(F-1) less than one and the attenuation is a negative number.īand Reject Filter – a filter that rejects (attenuates) one frequency band and passes both a lower and a higher frequency band. Filter attenuation is the ratio, at a given frequency, of the signal amplitude at the output of the filter over the signal amplitude at the input of the filter, defined asĪs is usually the case, for a given frequency, the output amplitude of the filter is smaller than the input amplitude, making the ratio in Eq. In this context, an allpass filter is sometimes called a phase equalizer.Īttenuation – an amplitude loss, usually measured in dB, incurred by a signal after passing through a digital filter. Allpass filters are typically appended in a cascade arrangement following a standard IIR filter, H1(z), as shown in Figure F-1.įigure F-1 Typical use of an allpass filter.Īn allpass filter, Hap(z), can be designed such that its phase response compensates for, or equalizes, the nonlinear phase response of an original IIR filter. - Thus the phase response of the combined filter, Hcombined (z), is more linear than the original H1(z), and this is particularly desirable in communications systems. This material is an introduction to the terminology of digital filters.Īllpass filter – an IIR filter whose magnitude response is unity over its entire frequency range, but whose phase response is variable. Fortunately, the vocabulary of digital filters corresponds rather well with the mother tongue used for continuous (analog) filters-so we don’t have to unlearn anything that we already know. The first step in becoming familiar with digital filters is to learn to speak the language used in the filter business. Lyons, UNDERSTANDING DIGITAL SIGNAL PROCESSING, (Appendix F – pages 494-505.) © 1997 by Addison Wesley Longman Inc., Reproduced by permission of Addison Wesley Longman. The magnitude response of the filter is displayed in the Filter Analysis area after the coefficients are computed.From: R. After setting the design specifications, click the Design Filter button at the bottom of the GUI to design the filter. Wpass and Wstop, in the Magnitude Specifications area are positive weights, one per band, used during optimization in the FIR Equiripple filter. Enter 0.2 for wpass and 0.5 for wstop in the Frequency Specifications area.Ħ. Select Normalized (0 to 1) in the Units pull down menu in the Frequency Specifications area.ĥ. Increasing the value creates a filter which more closely approximates an ideal equiripple filter, but more time is required as the computation increases. The FIR Equiripple filter has a Density Factor option which controls the density of the frequency grid. Select Specify order in the Filter Order area and enter 30.ģ. In general, when you change the Response Type or Design Method, the filter parameters and Filter Display region update automatically.Ģ. Select Lowpass from the dropdown menu under Response Type and Equiripple under FIR Design Method. We will use an FIR Equiripple filter with these specifications:ġ. We will design a low pass filter that passes all frequencies less than or equal to 20% of the Nyquist frequency (half the sampling frequency) and attenuates frequencies greater than or equal to 50% of the Nyquist frequency. You can right-click or click the What's This? button to get information on the different parts of the tool. The tool includes Context-sensitive help. Other panels can be displayed in the lower half by using the sidebar buttons. It controls what is displayed in the other two upper regions. The Design Panel, in the lower half is where you define your filter specifications.
The lower half of the GUI is the interactive portion of Filter Designer. The Filter Display region, in the upper right, displays various filter responses, such as, magnitude response, group delay and filter coefficients. It also provides access to the Filter manager for working with multiple filters. The Current Filter Information region, in the upper left, displays filter properties, namely the filter structure, order, number of sections used and whether the filter is stable or not. The upper half of the GUI displays information on filter specifications and responses for the current filter.
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