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Feedforward Comb Filter Amplitude Response

Comb filters get their name from the ``comb-like'' appearance of their amplitude response (gain versus frequency), as shown in Figures 2.25, 2.26, and 2.27. For a review of frequency-domain analysis of digital filters, see, e.g., [449].

Figure: Amplitude responses of the feed forward comb-filter $ H(z) = x(n) + g x(n-M)$ (diagrammed in Fig.2.23) with $ M=5$ and $ g=0.1$, $ 0.5$, and $ 0.9$. a) Linear amplitude scale. b) Decibel scale. The frequency axis goes from 0 to the sampling rate (instead of only half the sampling rate, which is more typical for real filters) in order to display the fact that the number of notches is exactly $ M=5$ (as opposed to ``$ 2.5$'').
\includegraphics[width=\twidth ]{eps/ffcfar}

The transfer function of the feedforward comb filter Eq.$ \,$(2.2) is

$\displaystyle H(z) = b_0+b_M\,z^{-M},$ (3.3)

so that the amplitude response (gain versus frequency) is

$\displaystyle G(\omega) \isdef \left\vert H(e^{j\omega})\right\vert = \left\vert b_0 + b_M e^{-j\omega M}\right\vert, \quad -\pi \leq \omega \leq \pi. \protect$ (3.4)

This is plotted in Fig.2.25 for $ M=5$, $ b_0=1$, and $ b_M=0.1$, $ 0.5$, and $ 0.9$. When $ b_0=b_M=1$, we get the simplified result

$\displaystyle G(\omega) = \left\vert 1 + e^{-j\omega M}\right\vert = \left\vert... ...ga M/2}\right\vert = 2\left\vert\cos\left(\omega\frac{M}{2}\right)\right\vert. $

In this case, we obtain $ M$ nulls, which are points (frequencies) of zero gain in the amplitude response. Note that in flangers, these nulls are moved slowly over time by modulating the delay length $ M$. Doing this smoothly requires interpolated delay lines (see Chapter 4 and Chapter 5).

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