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Band-limited digital FM systems employing discriminator detection are analyzed. The error-rate performance of binary FM with premodulation shaping and duobinary FM with the same occupied bandwidth are compared. At bandwidths above 1.1 times the bit rate, it is found that binary FM gives a lower error rate than duobinary FM. For binary FM to meet lower bandwidth requirements, frequency deviation ratios below 0.4 times the bit rate must be used. At these low deviations, binary FM does not perform as well as duobinary FM with the same bandwidth. In addition, if a more complex receiver is used which makes use of Viterbi decoding after the discriminator, the performance can be made better than binary FM even at the larger occupied bandwidths.