Group Delay Examples in Matlab

Figure 7.6 compares the group delay responses for a number
of classic lowpass filters, including the example of Fig.7.2.
The matlab code is listed in Fig.7.5. See, *e.g.*, Parks and
Burrus [64] for a discussion of Butterworth,
Chebyshev, and Elliptic Function digital filter design. See also
§I.2 for details on the Butterworth case. The various
types may be summarized as follows:

- Butterworth filters are maximally flat in middle of the passband.
- Chebyshev Type I filters are ``equiripple'' in the passband and ``Butterworth'' in the stopband.
- Chebyshev Type II filters are ``Butterworth'' in the passband and equiripple in the stopband.
- Elliptic function filters are equiripple in both the passband and stopband.

As Fig.7.6.4 indicates, and as is well known, the
Butterworth filter has the flattest group delay curve (and most gentle
transition from passband to stopband) for the four types compared.
The elliptic function filter has the largest amount of ``delay
distortion'' near the cut-off frequency (passband edge frequency).
Fundamentally, the more abrupt the transition from passband to
stopband, the greater the delay-distortion across that transition, for
any minimum-phase filter. (Minimum-phase filters are introduced in
Chapter 11.) The delay-distortion can be compensated by
*delay equalization*, *i.e.*, adding delay at other frequencies in
order approach an overall constant group delay versus frequency.
Delay equalization is typically carried out using an allpass filter
(defined in §B.2) in series with the filter to be
delay-equalized [1].

[Bb,Ab] = butter(4,0.5); % order 4, cutoff at 0.5 * pi Hb=freqz(Bb,Ab); Db=grpdelay(Bb,Ab); [Bc1,Ac1] = cheby1(4,1,0.5); % 1 dB passband ripple Hc1=freqz(Bc1,Ac1); Dc1=grpdelay(Bc1,Ac1); [Bc2,Ac2] = cheby2(4,20,0.5); % 20 dB stopband attenuation Hc2=freqz(Bc2,Ac2); Dc2=grpdelay(Bc2,Ac2); [Be,Ae] = ellip(4,1,20,0.5); % like cheby1 + cheby2 He=freqz(Be,Ae); [De,w]=grpdelay(Be,Ae); figure(1); plot(w,abs([Hb,Hc1,Hc2,He])); grid('on'); xlabel('Frequency (rad/sample)'); ylabel('Gain'); legend('butter','cheby1','cheby2','ellip'); saveplot('../eps/grpdelaydemo1.eps'); figure(2); plot(w,[Db,Dc1,Dc2,De]); grid('on'); xlabel('Frequency (rad/sample)'); ylabel('Delay (samples)'); legend('butter','cheby1','cheby2','ellip'); saveplot('../eps/grpdelaydemo2.eps'); |

Group Delays |

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