification{
+		ModType:    best,
+		Confidence: conf,
+		BW3dB:      bw,
+		Features:   feat,
+		SecondBest: second,
+	}
+}
diff --git a/internal/classifier/types.go b/internal/classifier/types.go
new file mode 100644
index 0000000..eccc905
--- /dev/null
+++ b/internal/classifier/types.go
@@ -0,0 +1,43 @@
+package classifier
+
+// SignalClass is a coarse modulation label.
+type SignalClass string
+
+const (
+	ClassAM      SignalClass = "AM"
+	ClassNFM     SignalClass = "NFM"
+	ClassWFM     SignalClass = "WFM"
+	ClassSSBUSB  SignalClass = "USB"
+	ClassSSBLSB  SignalClass = "LSB"
+	ClassCW      SignalClass = "CW"
+	ClassNoise   SignalClass = "NOISE"
+	ClassUnknown SignalClass = "UNKNOWN"
+)
+
+// Features are lightweight spectral features derived from a signal slice.
+type Features struct {
+	// Spectral
+	BW3dB        float64 `json:"bw_3db_hz"`
+	BW90         float64 `json:"bw_90_hz"`
+	SpectralFlat float64 `json:"spectral_flat"`
+	PeakToAvg    float64 `json:"peak_to_avg_db"`
+	Symmetry     float64 `json:"symmetry"`
+	RolloffLeft  float64 `json:"rolloff_left_db_khz"`
+	RolloffRight float64 `json:"rolloff_right_db_khz"`
+}
+
+// Classification is the classifier output attached to signals/events.
+type Classification struct {
+	ModType    SignalClass `json:"mod_type"`
+	Confidence float64     `json:"confidence"`
+	BW3dB      float64     `json:"bw_3db_hz"`
+	Features   Features    `json:"features,omitempty"`
+	SecondBest SignalClass `json:"second_best,omitempty"`
+}
+
+// SignalInput is the minimal input needed for classification.
+type SignalInput struct {
+	FirstBin int
+	LastBin  int
+	SNRDb    float64
+}
diff --git a/internal/detector/detector.go b/internal/detector/detector.go
index b0bd2e2..e305bb7 100644
--- a/internal/detector/detector.go
+++ b/internal/detector/detector.go
@@ -4,18 +4,21 @@ import (
 	"math"
 	"sort"
 	"time"
+
+	"sdr-visual-suite/internal/classifier"
 )
 
 type Event struct {
-	ID         int64     `json:"id"`
-	Start      time.Time `json:"start"`
-	End        time.Time `json:"end"`
-	CenterHz   float64   `json:"center_hz"`
-	Bandwidth  float64   `json:"bandwidth_hz"`
-	PeakDb     float64   `json:"peak_db"`
-	SNRDb      float64   `json:"snr_db"`
-	FirstBin   int       `json:"first_bin"`
-	LastBin    int       `json:"last_bin"`
+	ID        int64                      `json:"id"`
+	Start     time.Time                  `json:"start"`
+	End       time.Time                  `json:"end"`
+	CenterHz  float64                    `json:"center_hz"`
+	Bandwidth float64                    `json:"bandwidth_hz"`
+	PeakDb    float64                    `json:"peak_db"`
+	SNRDb     float64                    `json:"snr_db"`
+	FirstBin  int                        `json:"first_bin"`
+	LastBin   int                        `json:"last_bin"`
+	Class     *classifier.Classification `json:"class,omitempty"`
 }
 
 type Detector struct {
@@ -23,32 +26,35 @@ type Detector struct {
 	MinDuration time.Duration
 	Hold        time.Duration
 
-	binWidth float64
-	nbins    int
+	binWidth   float64
+	nbins      int
+	sampleRate int
 
