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Chapters

Introduction to Digital Filters
    Elementary Audio Digital Filters
       Elementary Filter Sections
          One-Zero

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One-Zero

Figure B.1: Signal flow graph for the general one-zero filter
$ y(n) = b_0x(n) + b_1 x(n - 1)$.
\begin{figure}\input fig/kfig2p17.pstex_t \end{figure}

Figure B.1 gives the signal flow graph for the general one-zero filter. The frequency response for the one-zero filter may be found by the following steps:

\fbox{ \begin{tabular}{rl} Difference equation: & $y(n) = b_0x(n) + b_1x(n - 1)$... ...requency response: & $H(e^{j\omega T}) = b_0 + b_1e^{-j\omega T}$ \end{tabular}}

By factoring out $ e^{-j\omega T/2}$ from the frequency response, to balance the exponents of $ e$, we can get this closer to polar form as follows:

\begin{eqnarray*} H(e^{j\omega T}) &=& b_0 + b_1 e^{-j\omega T}\\ &=& (b_0 - ... ...omega T/2)\\ &=& (b_0 - b_1) + e^{-j\pi f T} 2b_1\cos(\pi f T) \end{eqnarray*}

Figure B.2: Family of frequency responses of the one-zero filter
$ y(n) = x(n) + b_1 x(n - 1)$
for various values of $ b_1$. (a) Amplitude response. (b) Phase response.
\begin{figure}\input fig/kfig2p19.pstex_t \end{figure}

We now apply the general equations given in Chapter 7 for filter gain $ G(\omega)$ and filter phase $ \Theta(\omega)$ as a function of frequency:

\begin{eqnarray*} H(e^{j\omega T}) &=& b_0 + b_1e^{-j\omega T}\\ &=& b_0 + b_1... ...left[\frac{-b_1 \sin(\omega T)}{b_0 + b_1 \cos(\omega T)}\right] \end{eqnarray*}

A plot of $ G(\omega)$ and $ \Theta(\omega)$ for $ b_0 = 1$ and various real values of $ b_1$, is given in Fig.B.2. The filter has a zero at $ z = -b_1/b_0 = -b_1$ in the $ z$ plane, which is always on the real axis. When a point on the unit circle comes close to the zero of the transfer function the filter gain at that frequency is low. Notice that one real zero can basically make either a highpass ( $ b_1/b_0 < 0$) or a lowpass filter ( $ b_1/b_0 > 0$). For the phase response calculation using the graphical method, it is necessary to include the pole at $ z=0$.

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Next: One-Pole
written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.


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