Having seen how you can use the Arduino for performing some digital effects, I want to look at a specific form of effect, namely the digital filter. A filter is a circuit that changes the amplitude of each frequency depending on what value it is. Traditionally, these filters were implemented with analogue electronics and constructed of capacitors, resistors inductors, and operational amplifiers. A new way of implementing filters with analogue circuits has been developed in recent years, and is known as a switched capacitor filter. However, the development of digital filters offers a cheap and flexible way of creating a filter that can be implemented with a processor. As CPU resources become increasingly cheap, this type of filter is becoming more useful.