Fix bug 617 based on Steve's patch.

src/effects/ChangeTempo.cpp

@@ -51,6 +51,11 @@ EffectChangeTempo::EffectChangeTempo()
    m_ToLength = 0.0;	
 }
 
+double EffectChangeTempo::CalcPreviewInputLength(double previewLength)
+{
+   return previewLength * (100.0 + m_PercentChange) / 100.0;
+}
+
 wxString EffectChangeTempo::GetEffectDescription() { 
    // Note: This is useful only after change amount has been set. 
    return wxString::Format(_("Applied effect: %s %.1f%%"), 

src/effects/ChangeTempo.h

@@ -60,6 +60,8 @@ class EffectChangeTempo : public EffectSoundTouch {
    virtual bool CheckWhetherSkipEffect() { return (m_PercentChange == 0.0); }
    virtual bool Process();
 
+   double CalcPreviewInputLength(double previewLength);
+  
  private:
    double			m_PercentChange;	// percent change to apply to tempo
 												// -100% is meaningless, but sky's the upper limit

src/effects/Effect.cpp

@@ -403,6 +403,11 @@ long TrapLong(long x, long min, long max)
       return x;
 }
 
+double Effect::CalcPreviewInputLength(double previewLength)
+{
+   return previewLength;
+}
+
 wxString Effect::GetPreviewName()
 {
    return _("Pre&view");
@@ -422,7 +427,7 @@ void Effect::Preview()
    WaveTrack *mixRight = NULL;
    double rate = mProjectRate;
    double t0 = mT0;
-   double t1 = t0 + previewLen;
+   double t1 = t0 + CalcPreviewInputLength(previewLen);
 
    if (t1 > mT1)
       t1 = mT1;
@@ -488,6 +493,8 @@ void Effect::Preview()
       if (mixRight)
          playbackTracks.Add(mixRight);
 
+      t1 = wxMin(mixLeft->GetEndTime(), t0 + previewLen);
+
 #ifdef EXPERIMENTAL_MIDI_OUT
       NoteTrackArray empty;
 #endif

src/effects/Effect.h

@@ -128,6 +128,10 @@ class AUDACITY_DLL_API Effect {
    // Only override it if you need to do preprocessing or cleanup.
    virtual void Preview();
 
+   // Most effects just use the previewLength, but time-stretching/compressing 
+   // effects need to use a different input length, so override this method. 
+   virtual double CalcPreviewInputLength(double previewLength);
+
    // Get an unique ID assigned to each registered effect.
    // The first effect will have ID zero.
    int GetID() {