diff --git a/internal/audio/source.go b/internal/audio/source.go
new file mode 100644
index 0000000..cad96ce
--- /dev/null
+++ b/internal/audio/source.go
@@ -0,0 +1,37 @@
+package audio
+
+import (
+    "math"
+)
+
+type ToneSource struct {
+    LeftFreq   float64
+    RightFreq  float64
+    SampleRate float64
+    LeftPhase  float64
+    RightPhase float64
+    Amplitude  float64
+}
+
+func NewToneSource(sampleRate float64) *ToneSource {
+    return &ToneSource{
+        LeftFreq:   1000,
+        RightFreq:  1600,
+        SampleRate: sampleRate,
+        Amplitude:  0.4,
+    }
+}
+
+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)
+    s.LeftPhase += s.LeftFreq / s.SampleRate
+    s.RightPhase += s.RightFreq / s.SampleRate
+    if s.LeftPhase >= 1 {
+        s.LeftPhase -= math.Floor(s.LeftPhase)
+    }
+    if s.RightPhase >= 1 {
+        s.RightPhase -= math.Floor(s.RightPhase)
+    }
+    return NewFrame(Sample(left), Sample(right))
+}
diff --git a/internal/audio/source_test.go b/internal/audio/source_test.go
new file mode 100644
index 0000000..e92a8c6
--- /dev/null
+++ b/internal/audio/source_test.go
@@ -0,0 +1,12 @@
+package audio
+
+import "testing"
+
+func TestToneSourceNextFrame(t *testing.T) {
+    src := NewToneSource(48000)
+    a := src.NextFrame()
+    b := src.NextFrame()
+    if a == b {
+        t.Fatal("expected varying frames")
+    }
+}
diff --git a/internal/offline/generator.go b/internal/offline/generator.go
index cf42a6b..99e5ef7 100644
--- a/internal/offline/generator.go
+++ b/internal/offline/generator.go
@@ -55,16 +55,13 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
     })
     rdsSamples := rdsEnc.Generate(samples)
 
-    leftFreq := 1000.0
-    rightFreq := 1600.0
-    stereoCarrierFreq := 38000.0
+    source := audio.NewToneSource(sampleRate)
 
     for i := 0; i < samples; i++ {
         t := float64(i) / sampleRate
-        left := audio.Sample(0.4 * math.Sin(2*math.Pi*leftFreq*t))
-        right := audio.Sample(0.4 * math.Sin(2*math.Pi*rightFreq*t+math.Pi/3))
-        comps := stereoEncoder.Encode(audio.NewFrame(left, right))
-        stereoDSB := comps.Stereo * math.Sin(2*math.Pi*stereoCarrierFreq*t)
+        in := source.NextFrame()
+        comps := stereoEncoder.Encode(in)
+        stereoDSB := comps.Stereo * math.Sin(2*math.Pi*38000.0*t)
         rdsValue := 0.0
         if g.cfg.RDS.Enabled && i < len(rdsSamples) {
             rdsValue = rdsSamples[i]