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A telemetry data processing system based on crosscorrelation techniques and maximum-likelihood detection principles and designed to decode a special type of pulse-code modulated (PCM) signals, pulse-frequency modulation (PFM), is discussed. The decoding system employs a high-speed multiple-pass tape scanner to sequentially generate a set of correlation indices for each data word using the alphabet of possible data words. An on-line digital computer functions as the decision device to select the greatest of the indices as representing the transmitted data word. The data processing system combines analog-digital hybrid techniques for relatively high-speed signal processing and yet requires only a small digital computer memory.