Filters

In electronics, a filter is a group of components in a circuit that performs signal processing. They are made from basic components such as resistors, inductors, and capacitors. They also include microprocessors in the case of digital filters. Parts of the input signal may either be blocked or have their frequency and amplitude modified depending on the arrangement of these components in the filter circuit. There are a wide range of filter categories including passive, active, linear, non-linear, analogue, and digital.

Passive and Active Filters

Passive filters use only the energy of the input signal to change the signal properties, while active filters additional energy. Passive filters are made from basic components like resistors, inductors, and capacitors. Active filters also use these components, along with others that require a power source such as transistors, op-amps, and voltage amplifiers. Passive filter are often used to block or modify parts of the input signal, while active filters are mostly used for amplification. Both filter types are fundamental to circuit design and can be found in almost all electronic devices. One example is an audio amplifier, which uses many passive and active filter stages to remove noise from the input signal, as well as increasing its power so it can drive the speakers.

Linear and Non-Linear Filters

Linear filters produce an output signal that is a linear function of the input signal, while the output signal of non-linear filters is not a linear function.

Linear filters can be designed to remove specific frequencies or bands of frequencies. Low-pass linear filters remove frequencies above a certain threshold, while high-pass linear filters remove those frequencies below it. Band-pass linear filters remove all frequencies except for a specific group, while band-stop linear filters only remove the specific group of frequencies. Non-linear filters are often used to remove signal spikes, such as the static noise created by the electrical interference of other devices.

Analogue and Digital Filters

Analogue filters directly modify signals that change continuously over time. Raw audio and video feeds that have not been converted to digital are examples of these signals. Analogue filters were once widely used in radios and stereos to change the bass, treble, and volume of the sound. They were are also used in televisions to change the brightness and contrast of the picture, as well as the sound volume. However, signal processing in modern devices is mostly done by digital filters.

Digital filters modify signals that change discretely over time, which means their amplitude and frequency changes in steps. An analogue-to-digital converter is first used to turn the input signal into a digital approximation, which is then modified by the microprocessors in the digital filter. The output signal is sometimes turned back into an analogue signal by a digital-to-analogue converter. Digital filters provide greater flexibility over how the signal is modified and stored. They are used to perform digital signal processing (DSP) which has applications in telecommunications, radar, medical imaging, computer graphics, and many other fields. While digital signals are not perfect copies of analogue signals, the distortion is usually small enough to be ignored.