DSP System Toolbox
This example shows how to remove a 250 Hz interfering tone from an audio signal by using a notch filter. To listen to the effect of the filter on the audio, the center frequency and 3 dB bandwidth of the notch filter can be tuned as the audio is playing.
A notch filter is used to eliminate a specific frequency from a given signal. In their most common form, the filter design parameters for notch filters are center frequency for the notch and the 3 dB bandwidth. The center frequency is the frequency point at which the filter has a gain of zero. The 3 dB bandwidth measures the frequency width of the notch of the filter computed at the half-power or 3 dB attenuation point.
In this example, you tune a notch filter in order to eliminate a 250 Hz sinusoidal tone corrupting an audio signal. You can control both the center frequency and the bandwidth of the notch filter and listen to the filtered audio signal as you tune the design parameters.
The command audioToneRemovalExampleApp launches a user interface designed to interact with the simulation. It also launches a SpectrumAnalyzer to view the spectrum of the audio with and without filtering along with the magnitude response of the notch filter.
The notch filter itself is implemented in dsp.NotchPeakFilter. The filter has two specification modes: 'Design parameters' and 'Coefficients'. The 'Design parameters' mode allows you to specify the center frequency and bandwidth in Hz. This is the only mode used in this example. The 'Coefficients' mode allows you to specify the multipliers or coefficients in the filter directly. In the latter mode, each coefficient affects only one characteristic of the filter (either the center frequency or the 3 dB bandwidth). In other words, the effect of tuning the coefficients is completely decoupled.
Using MATLAB Coder, you can generate a MEX file for the main processing algorithm by executing the command HelperAudioToneRemovalCodeGeneration. You can use the generated MEX file by executing the command audioToneRemovalExampleApp(true).