By Diksha Nama, contributor, Engineers Garage
An audio system is designed to receive audio signals (via microphone), record audio in some storage, transmit audio (through wired or wireless communication channels), and reproduce audio signals (via speakers). So, the audio circuits perform signal processing for representing the sound in the form of electrical signals, manipulate the electrical (audio) signals like amplifying, filtering, or mixing, reproduce sound from the audio signals, store audio into computer files or reproduce audio from an audio file. The following block diagram can represent a general audio system.
Like microphones or audio sources and speakers, audio filters are also the basic building block of an audio system. The audio filters are actually amplifiers or passive circuits having distinct frequency responses. They can amplify or attenuate a range of frequencies from the audio input. This is different from a simple audio amplifier or input source, which does not have a frequency dependant functioning. Any simple audio amplifier amplifies the complete input audio signal irrespective of its frequency, or an audio source delivers the audio signal irrespective of the frequencies in the signal.
By amplifying or attenuating a specific range of frequencies in the audio signal, you can creatively enhance the audio input tone. The audio crossover and equalizer are also types of audio filters. The Audio Crossover is an electronic filter used to split the input audio signal into different frequency ranges to be sent to different drivers (Twitter, Mid Range, and Woofers). The audio equalizer is an electronic filter used to amplify the audio signal according to a frequency dependant function. So, that the output from an equalizer has different amplified levels for different frequencies. The Crossover and Equalizer play a major role in the audio devices. In this tutorial, we will discuss different types of filters and terms associated with them.
What is an audio filter?
The audio filters are the electronic circuits designed to amplify or attenuate a certain range of frequency components. This helps eliminate the unwanted noise from the audio signal and improves the tone of the output audio. Filters play a major role in telecommunication and audio electronics.
Types of filters
The filters are a special type of amplifiers or passive circuits which have frequency dependant output. The filters can be classified in many ways, like construction, frequency response, or both.
Based on construction, the audio filters are classified as follows:
1) Passive Filter
2) Active Filter
The terms Passive and Active are commonly used in context to electronic components. A component which needs a power supply for its operation is called an active component like transistors and OPAM. Those electronic components that do not require any power source for their operation are called passive components like resistor, capacitor, and inductor.
Passive Filter – A passive filter is designed using passive components like resistor and capacitor or resistor and inductance. The impedance of capacitors and inductances is frequency dependant which allows constructing filters using resistor-capacitor, resistor-inductance, or resistor-capacitor-inductor combinations. These filters do not require any power source for their operation that why they are called passive filters.
Active Filters – The active filters are designed using active components like transistors or operational amplifiers. The transistors or operational amplifiers require a DC power source for their biasing. By using active components, there remains no need to use inductance to construct the filter. This reduces the size and cost of the circuit and improves the efficiency of the filter. Since these filters require a DC biasing source for their active components, these are called active filters.
The filters can also be classified based on their frequency response. The range of frequencies which are amplified or allowed to pass by a filter is called its passband. The passband is the region in the frequency curve of filters where the circuit’s voltage or power is maximum. Based on the frequency band allowed to pass by the filters, they are categorized as follows:
1) Highpass Filter
2) Lowpass Filter
3) Bandpass Filter
4) Bandstop Filter
5) Notch Filter
6) Allpass Filter
7) Equalization Filter
Highpass Filter – This filter passes all the frequencies above its cut-off frequency and blocks all the frequencies below the cut-off frequency. The cut-off frequency is when the signal’s voltage or amplitude falls to 0.707 or 3 dB of the passband voltage. At this point, the power output of the circuit starts falling. The typical frequency curve of a highpass filter is shown below.
Fig. 2: Image showing Frequency Response of a Highpass Audio Filter
As can be seen from the frequency response graph, the low-frequency signals are not completely attenuated at the cutoff frequency. Frequencies below cut-off frequencies are also passed by this highpass filter but with very less gain. So there is a roll-off in the cut-off frequency. That’s why this is sometimes called a roll-off frequency.
