Some Extensions of Nyquist's Telegraph Transmission Theory

Nyquist's classic paper 1 considered the conditions necessary for digital data transmission without intersymbol distortion, and these conditions have provided the guides for system design for many years. However, Nyquist treated the case in which no energy is transmitted at a frequency above twice the signaling speed, although he mentioned the general case in passing. As a consequence, his results cannot be applied directly to cases in which the amplitude characteristics extend beyond twice the signaling speed (gradual cutoff systems) or baseband systems without low-frequency components (bandpass). In addition, Nyquist's theory has been incompletely exploited in practice. The usual application of the principles of channel shaping amounts to equalizing the phase to make it linear across the band, and equalizing the amplitude to produce a symmetric roll-off characteristic. This procedure is valid and consistent with the theory, but is only a special application of the theory. 1487 1488 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 19G5