Patent GB-401728-A (GB)
Title: Improvements in and relating to wireless signalling systems
IPC: H04L27/02
Filed: 1932-05-19
Abstract: 401,728. Wireless signalling. STANDARD TELEPHONES & CABLES, Ltd., Connaught House, Aldwych, London.- (Assignees of Materiel Telephonique Soc. Anon. ; 46, Quai de Boulogne, Boulogne-sur-Seine, France.) May 19, 1932, No. 14245. Convention date, May 20, 1931. [Class 40 (v).] In a single side-band system of wireless transmission, or a system employing a partiallysuppressed carrier, one or more pilot waves are transmitted for the purpose of controlling in frequency and phase the local oscillator at the receiver ; and the control effect is delayed to a variable extent according to the degree of fading experienced by the pilot wave. Fig. 1 shows a transmitting system in which the signal e.g. speech from a microphone T, passes through an amplifier B and filter F1 to a modulator M1 fed by an oscillator O1 of say 16 kc. One side band, say 16-19 kc. is selected by a filter F3, and this band together with a pilot wave of say 20 kc. from the oscillator 02 is amplified at A2 and fed to a modulator M2 supplied from an oscillator 03 of say 249 kc. One side band say 265-269 kc. is selected by the filter Fj, amplified at A3 and fed to a modulator H generating a short wave of say 10000 kc. A single side-band is again selected by the filter F4 and passed through amplifiers to the aerial AT. The modulator H preferably functions also as a multiplier of the frequency supplied by an oscillator M. At the receiving station, Fig. 2, the received side-band is reduced in frequency by an oscillator 09 and detector D1 and is fed through amplifiers and filters to a second detector D2 supplied by a further oscillator 06. This reduces the side-band frequencies to 16-20 kc., from which the band 16-19 kc. representing the signal is selected by a filter F1, and 20 kc. representing the pilot wave is selected by a filter F3. The signal is reduced to audibility, 0-3 kc., by an oscillator 0, of say 16 kc. and detector D, and passes through a filter F2 and an amplifier to the reproducer TR. The pilot wave from the filter F3 passes to a detector D4 which is fed by an oscillator 0 of say 24 kc. thus reducing the pilot wave to about 4 kc. This wave passes through an amplifier A2 and a sharply-tuned filter F4 to two networks F5, F6 the attenuation of which varies in opposite directions when the input frequency moves either way from the predetermined figure of 4 kc. Rectifiers BD1, BD2 following the respective networks are so connected at their output sides that the point S2 varies in voltage in opposite directions according as the input frequency rises or falls from its normal level of 4 kc. This voltage is utilized through a delay network CR for correcting the frequency of the oscillator O6 until the networks F5 F6 produce equal attenuation. The strength of the received pilot wave is kept constant by automatic gain regulation of the detector D4 under the influence of a voltage at S1 which may be the mid-point of a resistance connected across the plates of the detectors. This gain regulation operates without time-lag. The output from the filter F3 is applied to a further detector D3 also supplied from the oscillator O. The pilot wave of 4 kc. is selected by a filter F7 and passed through an amplifier to a rectifier R and thence through delay networks CR1-CR3 to the first signal detector D1 to provide slow-acting automatic gain regulation of the received signals. Fig. 3 shows a receiving diagram for a system in which a partially-suppressed carrier wave is used as a pilot wave, a further pilot wave being employed to counteract fading and to maintain constant phase relation between the transmitting and receiving oscillators. The received sideband together with a weak carrier wave are received by the aerial AR, and after amplification at HFA are reduced in frequency by the local source 01 and detector D1 so that the carrier frequency becomes say 20 kc. and the side band 20-23 kc. The signal passes through an amplifier A1 and filter BF to a second detector D which is supplied with a 20. kc. oscillation from a source 06. Thence the signal passes through a further amplifier A to the reproducer TR. In order to synchronise the oscillator 06, the carrier is selected from the output of amplifier A1 by a filter B sharply tuned to 20 kc., and is passed through condensers C to the oscillator 06. The output of the filter B is also passed to a detector D2 fed by an oscillator Oc of 24 kc. The beat of 4 kc. is amplified at Ap and is fed in parallel through two circuits tuned respectively to 3.8 kc. and 4.2 kc. to a balanced demodulator. The differential output is applied through a timedelay device CR1 to a control valve FC which regulates the frequency of the oscillator O1 in such a way that the beat is kept at 4 kc. A gaincontrol rectifier D3 may keep the 4 kc. beat at constant amplitude ; and the lead l<111> may apply gain control to the initial amplifier HFA. If it is desired to make provision for the fading of the carrier wave and consequent loss of synchronisation, a second pilot wave comprising a modulation of say 3.4 kc. on the carrier is employed. At the receiver this is selected as a wave of 23.4 kc. from the output of the amplifier A1 by a filter F2. A local oscillator CT combines a 3.4 kc. wave with the 23.4 kc. wave and the resultant at 20 kc. (equal to the first pilot wave) is selected by a filter B1 and is amplified and supplied through the condensers C to the oscillator O6 for synchronisation. In order to synchronise both in frequency and phase the 3.4 kc. source CT with the corresponding pilot oscillator at the transmitter, the 20 kc. output from the filter B and the 23.4 kc. output from the filter F2 are fed through condensers C1, C2 to a detector BD giving a beat of 3.4 kc. which is selected by a filter ABF and applied through a push-pull transformer to the grids of a balanced detector DFC which grids are also fed in parallel from the source CT. When this source is not in synchronism a voltage appears across the detector plates which operates through a time-delay device T on a control element FCT to adjust the frequency of the oscillator CT and to restore synchronism. The beat note from the filter ABF is also applied to a rectifier R which regulates the bias of the detector BD and so maintains the beat at constant strength. The degree of delay afforded by the network T and CR 1 may be varied in accordance with the amplitudes of the received pilot waves by causing the pilot wave to vary inductances or capacities in the network. Alternatively the plate-filament space of a valve may constitute a resistance element in a network ; and its ohmic value varied by adjusting grid voltage in accordance with the strength of the pilot wave. The pilot wave may be of audio frequency but beyond the necessary speech range of 0-3000 cycles, and may serve to mask the speech signals when received in an ordinary receiver.