-	active   map[int64]*activeEvent
-	nextID   int64
+	active map[int64]*activeEvent
+	nextID int64
 }
 
 type activeEvent struct {
-	id        int64
-	start     time.Time
-	lastSeen  time.Time
-	centerHz  float64
-	bwHz      float64
-	peakDb    float64
-	snrDb     float64
-	firstBin  int
-	lastBin   int
+	id       int64
+	start    time.Time
+	lastSeen time.Time
+	centerHz float64
+	bwHz     float64
+	peakDb   float64
+	snrDb    float64
+	firstBin int
+	lastBin  int
+	class    *classifier.Classification
 }
 
 type Signal struct {
-	FirstBin int     `json:"first_bin"`
-	LastBin  int     `json:"last_bin"`
-	CenterHz float64 `json:"center_hz"`
-	BWHz     float64 `json:"bw_hz"`
-	PeakDb   float64 `json:"peak_db"`
-	SNRDb    float64 `json:"snr_db"`
+	FirstBin int                        `json:"first_bin"`
+	LastBin  int                        `json:"last_bin"`
+	CenterHz float64                    `json:"center_hz"`
+	BWHz     float64                    `json:"bw_hz"`
+	PeakDb   float64                    `json:"peak_db"`
+	SNRDb    float64                    `json:"snr_db"`
+	Class    *classifier.Classification `json:"class,omitempty"`
 }
 
 func New(thresholdDb float64, sampleRate int, fftSize int, minDur, hold time.Duration) *Detector {
@@ -64,6 +70,7 @@ func New(thresholdDb float64, sampleRate int, fftSize int, minDur, hold time.Dur
 		Hold:        hold,
 		binWidth:    float64(sampleRate) / float64(fftSize),
 		nbins:       fftSize,
+		sampleRate:  sampleRate,
 		active:      map[int64]*activeEvent{},
 		nextID:      1,
 	}
@@ -100,21 +107,22 @@ func (d *Detector) detectSignals(spectrum []float64, centerHz float64) []Signal
 				peakBin = i
 			}
 		} else if in {
-			signals = append(signals, d.makeSignal(start, i-1, peak, peakBin, noise, centerHz))
+			signals = append(signals, d.makeSignal(start, i-1, peak, peakBin, noise, centerHz, spectrum))
 			in = false
 		}
 	}
 	if in {
-		signals = append(signals, d.makeSignal(start, n-1, peak, peakBin, noise, centerHz))
+		signals = append(signals, d.makeSignal(start, n-1, peak, peakBin, noise, centerHz, spectrum))
 	}
 	return signals
 }
 
-func (d *Detector) makeSignal(first, last int, peak float64, peakBin int, noise float64, centerHz float64) Signal {
+func (d *Detector) makeSignal(first, last int, peak float64, peakBin int, noise float64, centerHz float64, spectrum []float64) Signal {
 	centerBin := float64(first+last) / 2.0
 	centerFreq := centerHz + (centerBin-float64(d.nbins)/2.0)*d.binWidth
 	bw := float64(last-first+1) * d.binWidth
 	snr := peak - noise
+	cls := classifier.Classify(classifier.SignalInput{FirstBin: first, LastBin: last, SNRDb: snr}, spectrum, d.sampleRate, d.nbins)
 	return Signal{
 		FirstBin: first,
 		LastBin:  last,
@@ -122,6 +130,7 @@ func (d *Detector) makeSignal(first, last int, peak float64, peakBin int, noise
 		BWHz:     bw,
 		PeakDb:   peak,
 		SNRDb:    snr,
+		Class:    cls,
 	}
 }
 
@@ -158,6 +167,7 @@ func (d *Detector) matchSignals(now time.Time, signals []Signal) []Event {
 				snrDb:    s.SNRDb,
 				firstBin: s.FirstBin,
 				lastBin:  s.LastBin,
+				class:    s.Class,
 			}
 			continue
 		}
@@ -179,6 +189,11 @@ func (d *Detector) matchSignals(now time.Time, signals []Signal) []Event {
 		if s.LastBin > best.lastBin {
 			best.lastBin = s.LastBin
 		}
+		if s.Class != nil {
+			if best.class == nil || s.Class.Confidence >= best.class.Confidence {
+				best.class = s.Class
+			}
+		}
 	}
 
 	var finished []Event
@@ -204,6 +219,7 @@ func (d *Detector) matchSignals(now time.Time, signals []Signal) []Event {
 			SNRDb:     ev.snrDb,
 			FirstBin:  ev.firstBin,
 			LastBin:   ev.lastBin,
+			Class:     ev.class,
 		})
 		delete(d.active, id)
 	}