GB662227A - Improvements in or relating to electric suppression systems - Google Patents

Abstract

662,227. Sound reproducing circuits. ROBINSON, J. July 16, 1948 [June 17, 1947], No.2493/50. Divided out of 662,133. Class 40 (iv). [Also in Group XL (c)] When a carrier wave or a pulse train is modulated in frequency or phase by a modulating voltage, a multiplicity of side frequencies or side-bands are produced with frequency intervals equal to that of the modulating voltage. The amplitude of the pure carrier and of each side-band varies according to the modulating index. The curves 12, 13, 14 of Fig. 1 show the manner in which the pure carrier 12, and the first and second pair of side. bands 13, 14 vary in amplitude (ordinates) with change of modulation index (abscissae). The invention consists in applying a desired and an undesired signal jointly to the modulator, and in isolating a particular side-band (e.g. the first). By suitable choice of the operating conditions, the signal providing the lower modulating index is weakened relative to the other signal. If phase modulation is used, the weaker signal is further weakened. If frequency modulation is used, that signal is weakened which gives the lower ratio of carrier frequency deviation to modulating frequency. A preferred choice of conditions is that in which both modulating indices are less than 2, in which region the pure carrier 12 is falling and the first side-band 13 is rising. In the arrangement shown in Fig. 2, a carrier generated by the back-coupled oscillator 17, 18 is frequencymodulated by a reactance valve 19 to the grid of which the desired and undesired signals are applied. F.M. output is taken at 20 to an amplifier 21, from the anode circuit of which a filter 22 selects a particular side-band preferably the first. Both signals are present as components of this side-band, but the signal giving the lower modulation index is weakened relative to the other. The process may be repeated. If the signal giving the lower modulation index is the desired one, the undesired signal may be separated and fed back to balance out that signal in the mixed input, leaving only the desired signal. In a modification of the Fig. 2 circuit, the output signal 16 from the isolator 22 is fed back positively to the grid of the reactance valve 19, and a resistor shunted by a condenser is inserted in the cathode circuit, Fig. 5 (not shown). When the signals consist of the carrier and side-bands of an amplitudemodulated wave, the carrier being of greater strength than the side-bands tends to suppress them and to leave pure carrier. In gramophone reproduction, the low-intensity waves arising from needle scratch may be minimized by the above methods. As, however, there is a tendency to strengthen strong signals relatively to weak signals, the record should be prepared in such manner that large and small amplitudes are brought nearly to the same level.