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DC Blocker

The dc blocker is an indispensable tool in digital waveguide modeling and other applications.11.4 It is often needed to remove the dc component of the signal circulating in a delay-line loop. It is also often an important tool in multi-track recording, where dc components in the various tracks can add up and overflow the mix.

The dc blocker is a small recursive filter specified by the difference equation

$\displaystyle y(n) = x(n) - x(n-1) + R\, y(n-1)

where $ R$ is a parameter which is typically somewhere between $ 0.9$ and $ 1$ (for a 44.1 kHz sampling rate, $ R=0.995$ is good). The transfer function is

$\displaystyle H(z) = \frac{1-z^{-1}}{1-Rz^{-1}}, \protect$ (11.10)

thus, there is a zero at dc ($ z = 1$) and a pole near dc at $ z=R$. Far away from dc, the pole and zero approximately cancel each other. (Recall the graphical method for determining frequency response magnitude described in Chapter 8.)

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``Introduction to Digital Filters with Audio Applications'', by Julius O. Smith III, (August 2006 Edition).
Copyright © 2007-02-02 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
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