Patent GB-1308636-A (GB)
Title: Electrical filters enabling independent control of resonance frequenc  and of bandpass especially for a speech synthesizer
IPC: G10L25/00
Filed: 1970-08-19
Abstract: 1308636 Active filters; speech synthesizer; pulse modulation circuits AGENCE NATIONALE DE VALORIZATION DE LA RECHERCHE 19 June 1970 [20 June 1969] 29931/70 Headings H3T H3U H4L and H4R An electrical filter which enables independent control of resonance frequency and of bandwidth comprises an amplifier 21, Fig. 2, whose gain is of the order of three, having an input connected to two impedances Z1a, Z1b of a first type arranged in series with respect to the input and having adjustable and substantially equal values, there being a negative feedback loop between the output of the amplifier and the junction between the two impedances of the first type, and in which two other impedances Z2a, Z2b of a second type, which have adjustable and substantially equal values, arranged respectively in the negative feedback loop and shunted with respect to the input of the amplifier. As shown the impedances of first type comprise variable resistors R1, and each of the impedances of second type comprises a capacitor C2 shunted by a variable resistor R2, the arrangement comprising a low-pass filter. In a second embodiment comprising a high-pass filter the two types of impedance are interchanged and the arrangement comprises the feedback path of a further amplifier 31, Fig. 3, with signal input E2 and output S2. Each of the variable resistors may comprise two fixed resistors connected in series, one of each pair being shunted with a switch which may comprise a field-effect transistor (Figs. 4, 6, not shown); each switch is operated at a frequency appreciably higher than the maximum frequency passed by the filter, the effective value of each resistor being determined by the mark/space ratio of the switching. Triangular wave generator and pulse width modulator.-A circuit for deriving the switching waveform comprises a triangular wave generator 39 and comparator 40, Fig. 8. The triangular wave generator 39 comprises a bi-stable circuit 47 the output 53 from which feeds amplifier 52 with capacitive feedback, forming an integrator. The output of integrator 52 is fed back to the out-of-phase input of a comparator 41 and the in-phase input of a second comparator 43. The in-phase input of comparator 41 is earthed and the out-of-phase input of comparator 43 connected to a source of positive supply: the outputs of the comparators are fed to the two inputs 48, 49 respectively, of bi-stable circuit 47. The triangular waveform produced is fed to the inphase input of comparator 40, to the out-of-phase input of which is supplied a control voltage V c1 : the output of comparator 40 comprises a rectangular pulse train the markspace ratio of which depends upon the value of the voltage V c1  (Fig. 11, not shown). Alternatively, numerical control means (Figs. 9, 12, not shown) may be used to energize the switching means, instead of an analogue arrangement. Speech synthesizer.-The filters are intended for use in a speech synthesizer. This comprises a variable frequency generator 1 and a white noise source 2, coupled to amplifiers 3, 4 and 5,6 respectively, which are of variable gain. Amplifier 3 feeds into a "nasal" channel comprising filter circuits 8-12 while amplifier 6 feeds into a noise channel comprising filters 16-18. The outputs from amplifiers 4, 5 are combined in a mixer 7 and fed to a "vocal" channel comprising filters 13-15. The outputs from filters 12, 15, 18 are combined in a mixer 19 and fed to reproducer 20. Filter circuits 8, 16 have transfer functions which contain a complex zero (anti-resonant circuits) while the remaining filters have a transfer function containing complex poles (resonant circuits).