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

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

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

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