anchor.diff:      _filter.frequency.setValueAtTime(frequencyCutoff, now);
       _filter.frequency.exponentialRampToValueAtTime(rampTo, now + 1.0);
       
  -    prevFrequencyCutoff = frequencyCutoff;
  +    _prevFrequencyCutoff = frequencyCutoff;
       _prevQResonance = _qResonance;
     }
   }
positive.code:   AudioContext _context;
    
    double frequencyCutoff = 25000.0;
    double prevFrequencyCutoff = 0.0;
    // Higher values lead to more distortion.
    double _qResonance = 0.001;
    double _prevQResonance = 0.0;
  
negative.code: 
      bool valueChanged = false;
      
      if (frequencyCutoff != _prevFrequencyCutoff) {
        _filter.frequency.cancelScheduledValues(now);
        _filter.frequency.setValueAtTime(frequencyCutoff, now);
        valueChanged = true;
  