Patent GB-1070247-A (GB)
Title: Sound analysing system
IPC: G10L25/90
Filed: 1966-01-18
Abstract: 1,070,247. Speech recognition. INTERNATIONAL BUSINESS MACHINES CORPORATION. Jan. 18, 1966 [Jan. 22, 1965], No. 2227/66. Heading G4R. A sound analysing system produces a digital signal representation of each transition of a formant from one frequency band to an adjacent band. Speech signals from a microphone (1) are applied to a preamplifier (2) having a manual sensitivity control (3) settable to remove background noise and an automatic gain control (35) to produce a constant level output (30) to frequency selectors (F1-F14), a fricative selector (60) and voice selector (59). The frequency selectors (F1-F14) divide up the frequency range from 260 to 3750 c.p.s. on a log scale and each comprise a difference amplifier and a twin-T filter network. The selector outputs are rectified (R1-R14) then compared in adjacent pairs in balance detectors (BD1- BD13) each of which produces an output on one of two lines depending on which of its two inputs is the larger. These output lines go, generally in pairs, to AND gates (120a-n) also enabled by a second manual control (PT). The AND gate outputs are integrated (IPS1-IPS14) to remove undesired transients and indicate in which frequency bands peaks in the frequency spectrum (formants) occur (M1-M14). These outputs are fed directly and via differentiators (DF1-DF14) to latches (1F-13F, 1R-14R) requiring coincident inputs, the latches indicating which frequency bands a formant has moved to the next lower (1F-13F) or higher (1R- 13R) band from. Outputs of the latches are NORed to control first inputs of further latches (1S-14S) requiring coincident inputs and the other inputs of which are controlled via differentiators (D2F1-D2F14) from the previously mentioned differentiators (DF1-DF14). These further latches indicate in which frequency bands a formant existed which did not move to a higher or lower band, a latch being set if a formant disappears in its band without a formant concurrently appearing in an adjacent band. All these latches indicate vowel characteristics. Most of the signals indicating which bands formants occur in (M1-M14) are also fed (M1a-M13a) to a formant drive unit (FD) which logically combines them on to fewer lines (FDa-FDe) to latches requiring coincident inputs and indicating consonant features. The other inputs to these latches are signals representing F.V, #F.#V, F.#V, #F.V where F and V mean presence of fricative and voice components respectively. Signals representing F and V are obtained by the fricative and voice selectors (60, 59) which pass 4,000 to 10,000 c.p.s. and 100 to 250 c.p.s. respectively to respective integrators (70, 70a), the outputs of which, after gating by the second manual control (PT) and integrating (IPSF, IPSV), constitute the F and V signals. A slope detector (145) produces an output if a sharp enough negative transient in the automatic gain control (145) occurs, indicating a sudden burst in voice intensity. The detector (145) output is gated by the second manual control (PT) to set a burst latch. The outputs of all the latches mentioned are displayed on lamps and used for speech recognition. A switch (C.S) enables all the signals F.V, F.V, F.V, F.V to be replaced by zero, thereby preventing any of the consonant latches from being set.