A new method of wide-band modulation of pulses

In a pulse-modulation system it may be required to convey a wide band of modulation-frequency components with a high degree of fidelity, as when the modulating signal consists of a multichannel carrier wave having its intelligence channels separated on a frequency basis. In this case the occurrence in the pulse system of any appreciable non-linear distortion introducing intermodulation amongst the pulse modulating tones cannot be tolerated, since it leads to mutual interference between the sub-modulation signals. The particular problem investigated theoretically and experimentally by the authors was the design of a modulator and a demodulator for a single-channel pulse system to operate as a link in a multichannel carrier-telephony system (0.3¿32 kc/s) having low overall noise and crosstalk requirements. The method employed, of scanning the modulating signal ¿periodically¿ instead of ¿synchronously¿, enables a low recurrence frequency to be used (theoretically twice the highest pulse modulating frequency, but in this case 80 kc/s to permit ease of filtering). There is a consequent gain of signal/noise time excursions, although each pulse position is limited to one recurrence period.