Patent GB-245516-A (GB)
Title: Improvements in modulating systems for electric signalling and the like
IPC: H03C1/52
Filed: 1924-10-09
Abstract: 245,516. Western Electric Co., Ltd., (Western Electric Co., Inc.). Oct. 9, 1924. Inverting and transposing frequencies of telephone currents.-A carrier wave and a signal wave are jointly impressed on a modulating or distorting medium the characteristic curve of which is of such shape as to produce side-bands of higher than the second order, one or more of the side-bands being utilized for the signal transmission. If p is the carrier frequency and q the signal frequency, the frequency of all the sidebands is given by m p + n q where m and n are integers. The order of the side-band is defined as m Œ n, and in the system of the present invention m + n is always greater than 2. In a more specific form of the invention n is unity, and m is equal to 2, thus utilizing third-order modulation or third-order side-bands. Tho system enables an upper and/or lower side-band to be readily transmitted without the carrier wave. Multiplex and duplex transmission may be effected by employing different orders of modulation for the different channels. Secrecy in telephony may be obtained by employing a carrier wave p slightly higher in frequency than the upper speech limit, and transmitting the third-order side-band 2p-q together with the carrier p. Ordinary detection or demodulation gives inverted speech p - q whereas a third-order demodulator arranged to exclude second-order effects, will suppress the inverted speech and reproduce the original signal wave q. The modulators and demodulators may be of the thermionic magnetic, or other type. Fig. 2 shows a thermionic modulating system, and Fig. 4 is a corresponding diagram of characteristic curves. Referring first only to the upper part of Fig. 2 (the switch s being assumed to be closed), the input waves to be modulated are applied through transformer 5, 6 to the valve 7. The grid circuit includes the secondary coil 6 and a diode valve 8, the filaments of the valves 7, 8 being connected. A leak resistance 9 is included between grid and filament. In Fig. 4, the curve a, b, c represents the normal characteristic of the valve 7, and the presence of the valve 8 causes the upper curved part c to be cut off, and a horizontal part d to be substituted. This is due to the fact that during the half-wave that tends to make the grid positive, the valve 8 is non-conducting, and therefore the whole voltage is across the valve 8 and none on the grid. During the other half-wave, the valve 8 conducts and most of the voltage appears at the grid. A similar effect may be produced by substituting an ohmic resistance for the diode valve 8. If the normal grid potential is adjusted to the angle b, d of the characteristic, a small sinusoidal input potential will give an output current in which the upper half-waves are cut off. These conditions favour the production of harmonics and side-bands of even order. When the input potentials are larger so as to overrun the base portion e of the curve, the output current becomes more angular in shape, and eventually when the amplitude of the input potential is very large compared with the rising portion a, b of the curve, the output current becomes rectangular. Under these conditions harmonics and side-bands of odd order are present in the output current to the exclusion of those of even order, as the characteristic curve is practically symmetrical about the working-point. When the lower valves of Fig. 2 are put into circuit by opening the switch s, and the output current taken jointly from their plate circuits, a more precise symmetry of the characteristic curve may be obtained. The input potentials applied to the grid of the lower valve are reversed relatively to the upper valve, so that the characteristic of the lower valve may be represented by the curve d<1>, b<1>, a', e' of Fig. 4. By adding the ordinates of the two curves algebraically, the characteristic of the combination is found to be completely symmetrical about the origin, and to comprise a rising portion bending at each end to horizontal portions, thus giving odd order modulation. If the two halves of the output winding oppose each other in their effect on the output circuit 10, a different joint characteristic is obtained resulting in even-order modulation only. The characteristic curve of an unpolarized magnetic modulator hereinafter described possesses the symmetry in the first and third quadrants that is necessary for securing modulation of odd order only. It is preferred to use a core of " Permalloy " containing 80 per cent nickel and 20 per cent iron subjected to a special heat treatment. The core may be formed from ribbon of this material, and may be approximately the size of a finger ring. It is designed for saturation by small values of magnetizing current. As shown in Fig. 8, the modulator 12 has a single winding traversed by signal current from the microphone 13 and carrier current from the source 14, filters FL, FC preventing interaction except in the modulator. The output is transmitted to a line or aerial through a filter FB which selects the upper and/or lower side-bands of the desired order. In the figure the output is transferred through a balanced transformer 19 to the line 18, which is balanced by a network 21, and the carrier or its harmonics are supplied to the line by a circuit 20 connected across the midpoints of the transformer 19. The switch 23 may be opened to put in circuit the harmonic generator GH; or it may be closed to send the original carrier wave to line. Rheostats 15, 17, 22 control the amplitude of currents in their respective circuts. Assuming that the switch 23 is closed, the demodulator DM at the receiving station must be arranged for demodulation of the same order as the modulation at the transmitter. The resulting signal wave is passed by a low-pass filter FL, to the phones 23a. The pure carrier wave may be supplied by a local source 27 or may be transmitted over a separate line. If third-order modulation is employed in the modulator 12 so as to transmit the side-bands 2p - - q, an ordinary second-order demodulator DM may be used at the receiver provided that the harmonic 2p is either transmitted over the line or supplied by the source 24 at the receiver. For secret working, p is approximately equal to the upper limit of speech frequency, and the lower third-order side-band 2p - - q is transmitted together with p, the harmonic 2p being suppressed. Any signal received by ordinary demodulation would be masked by the inverted speech p - - q. A demodulator giving pure third-order demodulation will however give the original signal q undistorted. Separate coils may be arranged on the modulator for carrying the two current waves to be modulated, and the modulator may be connected in series in the line or across it. The current sources and filters may also be arranged in series or in parallel. Fig. 11 is an example of a circuit used both for modulation and demodulation. For transmitting purposes, the signal wave is supplied through the low-pass filter FL, and the carrier wave through filter FC. The modulator 12 is across the line, and the outgoing modulated wave passes through a band filter FB. For receiving, the incoming modulated wave enters through the filter FB, and is combined in the modulator with the local wave FC, the resulting signal wave passing through a filter FL to the phones. A condenser C may be connected across the single modulator coil 12 as shown in Fig. 8, or the condenser may be connected across a, separate coil on the modulator, and the circuit tuned to the carrier frequency. In an alternative form of modulator, the windings carrying the respective currents are arranged to produce fluxes which cross each other at right angles, thereby avoiding mutual induction between the windings.