Lowpass Filter – This filter passes all the frequencies below its cut-off frequency and blocks the frequencies above it. The frequency response of a lowpass filter is as follows:
It can be seen from the frequency response graph that at the cutoff frequency, the high-frequency signals are not completely attenuated. Frequencies above cut-off frequencies are also passed by this lowpass filter but with very less gain.
Bandpass Filter –This filter only passes a band of frequencies in its cut-off frequency range. The bandpass filter has two cut-off frequencies, one is lower cut off, and another one is upper cut off frequency. The center frequency and bandwidth of the filter decide the lower and upper cut-off frequencies shown in the frequency response graph below.
Bandstop Filter – A bandstop filter passes all the frequencies except a specific range of frequencies. It passes all the frequencies below its lower cut-off and all the frequencies above its higher cut-off but not frequencies ranging between lower and higher cut-off. The higher cut-off and lower cut-off frequencies are deviations of a center frequency for which the gain of the filter circuit is ideally zero (practically minimum).
Notch Filter – A notch filter is a bandstop filter with a very narrow stopband. Due to the very narrow stopband, these filters have a very high Quality Factor.
Allpass Filter – An allpass filter allows passing all the frequencies but modifies the phase relationship between them. So, at the allpass filter’s output, different frequency ranges have phase differences with each other. The frequency response graph of an allpass filter has phase-shifted frequencies, as shown in the graph below.
Equalizer Filter – An equalizer filter does not completely attenuates or pass a specific range of frequencies but variably amplifies frequencies according to a frequency dependant function.
Based on both frequency response and the construction, the filters can be classified as follow –
1) Passive Highpass Filter
2) Active Highpass Filter
3) Passive Lowpass Filter
4) Active Lowpass Filter
5) Passive Bandpass Filter
6) Active Bandpass Filter
7) Passive Bandstop Filter
8) Active Bandstop Filter
Passive Highpass Filter – A highpass filter blocks the lower frequency components and allows higher frequency components. A passive highpass filter can be constructed using an RC network. This type of filter is generally used to direct high-frequency components of an audio signal to a tweeter. A simple passive highpass filter is shown below.
For the RC network shown above, the cut off frequency is related to the resistor and capacitor as follows:
fh = 1/ (2πRC)
So, by setting the resistor and capacitor’s value, a highpass filter with desired cut-off frequency can be designed. In the above circuit, the cut-off frequency will be 160 Hz approximately. The above highpass filter will pass all the frequencies above 160 Hz and attenuate the frequencies below it.
A passive filter does not have any bandwidth limitation and can be designed by selecting the resistor and capacitor’s suitable value. It does not require any power source for DC biasing. Such a filter requires fewer components to design and has a high current output. However, these filters cannot amplify the audio signal, and if an inductor is used for their construction, they are costly and bulky.
Active Highpass Filter – An active highpass filter can be designed using transistors or operational amplifiers. A simple active highpass filter (first-order filter) is shown below
This filter uses an OPAM (Operational Amplifier) at the RC network’s output, making it an active filter. While the RC network is blocking the low-frequency components, the OPAM amplifies the allowed frequency range. Since the RC network is connected at the non-inverting input pin of the operational amplifier, its output is not inverted. However, if it would have been connected at inverting pin of the OPAM, the output audio signal would have been out of phase by 180 degrees from the input audio signal.
This filter has no loading effect. The OPAM has high input impedance and low output impedance, so they do not suffer from the loading of source and load. The filter has non-unity gain, which is generally very high. So, the output audio signal is not only noise-free, but it is also well amplified. These filters are also small in size, and generally, the ICs or transistors used in their design are not bulky. However, an active filter design involves more components that require a DC source for their biasing. So, the filter circuit requires an external power supply for its operation. Also, due to the use of an operational amplifier, the filter circuit has bandwidth limitations.
