Patent GB-283264-A (GB)
Title: Improvements in radio transmission systems
IPC: H03C1/62
Filed: 1926-10-05
Abstract: 283,264. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Oct. 5, 1926. Modulating systems and apparatus; transmitting systems and apparatus.-In modulating or transmitting systems for carrier wave signalling, the amplitude of the carrier wave component is varied in accordance with the variations in signal volume. The amplitude of the side band components is either maintained constant during, or is varied in opposite sense to, the variations in signal volume. When the components are recombined at the receiving station by the ordinary methods of detection, the resulting signal correctly reproduces the volume variations of the original signal. In the arrangement shown in Fig. 1, a high-frequency source 1 is coupled to a modulator M which energizes an aerial AT. Signal currents from a line L1 are supplied through an amplifier LA to the grid circuit of a valve A1, the plate circuit of which is energized by a battery B through a choke 2, both of which are also in the plate supply of the valve M. The signal thus modulates the output from the valve M by the known choke-modulation method. Signal current also passes from the amplifier LA to a rectifier R, which is included in' the plate supply circuit of the valves A1, M. The voltage applied to the plates of these valves is thus varied in accordance with the intensity of the signals. When the signals increase in intensity, the carrier wave amplitude is increased owing to increased output trom the valve M, whilst the modulating current from the valve A1 is relatively decreasel owing to increased negative bias being applied to its grid, due to the greater voltage drop across the resistance 5. In this way the carrier component increases and decreases in accordance with signal intensity, whilst the side bands tend to maintain a constant intensity. A filter LF prevents speech-frequency variations from reaching the plates of the valves A1, M. Fig. 2 shows another arrangement, in which the carrier wave 1 is applied in phase agreement to a push-pull pair of valves 9, 10 and the signal wave L1 in phase opposition. The grid circuits include a common bias battery C and separate resistances 11, 12; and a neon tube or the like N is connected between the grid of one valve 9 and its filament, the normal adjustment being such that the tube N is non-conductive, but is on the verge of discharge. Under such conditions the carrier-wave from the source 1 does not appear in the output circuit L2. Signal current in the line L1, however, creates a discharge across the tube N and the resulting current in the resistance 11 varies the bias on the grid of the valve. 9. Thus a carrier component, as well as the side bands, appears in the output circuit L2. Fig. 3 shows a modification in which the pure carrier and the side bands are dealt with by separate valve pairs A2, M. The carrier wave source I is coupled to the grid circuits of the valves across inductances 16, 18. Signal currents from the line L1 are applied in the usual way to the modulators H, and the side bands (without carrier) pass through a filter F1, which may select a single side band, to the output line L2. A shunt path across the line L1 comprises a rectifier R, filter LF and two resistances 13, 14 across which a direct voltage proportional to signal intensity is created. This voltage is applied through the circuit 17 to the grids of the modulations M, thus counteracting the tendency for the amplitude of the side bands to increase with signal intensity. The voltage drops across the resistances 13, 14 are applied in opposite sense to the grids of the valves A2, causing the amplitude of the pure carrier to vary proportionally to signal intensitv. The pure carrier wave passes through a filter F2 to the output line L2. The circuit of Fig. 3 may be modified by using an inductive counling between the source 1 and the grid circuit of the valves M. The rectifier R mav be a threeelectrode valve arranged for grid-leak rectification, and the plate-filament path of this valve may be connected as a resistance in series with the source 1, thus controlling the side-band amplitude in accordance with signal intensity. The amplifier for the pure carrier wave is a single valve, the grid circuit of which is connected across the plate-filament path of the rectifier, thus increasing the carrier amplitude as the signal intensity increases. In another modification shown in Fig. 5, the signal currents are rectified at R and the resulting direct current passes through a solenoid 30 and operates a link 32 against the action of a spring 31. The link controls two potentiometers 28, 29, one of which varies the input from the carrier source 1 to the valve M, and the other varies the signal input from the line L1 to the valve 27. The valve 27 is connected for choke modulation to the valve M. When no signals are present in the line L1, the link 32 moves upwards and no carrier wave is applied to the valve M and to the aerial AT, so that the carrier and side bands are suppressed. As signal strength rises, the carrier amplitude increases, and the amplitude of the side-bands remains substantially constant. The potentiometers may comprise resistances with tap connections to studs, or they may be replaced by the plate-filament space of valves, the grids of which are biased according to the output of the rectifier R. Fig. 6 shows a modification in which the signal source L1 is separated from the aerial AT by a line 37 including repeaters RP1. The signal source L1 is connected to the line 37 through an amplifier LA, potentiometer 39, and amplifier A5, a volume indicator 36 being connected across the output coil of the valve A5. The line 37 is coupled to a push-pull modulator M and radio-amplifier RA to the aerial AT. The carrier wave is applied to the modulator M from a source 1 through a transformer T2. A potentiometer 44 across the source 1 applies a varying carrier wave voltage to the amplifier RA. A control current of suitable frequency from a source 41 is sent along the line 37' through repeaters RP2 to a solenoid 45, and its amplitude is regulated by a shunt resistance 40 variable jointly with the potentiometer 39. The solenoid 45 operates an arm 42 against spring pressure, and so varies the setting of the potentiometer 44. Increase of strength of signals in the line L1 is recorded on the indicator 36, and the potentiometers 39, 40 are operated either by hand or automatically until the reading of the indicator 36 is normal. As a result the signal current in the line 37. and therefore the side-band amplitude in the aerial AT, are kept constant. The cutting out of resistance 40 reduces the current through solenoid 45 and the movement of the potentiometer 44 allows an increased carrier voltage to reach the valve RA and aerial AT. A single line mav replace the two lines 37, 37' if the control current does not interfere with the signal current. The svstem of Fig. 6 may be used in conjunction with apparatus such as disclosed in Specifications 246.274 and 255.221 for varying the signal intensity impressed on the line in accordance with the volume of the original signal.