In Fig. 1 , we introduce a systematic diagram of a simple sampling-reconstructing system (SRS) where X„, is the sampling interval and xc. is the filter cutoff frecuency.We asume a non isoplanatic system.Therefore, one avoids applying linear system properties.The meaning of a MTF associated with the SRS is here not fully related with standard MTF definitions. We analyze the system response to an edge , and apply the well known transformation LSF/MTF. As the Sampling Theorem states , functions with non-compact support cannot be entirely reconstructed. Main problems are : a) lossing of high frecuencies. b) Modification in low frecuency carrier due to aliasing. Following the mentioned theorem we analyze the influence of various filter functions in the sampled reconstructed hard finite edge. Analyzing the corresponding responses we shall introduce the concept of frecuency modulation function as an alternative to standard MTF.The sampled finite hard edge and corresponding spectrum are defined:
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