The bi-quadratic filter, usually called the bi-quad filter, is arguably the most important and widely used filter in audio signal processing. The same design can be modified to create many different filter types including a low-pass filter, high-pass filter, band-pass filter, low shelf, high shelf, peaking/bell, and all-pass filter.
A bi-quad MATLAB function has been implemented and can be used to apply the desired filtering to a signal. The function can be downloaded here.
The following is the syntax for using biquadFilter:
[y] = biquadFilter(x,Fs,f0,Q,dBGain,type,form)
Input Variables:
x – input signal to be filtered
Fs – sampling rate
f0 – cut-off, center, or corner frequency depending on filter type
Q – bandwidth, resonance parameter
dBGain – amplitude change in units of decibels (dB)
type – filter type. Example: 'lpf' , 'hpf' , 'pkf' , 'apf' , 'bp1' , 'bp2', 'lsf' , 'hsf'
form – filter topology, 1 (Direct Form I), 2 (DFII), 3 (Transposed DFII)
Output Variable:
y – processed, output signal
The filter has two feedforward delay lines and two feedback delay lines. By manipulating the relative gain values, most common types of filters can implemented using the same design.