Allpass from Two Combs

Figure 2.30: A combined feedback/feedforward comb filter which gives an allpass filter when $b_0 = a_M$ .

An allpass filter can be defined as any filter having a gain of $1$ at all frequencies (but typically different delays at different frequencies).

It is well known that the series combination of a feedforward and feedback comb filter (having equal delays) creates an allpass filter when the feedforward coefficient is the negative of the feedback coefficient.

Figure 2.30 shows a combination feedforward/feedback comb filter structure which shares the same delay line.^3.13 By inspection of Fig.2.30, the difference equation is

$$\begin{eqnarray*} v(n) &=& x(n) - a_M v(n-M)\\ y(n) &=& b_0 v(n) + v(n-M). \end{eqnarray*}$$

This can be recognized as a digital filter in direct form II [452]. Thus, the system of Fig.2.30 can be interpreted as the series combination of a feedback comb filter (Fig.2.24) taking $x(n)$ to $v(n)$ followed by a feedforward comb filter (Fig.2.23) taking $v(n)$ to $y(n)$ . By the commutativity of LTI systems, we can interchange the order to get

$$\begin{eqnarray*} v(n) &=& b_0 x(n) + x(n-M)\\ y(n) &=& v(n) - a_M y(n-M). \end{eqnarray*}$$

Substituting the right-hand side of the first equation above for $v(n)$ in the second equation yields more simply

$$y(n) = b_0 x(n) + x(n-M) - a_M y(n-M). \protect$$ (3.15)

This can be recognized as direct form I [452], which requires $2M$ delays instead of $M$ ; however, unlike direct-form II, direct-form I cannot suffer from ``internal'' overflow--overflow can happen only at the output.

The coefficient symbols $b_0$ and $a_M$ here have been chosen to correspond to standard notation for the transfer function

$$H(z) = \frac{b_0 + z^{-M}}{1 + a_M z^{-M}}.$$

The frequency response is obtained by setting $z = e^{j\omega T}$ , where $\omega$ denotes radian frequency and $T$ denotes the sampling period in seconds [452]. For an allpass filter, the frequency magnitude must be the same for all $\omega\in[-\pi/T,\pi/T]$ .

An allpass filter is obtained when $b_0 = \overline{a_M}$ , or, in the case of real coefficients, when $b_0 = a_M$ . To see this, let $a\isdef a_M=\overline{b_0}$ . Then we have

$$\left\vert H(e^{j\omega T})\right\vert = \left\vert\frac{\overline{a} + e^{-j\omega MT}}{1 + a e^{-j\omega MT}}\right\vert = \left\vert\frac{\overline{a} + e^{-j\omega MT}}{e^{j\omega MT} + a}\right\vert = \left\vert\frac{\overline{a + e^{j\omega MT}}}{a+e^{j\omega MT}}\right\vert = 1.$$