BiquadFilter = function(audiolet, frequency) {
    AudioletNode.call(this, audiolet, 2, 1);

    // Same number of output channels as input channels
    this.linkNumberOfOutputChannels(0, 0);

    this.frequency = new AudioletParameter(this, 1, frequency || 22100);
    this.lastFrequency = null; // See if we need to recalculate coefficients

    // Delayed values
    this.xValues = [];
    this.yValues = [];

    // Coefficients
    this.b0 = 0;
    this.b1 = 0;
    this.b2 = 0;
    this.a0 = 0;
    this.a1 = 0;
    this.a2 = 0;
}
Generic biquad filter.  The coefficients (a0, a1, a2, b0, b1 and b2) are set
using the calculateCoefficients function, which should be overridden and
will be called automatically when new values are needed.

**Inputs**

- Audio
- Filter frequency

**Outputs**

- Filtered audio

**Parameters**

- frequency The filter frequency.  Linked to input 1.

@constructor
@extends AudioletNode
@param {Audiolet} audiolet The audiolet object.
@param {Number} frequency The initial frequency.