DC Blocker Software Implementations

In plain C, the difference equation for the dc blocker could be written as follows:

  y = x - xm1 + 0.995 * ym1;
  xm1 = x;
  ym1 = y;

Here, x denotes the current input sample, and y denotes the current output sample. The variables xm1 and ym1 hold once-delayed input and output samples, respectively (and are typically initialized to zero). In this implementation, the pole is fixed at $R=0.995$, which corresponds to an adaptation time-constant of approximately $1/(1-R) = 200$ samples. A smaller $R$ value allows faster tracking of ``wandering dc levels'', but at the cost of greater low-frequency attenuation.

A complete C++ class implementing a dc blocking filter is included in the free, open-source Synthesis Tool Kit (STK) [15]. (See the DCBlock STK class.)

For a discussion of issues and solutions related to fixed-point implementations, see [7].