GB1364056A - Method and apparatus for signal processing - Google Patents

Abstract

1364056 Character recognition systems C F QUATE 15 June 1971 [15 June 1970] 28046/71 Heading G4R [Also in Divisions G1 and H3] A method of processing signals within a piezo-electric medium, for use in character recognition systems, comprises so applying first and second modulated signals to the medium as to establish therein two acoustic waves having different velocities or travelling in opposite directions and an electric polarization related to the two acoustic waves by parametric coupling within the medium. The acoustic waves may be volume, surface, flexural, torsional or Love waves. Evaluating cross-correlation functions (Fig. 3). One of two modulated microwave-frequency electric signals, Signal 4, is applied to a piezoelectric transducer 22 disposed at one end of the main piezo-electric crystal 26, setting up a shear acoustic wave A4 in the crystal 26. The second signal, Signal 3, is applied to a further transducer 24 a short time later so as to set up a longitudinal acoustic wave A3 which travels faster than the wave A4. As they overtake, the time delay between the modulations on the two acoustic waves changes slowly. Meanwhile, assuming their carrier frequencies are matched, parametric coupling of the two waves will occur whenever the carriers are in phase, and this will result in an electric polarization D across the crystal which, at any point along the length of the crystal, has a magnitude proportional to the product of the magnitudes (representing the modulations) of the two acoustic waves at that point. If the two acoustic waves are such that the resultant polarization D has zero propagation vector, a tuned cavity coupled to the crystal will provide an electromagnetic output signal which, at any instant, is proportional to the integral of D over the length of the crystal. This output therefore represents the cross correlation of the modulations on Signals 3, 4. As the two acoustic signals overtake the delay between their modulations will vary, and hence the complete correlation function will be obtained. Modifications.-(i) If the acoustic waves are such that the polarization D propagates, a folded strip line (13, Fig. 5, not shown) is used in place of the tuned cavity to achieve the required integration. (ii) Instead of propagating the two acoustic waves from the same end of the crystal 26, one of the two electric signals may first be reversed in time, and the two acoustic waves then propagated from opposite ends. (iii) The system may be used in reverse, to obtain an output from one transducer when inputs are applied via the other transducer and via the tuned cavity. Application to character recognition (Fig. 11).- The letter of the alphabet represented by the output of an optical scanner 74 is determined by correlating the output with reference signals (70) representing the twenty-six letters, using the modification (ii) above. The scanner output is reversed in time (78) and applied to one end of the crystal. The sequence of twenty-six reference signals is applied to the other end, and the timing of the output signal identifies the particular reference signal with which the scanner output has the best correlation.