Patent GB-511222-A (GB)
Title: Improvements in electric signalling systems
IPC: H03K7/08
Filed: 1938-06-07
Abstract: 511,222. Transmission. systems. STANDARD TELEPHONES & CABLES, Ltd. June 7. 1938, No. 16921. Convention date, June 18, 1937. [Class 40 (iv)] [Also in Groups] XL, XXXV and XXXVI] In a system in which a signal wave is converted into a succession of pulses of constant amplitude as explained below, each pulse. is transmitted as a short train of high frequency oscillations generated by a shock excitation of a device having a predetermined natural periodicity. A signal wave 32, Fig. 2, may be converted into pulses of the form shown at 33 or 34 in which the duration of the pulses represents the instantaneous amplitude of the wave at regularly recurring intervals. Such pulse forms may be converted into double short pulses marking the beginning and end of the single pulses as shown at 37, or the variable duration pulses may be represented by single short pulses marking the beginning or end thereof together with single pulses at suitable constant time-intervals, or the constant timeinterval pulses may be omitted. The invention is concerned with the transmission of such short double or single pulses and is described in connection with the transmission of the double pulses shown at 37. As shown in Fig. 1, an input signal wave which may be telephony, is amplified at 45 and applied to one grid of a multivibrator 49, the outputs in the plate circuits of which are as shown at 33, 34, Fig. 2. The condensers 23, 24 in the plate circuits convert the signals to the double pulse form shown at 35, 26, and after the negative pulses have been removed by rectifiers 25, 26, the two sets of pulses are combined (see 37) and applied to the grid of a mercury vapour tube 56. The tube is trigged by the pulses and surge current is caused by discharge of condenser 55, the charging circuit of which is arranged so that condenser 55 is not replenished with sufficient rapidity to maintain plate voltage at a value at which the discharge is continuous, so that the surge ceases when the pulse ceases. The surge circuit is tuned to the desired carrier frequency, e.g. 40Mc and passes to the aerial through a band filter say 200 kc. in width. The Specification as open to inspection under Sect. 91 comprises also the arrangements described in the following paragraphs. A transmitter, Fig. 2 (Cancelled) and receiver Fig. 3 (Cancelled) for pulses of the variable duration form shown in Fig. 1. The modulating wave, e.g. speech, is amplified at 6 and applied to the grid of one valve of the multivibrator 5 which normally produces rectangular pulses of constant width at say 15 k.c. The effect of the wave is to vary the width of the pulses at the output of the other valve of the vibrator which is used to modulate the carrier. The receiver comprises amplifying and frequency changing stages 7-9, the intermediate frequency output of 9 being applied to a trigger circuit 10 which is stable in one position so that when operated by a pulse it returns to that position on cessation of the pulse. The output of the trigger circuit comprising variable width pulses is demodulated at 11 and passed through a filter 12 to remove pulse frequencies resulting in an output reproducing the original speech. A receiver, Fig. 7 (Cancelled) (not shown), for the double pulse system used in Fig. 1, and comprising amplifying, frequency changing and demodulating stages, the resultant double pulses (negative) being applied to both grids of a trigger circuit having two stable conditions which are changed by the pulses resulting in a trigger output of rectangular variable width pulse form which is passed through a filter to reproduce the original modulating wave. Maintaining the receiving and transmitting triggers in step. If one of the double pulses in the receiving system of Fig. 7 (Cancelled) is accidentally suppressed or if a pulse is added by noise, the receiver operates out of phase. This is of no importance for telephony, but in cases where correction is desired, the amplitude of one of the pulses is made double that of the other and the smaller amplitude pulse is prevented by a suitably biassed rectifier from reaching one of the grids. A large pulse thus changes over the trigger circuit whatever its condition whereas a small pulse can only change it from one condition. Modified double pulse system in which one of the pulses is not transmitted but is replaced by a pulse of regular frequency at the receiver. The transmitter, Fig. 9 (Cancelled and not shown) is similar to that of Fig. 1, except that only one of the output circuits of the vibrator is utilized. The receiver, Fig. 10 (Cancelled and not shown) is similar to the double pulse receiver previously described but the output of the demodulator is fed also to a back-coupled or other oscillating valve tuned to the pulse frequency and controlling a trigger circuit which produces rectangular pulses of constant width. These are treated with a condenser-rectifier combination to produce sharp impulses corresponding to the end of the rectangular impulses (if the incoming variably timed impulses represent the beginning of rectangular impulses) and these together with the incoming impulses are fed to the trigger circuit. It is preferably arranged that an unmodulated transmission produces pulses and spaces of equal duration at the trigger circuit. Noise suppression. This may be effected in known manner by the use of two receivers, one tuned to the carrier and the other to an adjacent wave length, the outputs being combined in opposition. Alternatively, Fig. 11 (Cancelled and not shown) comprises a modified receiver in which a suppression valve is inserted between the demodulator and the trigger circuit. This valve comprises a double triode with its grids biassed to v + 5 and v + 7 volts respectively where v is the cut-off bias, the valve being such that saturation occurs on grid voltages slightly less than the cut-off value, e.g. on signal voltages of 6 and 8 volts respectively. The signals are applied to both grids and owing to the rapid saturation effect, a difference in output arises only on signals of 6 to 7 volts, this difference output being applied to the trigger. In a further modification of Fig. 12 (Cancelled and not shown), the output of the demodulator is first limited to a desired peak value by a biassed shunt metal rectifier and is then divided, one part being fed direct to the grid of a valve and the other through a delay define, the delay time being equal to the period between a pair of commencing or terminating pulses and the voltages applied to the grid over the two valves being substantially equal and each unable by itself to produce an output (the valve is suitably biassed beyond cut-off). The result is that the first commencing or terminating pulse received does not operate the valve, but when the second arrives, the combined effect of the second and the delayed first pulse provides an output which is fed to the trigger. The period between pairs of successive pulses of the same sort (commencing and terminating) is substantially that of the original pulse frequency and is unlikely to occur in noise pulses. The output of the multivibrator at the-transmitter may be transmitted directly. In the case of high frequency transmission, removal from the modulated carrier of frequencies outside the side band range of the original signal wave results in ordinary amplitude modulation. Applications to multiplex systems with or without the use of distributors and to the reproduction of sound films are also mentioned, and negative feed back may be used to obtain linearity in amplifiers or modulators. The subject-matter of the above paragraphs does not appear in the Specification as accepted.