Patent GB-535860-A (GB)
Title: Electric signalling systems
IPC: H04B14/04
Filed: 1939-10-23
Abstract: 535,860. Radio signalling; valve circuits. STANDARD TELEPHONES & CABLES, Ltd. (Materiel Telephonique Soc. Anon.). Oct. 23, 1939, No. 28480.' [Class 40 (v)] [Also in Group XXXIX] A complex wave such as speech, is transmitted by arranging a finite number of graded predetermined amplitudes to cover the amplitude range, and transmitting the instantaneous amplitudes of the wave, at predetermined intervals, in the form of signals representing the nearest predetermined amplitude above or below said instantaneous amplitudes. In general, the signals comprise n different signal elements transmitted over each of m channels. The channels may comprise different carrier frequencies or may be obtained by using a distributer system, and the signal elements may comprise amplitude, frequency or phase modulation of the carrier, or impulses or intervals between impulses of different duration. The two embodiments described, comprise firstly a single channel system in which the signal elements consist of pulses of 32 predetermined durations (n = 32) and secondly, a five channel system in which the signal elements in each channel consist of " on " and " off" conditions (n = 2) so that 32 signal combinations are provided. The Specification discusses the merits of systems using various values of m and n for obtaining good signal-to-noise ratios ; a multichannel system, which in effect increases the band width, is advantageous when the initial signal to noise ratio is poor. Single channel system, Figs. 2 and 3. The original wave is applied to the grids of a multivibrator A operating at say 6 k.c. so that in the absence of any other control the outputs in the plate circuits comprises pulses at that frequency varied in width in accordance with the instantaneous amplitudes of the wave. One anode however is loosely coupled over a variable condenser C to an oscillator B of say 600 k.c. and it is arranged that a pulse can be commenced by A only at the moment of reception of a peak from B. With the frequency pulses as above, there are thus 100 points of time in a cycle of A at which a pulse can commence and the amplitude range of the wave is assumed to require 32 different durations. The output across load resistance R, as explained with reference to Fig. 2A (not shown), comprises pairs of negative pulses with variable interval duration, the pulses occurring at instants of increase of current in the anode circuits of the multi-vibrator. The smaller positive pulses due to gradual delay of the current are removed by rectifier F. This output is taken to a wireless transmitter which may also transmit some 600 k.c. or a subharmonic to synchronise the receiver. The signals from the wireless receiver are applied to a multi-vibrator AR, Fig. 3, controlled by an oscillator ER synchronized with the 600 k.c. oscillator at the transmitter, or a wave for controlling the multi-vibrator may be obtained from the transmittter 600 k.c. wave or its sub-harmonic. The multi-vibrator AR does not respond to receiver pulses or the 600 k.c. wave, alone. An output of AR comprising variable duration pulses is coupled by valve CR to a low pair filter BR in the output of which the original wave form appears. Five-channel systems, Figs. 4 and 5. The speech input BT at the transmitter, Fig. 4, is applied to the multi-vibrator AT to produce pulses of variable duration as in Fig. 2, the pulses on grid GT being applied to a doubletriode valve ET which couples a 600 k.c. oscillator DT to a cascade arrangement C1 ... C5 of multivibrators operating as described in Specification 296,827, [Class 40 (v)]. The valve ET is active during the periods of the pulses and it is arranged that the longest pulse occupies 32 periods of GT. Normally, the multivibrators are in the condition in which current flows only in the left-hand plate circuits (condition 1) and the output of each stage is of half the frequency of its input. The oscillator DT is coupled by ET to the 1st stage at the commencement of a pulse and a varying number of cycles not exceeding 32 is fed thereto. At the end of the pulse, the ET is ineffective and certain stages have been operated to condition 2 (current in right-hand half) depending on the number of cycles fed to C1. On termination of a pulse on GT, valve KT is trigged from one condition to the other and restores after an interval depending on the constants of the circuit a similar valve NT being thereupon trigged and restored, the time of these operations being such that they are completed before the time of arrival of the next pulse. Each stage of C1 ... C5 is associated with an oscillator O1 ... O5 of different frequency and an amplifier P1 ... P5 which is only effective when the grids 1, 2 are made less negative simultaneously. Grid 2 is connected to the respective multi-vibrator stage and all the grids 1 are connected over battery R to the trigger valve NT so that a combination of amplifiers is prepared by the stages in condition 2 and is made effective at the end of a pulse when valve NT is trigged. When NT restores, a restorihg pulse is applied to all the stages over circuits T. In the result, a combination of frequencies representative of a pulse duration is transmitted. At the receiver, Fig. 5, the frequencies are separated by filters B1 ... B5 and, after rectification, are applied to multi-vibrators stages LC1 ... LC5 normally in condition 2 but selectively changed to condition 1 by the incoming frequencies. The first stage is fed by a 600 k.c. oscillator DL over a coupling valve EL made effective when a double stability circuit HL is trigged to condition 2 in response to a pulse from GL trigged by an incoming pulse combination. The multi-vibrator LC1 continues to receive input from oscillator DL until all the stages are restored to condition 2 whereupon LC5 transmits a pulse to HL to restore it to condition 1. The number of cycles of DL required to restore the multi-vibrator stages varies inversely as the number of pulses from DT used to set the vibrators at the transmitter and as the numbers at the receiver correspond to durations of pulses of HL, the speech wave is restored by applying the pulses to valve VL; inverting them by a transformer and eliminating the pulse frequency by filter VL.