Passive Lowpass Filter – A passive lowpass filter can be constructed using an RC network or RL network (for the first-order filter). A second-order lowpass filter can be constructed using an RLC network. Higher-order lowpass filters that filter the audio signal more precisely can be designed by combining many first-order filters in series. In a simple passive lowpass filter, the input audio signal is passed through the resistor (instead of a capacitor like in a highpass filter). The capacitor is connected between the resistor and the ground.
The following equation gives the cut-off frequency of such filter:
fl = 1/ (2πRC)
The filter allows all the frequencies below the cut-off frequency to pass but attenuates the frequencies above the cut-off frequency. These filters do not have any bandwidth limitation and do not require any power source for their operation. These are generally used to drive low-frequency components of an audio signal to the woofers.
Active Lowpass Filter – The active lowpass filter uses an operational amplifier or transistor amplifier at the output before the lowpass RC, RL, RLC, or multiple order passive filters. The operational amplifier amplifies the allowed low-frequency components before delivering to a power amplifier or the speaker. The gain provided by the OPAM is the main advantage of such a filter. However, such a filter has a bandwidth limitation and requires a DC source for biasing the OPAM or transistor circuit.
Passive Bandpass Filter – A bandpass filter is designed by combining a lowpass and highpass filter. It is generally constructed using an RLC network. A simple passive bandpass filter is shown below.
In the bandpass filter shown above, a highpass filter is connected in series with a lowpass Filter. The highpass filter’s cut-off frequency is actually the lower cut-off frequency of this bandpass filter. The cut-off frequency of the lowpass filter is actually the higher cut-off frequency of this bandpass filter. So, only the frequencies lying between the cut of frequencies of the combined highpass and lowpass filter are allowed to pass at the output.
These filters are generally used to direct a specific range of frequencies to mid-range drivers. Due to the increased number of components in their construction, these filters are quite bulky in size.
Active Bandpass Filter – The active bandpass filter has an operational amplifier or transistor amplifier connected before the output after the passive bandpass circuit. The operational amplifier amplifies the allowed band of frequencies. In such a filter, the bandwidth of the OPAM must match with the desired bandwidth of the bandpass filter.
Passive Bandstop Filter – A bandstop filter attenuates a range of frequencies and allows passing frequencies lower than its lower cut-off and higher than its higher cut-off. A (first-order) passive bandstop filter is generally constructed using an RLC network where the input signal is passed through the resistor. The LC network is connected between the resistor and the ground. A simple passive bandpass filter is shown below.
Such a circuit is a parallel combination of a highpass and a lowpass filter. The highpass filter’s cut-off frequency is the higher cut-off frequency of this bandstop filter, and the cut-off frequency of the lowpass filter is the lower cut-off frequency of this bandstop filter. So, only the frequencies excluding the frequencies between the cut-off frequencies of the combined highpass and lowpass filter are allowed to pass at the output. These filters are also called Band Reject Filters, Band Elimination Filters, and T-Notch Filters.
Active Bandstop Filters – An active bandstop filter has an operational amplifier or transistor amplifier at the output, which amplifies the allowed frequency signals before they are delivered to a power amplifier or audio driver. The OPAM using such an amplifier must have suitable bandwidth to match the band-stop filter’s desired frequency curve.
Few terms are frequently used in context to the audio filters. Some of these terms are explained below.
1) Bandwidth – This is the range of frequencies allowed to pass by the filter. The bandwidth can be defined as the difference in upper and lower cut-off frequency. Sometimes it is also known as Pass Band Bandwidth. The bandwidth determines the frequency response of the filter within the set range of frequencies. The bandwidth of a lowpass filter from its frequency response curve is shown in the figure below –
2) Quality Factor (Q-Factor) – The Quality Factor describes the losses in the resonator circuit. The ratio of energy stored at the resonator to the energy supplied to it per cycle to maintain the signal constant amplitude. The more Q means fewer losses and vice versa.
Q = (Energy stored / Energy lost per cycle)
In terms of bandwidth, the Q is determined by the following equation:
Q = (fc/ BW)
fc = Resonant Frequency
BW = Bandwidth or Resonance Width
The Q-factor can be determined using the frequency curve of the audio filter: