Here is a collection of one liner IIR filters.

Each filter has been transformed into a single C++ expression.

The filter parameter is f or g, and the state variable that needs to be kept around between interations is s.

- Christian

101 Leaky Integrator

a0 = 1

b1 = 1 - f

out = s += in - f * s;

102 Basic Lowpass (all-pole)

A first order lowpass filter, by finite difference appoximation (differentials --> differences).

a0 = f

b1 = 1 - f

out = s += f * ( in - s );

103 Lowpass with inverted control

Same as above, except for different filter parameter is now inverted.

In this case, g equals the location of the pole.

a0 = g - 1

b1 = g

out = s = in + g * ( s - in );

104 Lowpass with zero at Nyquist

A first order lowpass filter, by via the conformal map of the z-plane (0..infinity --> 0..Nyquist).

a0 = f

a1 = f

b1 = 1 - 2 * f

s = temp + ( out = s + ( temp = f * ( in - s ) ) );

105 Basic Highpass (DC-blocker)

Input complement to basic lowpass, yields a finite difference highpass filter.

a0 = 1 - f

a1 = f - 1

b1 = 1 - f

out = in - ( s += f * ( in - s ) );

106 Highpass with forced unity gain at Nyquist

Input complement to filter 104, yields a conformal map highpass filter.

a0 = 1 - f

a1 = f - 1

b1 = 1 - 2 * f

out = in + temp - ( s += 2 * ( temp = f * ( in - s ) ) );

107 Basic Allpass

This corresponds to a first order allpass filter,

where g is the location of the pole in the range -1..1.

a0 = -g

a1 = 1

b1 = g

s = in + g * ( out = s - g * in );