spandsp: Removing DC bias from a signal

spandsp 0.0.6

Removing DC bias from a signal

# What does it do?

Telecoms signals often contain considerable DC, but DC upsets a lot of signal processing functions. Placing a zero DC restorer at the front of the processing chain can often simplify the downstream processing.

# How does it work?

The DC restorer uses a leaky integrator to provide a long-ish term estimate of the DC bias in the signal. A 32 bit estimate is used for the 16 bit audio, so the noise introduced by the estimation can be keep in the lower bits, and the 16 bit DC value, which is subtracted from the signal, is fairly clean. The following code fragment shows the algorithm used. dc_bias is a 32 bit integer, while the sample and the resulting clean_sample are 16 bit integers.

    dc_bias += ((((int32_t) sample << 15) - dc_bias) >> 14);
    clean_sample = sample - (dc_bias >> 15);