setInput(source) {
    if (!source) {
      p5sound.fftMeter.connect(this.analyser);
    } else {
      if (source.output) {
        source.output.connect(this.analyser);
      } else if (source.connect) {
        source.connect(this.analyser);
      }
      p5sound.fftMeter.disconnect();
    }
  }
Set the input source for the FFT analysis. If no source is
 provided, FFT will analyze all sound in the sketch.

 @method  setInput
 @for p5.FFT
 @param {Object} [source] p5.sound object (or web audio API source node)