GB1146185A - Filter - Google Patents

Abstract

1,146,185. Vocoder; variable-bandwidth circuits. STANDARD TELEPHONES & CABLES Ltd. 1 July, 1966, No. 29685/66. Headings H3T and H4R. A speech synthesizer channel includes a resonant filter with an electronically variable damping resistor. Fig. 2 shows an embodiment of the filter which comprises a parallel resonant circuit including inductor L1 and capacitor C1 fed, via transistors Q3 and Q4, with pitch pulses or noise from the respective channel modulators, not shown. A transistor Q5 is connected in series with a resistor R1 so that critical damping is applied to the resonant circuit whenever Q5 conducts, which is arranged to occur for a short time immediately before each pitch pulse occurs, so that interference between successive pitch pulses is avoided. A further transistor Q6 is arranged in the resonant circuit and has an impedance dependent on the control voltage applied to its base so that the Q of the resonant circuit may be varied to affect the quality of the reproduced speech. The transistors Q5 and Q6 may be symmetrical transistors as shown or field effect transistors. In an alternative arrangement, Fig. 5 (not shown), a series resonant circuit is used and the variable damping may be applied to the secondary of a transformer whose primary forms the resonant circuit inductor. A resonant filter as described above may be provided in each channel of a vocoder synthesizer, Fig. 1 (not shown), or a number of filters, one for each formant, may be arranged in series, Fig. 11 (not shown), in an artificial speech synthesizer.