diff --git a/docs/README.md b/docs/README.md
index bf66fb4..5981e5a 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -10,6 +10,13 @@
 - `go run ./cmd/fmrtx --simulate-tx --simulate-output build/sim/simulated-soapy.iqf32 --simulate-duration 250ms`
 - `go run ./cmd/offline -duration 500ms -output build/offline/composite.iqf32`
 
+### Tone configuration
+
+The current no-hardware source can be parameterized via config:
+- `audio.toneLeftHz`
+- `audio.toneRightHz`
+- `audio.toneAmplitude`
+
 ### Internal DSP module
 - `cd internal`
 - `go test ./...`
diff --git a/docs/config.sample.json b/docs/config.sample.json
index 4397599..23ecaf4 100644
--- a/docs/config.sample.json
+++ b/docs/config.sample.json
@@ -2,7 +2,10 @@
   "audio": {
     "inputPath": "",
     "sampleRate": 48000,
-    "gain": 1.0
+    "gain": 1.0,
+    "toneLeftHz": 1000,
+    "toneRightHz": 1600,
+    "toneAmplitude": 0.4
   },
   "rds": {
     "enabled": true,
diff --git a/internal/audio/source.go b/internal/audio/source.go
index cad96ce..7f95153 100644
--- a/internal/audio/source.go
+++ b/internal/audio/source.go
@@ -22,6 +22,14 @@ func NewToneSource(sampleRate float64) *ToneSource {
     }
 }
 
+func NewConfiguredToneSource(sampleRate, leftHz, rightHz, amplitude float64) *ToneSource {
+    src := NewToneSource(sampleRate)
+    src.LeftFreq = leftHz
+    src.RightFreq = rightHz
+    src.Amplitude = amplitude
+    return src
+}
+
 func (s *ToneSource) NextFrame() Frame {
     left := s.Amplitude * math.Sin(2*math.Pi*s.LeftPhase)
     right := s.Amplitude * math.Sin(2*math.Pi*s.RightPhase)
diff --git a/internal/config/config.go b/internal/config/config.go
index ff3b799..a300a12 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -15,9 +15,12 @@ type Config struct {
 }
 
 type AudioConfig struct {
-    InputPath  string  `json:"inputPath"`
-    SampleRate int     `json:"sampleRate"`
-    Gain       float64 `json:"gain"`
+    InputPath      string  `json:"inputPath"`
+    SampleRate     int     `json:"sampleRate"`
+    Gain           float64 `json:"gain"`
+    ToneLeftHz     float64 `json:"toneLeftHz"`
+    ToneRightHz    float64 `json:"toneRightHz"`
+    ToneAmplitude  float64 `json:"toneAmplitude"`
 }
 
 type RDSConfig struct {
@@ -50,7 +53,7 @@ type ControlConfig struct {
 
 func Default() Config {
     return Config{
-        Audio: AudioConfig{SampleRate: 48000, Gain: 1.0},
+        Audio: AudioConfig{SampleRate: 48000, Gain: 1.0, ToneLeftHz: 1000, ToneRightHz: 1600, ToneAmplitude: 0.4},
         RDS: RDSConfig{Enabled: true, PI: "1234", PS: "FMRTX", RadioText: "fm-rds-tx", PTY: 0},
         FM: FMConfig{FrequencyMHz: 100.0, StereoEnabled: true, PilotLevel: 0.1, RDSInjection: 0.03, OutputDrive: 0.5, CompositeRateHz: 228000},
         Backend: BackendConfig{Kind: "file", OutputPath: "build/out/composite.f32"},
@@ -80,6 +83,12 @@ func (c Config) Validate() error {
     if c.Audio.Gain < 0 || c.Audio.Gain > 4 {
         return fmt.Errorf("audio.gain out of range")
     }
+    if c.Audio.ToneLeftHz <= 0 || c.Audio.ToneRightHz <= 0 {
+        return fmt.Errorf("audio tone frequencies must be positive")
+    }
+    if c.Audio.ToneAmplitude < 0 || c.Audio.ToneAmplitude > 1 {
+        return fmt.Errorf("audio.toneAmplitude out of range")
+    }
     if c.FM.FrequencyMHz < 65 || c.FM.FrequencyMHz > 110 {
         return fmt.Errorf("fm.frequencyMHz out of range")
     }
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index aae9e11..f2d6fd2 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -16,15 +16,15 @@ func TestDefaultValidate(t *testing.T) {
 func TestLoadAndValidate(t *testing.T) {
     dir := t.TempDir()
     path := filepath.Join(dir, "config.json")
-    if err := os.WriteFile(path, []byte(`{"fm":{"frequencyMHz":99.9},"backend":{"kind":"file","outputPath":"out.f32"},"control":{"listenAddress":"127.0.0.1:8088"}}`), 0o644); err != nil {
+    if err := os.WriteFile(path, []byte(`{"audio":{"toneLeftHz":900,"toneRightHz":1700,"toneAmplitude":0.3},"fm":{"frequencyMHz":99.9},"backend":{"kind":"file","outputPath":"out.f32"},"control":{"listenAddress":"127.0.0.1:8088"}}`), 0o644); err != nil {
         t.Fatalf("write config: %v", err)
     }
     cfg, err := Load(path)
     if err != nil {
         t.Fatalf("load config: %v", err)
     }
-    if cfg.FM.FrequencyMHz != 99.9 {
-        t.Fatalf("unexpected frequency: %v", cfg.FM.FrequencyMHz)
+    if cfg.Audio.ToneLeftHz != 900 {
+        t.Fatalf("unexpected left tone: %v", cfg.Audio.ToneLeftHz)
     }
 }
 
diff --git a/internal/offline/generator.go b/internal/offline/generator.go
index 99e5ef7..d150bd3 100644
--- a/internal/offline/generator.go
+++ b/internal/offline/generator.go
@@ -55,7 +55,7 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
     })
     rdsSamples := rdsEnc.Generate(samples)
 
-    source := audio.NewToneSource(sampleRate)
+    source := audio.NewConfiguredToneSource(sampleRate, g.cfg.Audio.ToneLeftHz, g.cfg.Audio.ToneRightHz, g.cfg.Audio.ToneAmplitude)
 
     for i := 0; i < samples; i++ {
         t := float64(i) / sampleRate
@@ -108,5 +108,5 @@ func (g *Generator) WriteFile(path string, duration time.Duration) error {
 }
 
 func (g *Generator) Summary(duration time.Duration) string {
-    return fmt.Sprintf("offline frame: freq=%.1fMHz sampleRate=%d duration=%s outputDrive=%.2f stereo=%t rds=%t", g.cfg.FM.FrequencyMHz, g.cfg.FM.CompositeRateHz, duration.String(), g.cfg.FM.OutputDrive, g.cfg.FM.StereoEnabled, g.cfg.RDS.Enabled)
+    return fmt.Sprintf("offline frame: freq=%.1fMHz sampleRate=%d duration=%s outputDrive=%.2f stereo=%t rds=%t toneL=%.1f toneR=%.1f", g.cfg.FM.FrequencyMHz, g.cfg.FM.CompositeRateHz, duration.String(), g.cfg.FM.OutputDrive, g.cfg.FM.StereoEnabled, g.cfg.RDS.Enabled, g.cfg.Audio.ToneLeftHz, g.cfg.Audio.ToneRightHz)
 }