GB1499697A - Narrow band communication system - Google Patents

Abstract

1499697 Baseband vocoder INTERNATIONAL STANDARD ELECTRIC CORP 15 April 1976 [16 April 1975] 15665/76 Heading H4R In a reduced bandwidth transmission system, wherein the baseband is transmitted direct and the higher harmonic range is transmitted as frequency- or time-multiplexed control signals for harmonic generators at the receiving end the harmonic generators comprise an arrangement of a generator MHG of frquency fg, which is controlled by variations in the fundamental voice frequency derived from the transmitted baseband signals, and a number of frequency dividers which each divide the frequency fg to produce one harmonic frequency. In a first embodiment (Fig. 2, not shown), a filter is used to separate the fundamental voice frequency fo from the baseband signal and a threshold circuit converts the filtered sine wave into a constant amplitude square wave from which a D.C. control voltage proportional to the frequency fo is derived. An oscillator of frequency fg is then controlled by this D.C. voltage to produce a high frequency fg which varies with the frequency fo, but is not a direct integral multiple thereof, nor is locked thereto. Figs. 3 and 4 (not shown) illustrate arrangements wherein the generator frequency fg is locked to an integer multiple of the frequency fo. The Fig. 4 arrangement employs a phase lock loop arrangement with a voltage controlled oscillator, whereas that of Fig. 3 uses a frequency multipler in combination with a filter and a threshold circuit to produce directly from the baseband signal an integer multiple of the frequency fo. In all arrangements the frequency fg is applied to dividers FD which produce the various harmonic frequencies for application to variolossers VA controlled by the transmitted control signals in known manner.