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An experimental multilevel narrow-band digital FM modem is described. The design is based on theoretical principles predicting better performance than that formerly thought to be typical. Various waveforms and eye patterns are shown under conditions of delay and slope distortion. Measured error rates vs. signal-to-noise ratio for additive Gaussian noise, with and without channel impairments, are presented for binary, duobinary, and quaternary systems. The experimental results agree closely with theory, and it is demonstrated that the theoretical performance can be approached closely by a physical model which retains the promised simplicity of FM instrumentation.