RASTA filtering and mean subtraction was introduced to support perceptual linear prediction (PLP) preprocessing.
It has also been applied to cepstrum feature-based preprocessing with both log spectral and cepstral domain filtering.
The numerator is a regression filter with N being the order (must be odd) and the denominator is an integrator with time decay.
RASTA-filtering can be changed to use mean subtraction, implementing a moving average filter.
The mean becomes the long term cepstrum and is typically computed on the speech part for each separate utterance.