Patent GB-786682-A (GB)
Title: Improvements in or relating to electric circuits for analyzing and synthesizing periodic complex waves
IPC: G10L25/90
Filed: 1955-10-04
Abstract: 786,682. Vocoder systems. WESTERN ELECTRIC CO., Inc. Oct. 4, 1955 [Oct. 20,. 1954], No. 28246/65. Class 40 (4). [Also in Group XL (c)] The fundamental period of a periodic complex wave is determined by feeding the wave to a tapped delay line, subtracting the input signal from the delayed signals, and determining the delay of the tapping for which the difference in signals is a minimum. As shown, speech is passed via a limiter 2 to a delay line 3 having tappings feeding devices 6 which produce voltages indicating the absolute difference between the tapped-off voltages and the input voltage. After integration in low-pass filters 8 the difference signals are fed to terminals 9 of a distributer 10 (which may be of the electronic type). The distributer output A (Fig. 4), has a minimum a near to zero in the form of a cusp corresponding to the tapping of the delay line corresponding to the period of the signal. The distributer feeds an inverter 15 to produce an output B the amplitudes and biases being so arranged that the cusp b corresponding to the cusp a occurs at about +5 volts. The output B is combined in an adder 25 with an output C from a multivibrator 20 whose period is twice that of the distributer. The resulting output D is caused to charge condenser 27 through a rectifier 26 to produce a curve G. The condenser 27 is discharged at the end of the second period by a negative pulse obtained by differentiating the waveform E obtained from the multivibrator 20. The level of the curve G, which was determined by the value of the minimum a, provides a reference level for determining when the next minimum occurs, which minimum should of course have a value very close to that of the previous minimum. Accordingly the output G from the cathode follower 28 is combined with the input voltage A and a bias of - 25 v. in a network 36 ... 38 to provide an output J. This is inverted in an amplifier 40 whose output is shown at K. From the manner in which the curve K is built up it will be seen that the cusp k<SP>1</SP> always occurs at approximately the same level. The interval between the start of the second cycle and the occurrence of the cusp k<SP>1</SP> is measured by a bistable trigger pair 41, 42 which is switched to the " on " condition by a pulse F obtained by differentiating the multivibrator output at the beginning of the cycle and switched " off " by the occurrence of the cusp k<SP>1</SP>. During the period L during which the multivibrator was " on," a D.C. signal is fed to the low-pass filter 45 whose output is a steady D.C. signal proportional to the length of the D.C. signals fed thereto, i.e. proportional to the angle of rotation of the distributer corresponding to the minimum signal received. Since early delay line tappings corresponding to improbable fundamental periods are not provided, the output of the low-pass filter 45 is connected in series with a battery 46 whose e.m.f. represents the delay corresponding to the first input to the distributer. The amplitudes of the curves B and C are adjusted so that rectifier 26 does not conduct except to pronounced minima, so that in the event of an aperiodic signal (hiss) being received, i.e. when no pronouned minima are received curve G remains flat and curve K is lowered and does not rise to a value high enough to switch off the trigger circuit. In this case the output on lead 48 assumes a maximum value. In the reconstructor, Fig. 3, the signal is inverted so that a maximum voltage is received from the anode of valve 56 when a minimum signal corresponding to high frequency signal. The anode voltage feeds a relaxation oscillator comprising storage condenser 58 and a thyratron 57, the frequency of the latter being proportional to the applied voltage. The output of this oscillator is fed via contacts of relay 61 to the spectrum synthesizer 52 which is controlled by signals derived from the spectrum analyser 50. In the event of an aperiodic signal, the maximum signal on lead 48 operates relay 61 to connect an aperiodic source 63 to the spectrum synthesizer. Specifications 466,327 and 547,505 are referred to.