Designing the Filter

This section is a step-by-step introduction to using the Filter Designer app to design an octave-band filter. An octave is the interval between two frequencies having a ratio of 2:1. An octave-band filter is a bandpass filter with high cutoff frequency approximately twice that of the low cutoff frequency. The class of an octave filter is determined by its allowable passband ripple and its stopband attenuation. Refer to the ANSI S1.11–2004 standard for more information.

Start the app from the MATLAB^® command line.
```
filterDesigner
```
The app opens with a default filter. Its filter information is summarized in the upper left (Current Filter Information) and its filter specifications are depicted in the upper right. In addition to displaying filter specification, this upper right pane displays filter responses and filter coefficients.
The bottom half of the app shows the Filter Design panel, where you specify the filter parameters. Other panels, such as Import filter from workspace and Pole/Zero Editor, which you access with the buttons on the lower left, are also displayed in this area. If you have other products installed, you may see additional buttons.
Note that when you open the app, Design Filter is not enabled. You must make a change to the default filter design in order to enable Design Filter. This is true each time you want to change the filter design. Changes to radio button items or drop down menu items such as those under Response Type or Filter Order enable Design Filter immediately. Changes to specifications in text boxes such as Fs, Fpass, and Fstop require you to click outside the text box to enable Design Filter.
In the Response Type pane, select Bandpass.
In the Design Method pane, select IIR, and then select Butterworth from the selection list.
For the Filter Order, select Specify order, and then enter 6.

Set the Frequency Specifications as follows:

Parameter	Setting	Description
Units	`Hz`	Units for the parameters
Fs	`48000`	Sampling frequency
Fc1	`22`	First cutoff frequency (i.e., the frequency preceding the passband at which the magnitude response is 3 dB below the passband gain)
Fc2	`45`	Second cutoff frequency (i.e., the frequency following the passband at which the magnitude response is 3 dB below the passband gain)

After specifying the filter design parameters, click the Design Filter button at the bottom of the design panel to compute the filter coefficients. The display updates to show the magnitude response of the designed filter.
Notice that the Design Filter button is disabled after you compute the coefficients for your filter design. This button is enabled again if you make any changes to the filter specifications.
Click the Store Filter button.
In the Store Filter dialog, change the filter name to Bandpass Butterworth-1 and click OK to save the filter in the Filter Manager.