The Digital Echo Modulation
Alice Croisier, J.-M. Pierret · IRE Transactions on Communications Systems · 1970
A new technique for the transmission of binary data over band-limited channels is described. The signaling method is a generalization of Nyquist's telegraph theory, it allows pulse modulation without intersymbol interference within a frequency spectrum centered at any multiple of one quarter of the signaling rate. The signals are derived from pulse trains generated by a combination of logic circuits and weighting resistors. The number of pulses constituting a signal element varies typically from 5 to 9 in function of the desired total bandwidth. Signaling at 80 percent of the Nyquist rate has been easily achieved with 40-dB out-of-band attenuation. The digital echo modulation eliminates the need for precise analog filters and minimizes the effects of component instability; in addition, it gives a great flexibility to the design, allowing special features such as efficient half-speed operation.