1. An implantable medical device (IMD) comprising: a memory; an analog to digital converter (ADC) configured to receive an analog waveform and sample the waveform into digitized data; a smoothing filter configured to process the sampled signal and classify each data point into a first type or a second type and to apply a smoothing function to the first type of classified data and to retain a sampled valued for the second type of classified data; a predictive encoding module operatively coupled with the ADC to generate an encoded waveform data point for each of an equivalent data point from a subset of the digitized data; and an entropy encoding module operatively coupled with the predictive encoder and configured to selectively replace an encoded waveform data point with a codeword for that data point; and wherein the smoothing filter is configured to determine variability between sampled data points adjacent to a given data point being classified and classifying the given data point into the first type if the variability is less than a predetermined variability value and into the second group if the variability is equal to or greater than the predetermined variability value.