摘要:

Frequency compression makes it possible to transmit intelligible speech in very much less than the usual waveband of 2000–3000 c/s. Two methods are discussed, of which only the first has been experimentally tested.In the first method, frequencies are compressed or expanded by the Doppler effect which arises when a continuously moving record, such as a sound film, is scanned by slits or the like which are themselves running. An experimental device is described which has been constructed for testing the quality of compressed and re-expanded speech. Compression to one-half, followed by re-expansion gives full intelligibility, but some roughness. Speech compressed to one-quarter, and even to one-sixth, remains intelligible, but becomes monotonous, as the inflections of the pitch are not transmitted. The intelligibility is lost only at about eightfold compression.Analysis of the causes of roughness and monotony leads to an improved principle of speech transmission by compression. In the original method only certain “preferred frequencies” are correctly transmitted, and the reproduction is good only if one of these happens to coincide with the pitch of the speech. In the improved method the preferred frequencies are not fixed but are variable, and adjust themselves automatically to the pitch, which is always correctly transmitted. This also ensures automatically the correct and undistorted transmission of its overtones. Suggestions are made for realizing this principle, which eliminates both roughness and monotony.Frequency-compression systems, however perfect, utilize only one-half of the capacity of a transmission channel, as they do not make use of the information contained in the phase of the spectral components. The concept of “phase” is analysed, and it is shown that it is possible, at least in principle, to transmit two independent messages through the same channel by a system called “quadrature transmission” or “sine-cosine duplex,” but that there are great technical difficulties in the way of realizing this system.

DOI:10.1049/ji-3-2.1947.0071

出版商:IEE    

年代:1947

数据来源: IET

摘要:

Measurements of the velocity of propagation of pulses of radio waves through the atmosphere using techniques developed for accurate radio aids to navigation are described. The time of travel of pulses between two ground stations is accurately determined in terms of calibration marks generated by an accurate crystal, account being taken of the measured delays in the receiving equipment. The mean value obtained for the velocity of propagation in the atmosphere at ground level is 299 695 ± 50 km/sec. This value does not differ significantly from that calculated from the best optical values for the velocity of light in free space by allowing for the refractive index of the atmosphere.

DOI:10.1049/ji-3-2.1947.0074

出版商:IEE    

年代:1947

数据来源: IET

摘要:

A method of finding the effective velocity of propagation of radio waves in the 100-kc/s region was developed by the Admiralty Signal Establishment in 1945, using the Decca system—a hyperbolic, phase-measuring navigational aid, at that time utilizing unmodulated continuous waves on 85, 113⅓ and 127½ kc/s from a triplet of stations arranged with two base-lines about 50 km long and situated in flat, low-lying country. The only quantities measured were (a) distances over the earth's surface between transmitting aerials, (b) phase-changes of the order of 100 complete revolutions (measured correct to ± 1/50 revolution) from end to end of the hyperbolic patterns, (c) a radio frequency. Phase readings were taken in an aircraft flying at 1000 ft; the transmitting aerials were 90 ft high.Measurements were taken beyond the “near zone,” that is from 10 to 100 km from the transmitters. As no variation of the velocity with distance could be detected outside the probable experimental error, all readings were grouped together. The final average for the velocity was 2.9925 ± 0.0004 × 108m/sec, which is lower by 1.4 parts in 1000 than the velocity of short waves through air. It may be proved that the velocity derived from this method is the phase velocity. The results may be correlated with theory.

DOI:10.1049/ji-3-2.1947.0075

出版商:IEE    

年代:1947

数据来源: IET

摘要:

The Oboe blind-bombing system probably provides the most accurate radar range-measuring system to date. The system has been used to measure the velocity of propagation of centimetre waves over sea and land paths at low level, in addition to paths between ground and aircraft at 10,000, 20,000 and 30,000 ft. The paper describes the experiments over a sea path between two shore-based stations each situated at a height of about 400 ft. The results give a value of 186,218±15 miles/sec for 9-cm waves over a 40-mile path. A subsidiary check over a 30-mile path gave a figure of 186,216.5±17 miles/sec. It is concluded that the final result is 186,217±16 miles/sec. Using a conversion factor of 1 m = 3.280843 ft, this is equivalent to 299,687±25 km/sec.

DOI:10.1049/ji-3-2.1947.0076

出版商:IEE    

年代:1947

数据来源: IET

摘要:

The principal magnetic properties of transformer core materials are generally expressed in terms of permeability and power loss per kilogramme. Another method, described in this paper, is to assign a complex value to the permeability (Sections 2 to 4). For many purposes this method possesses advantages; for example, very simple relations can be found (Sections 5 to 7) connecting the electrical characteristics of coils with the components of the vector permeability of their cores. Data for samples of nickel-iron and silicon-steel are given (Section 8) and, by way of an example of the use of the system, a simple method of securing the optimum design for a low-loss reactor is worked out (Section 9).

DOI:10.1049/ji-3-2.1947.0079

出版商:IEE    

年代:1947

数据来源: IET

摘要:

A conventional end-fire array has a linear phase change of 360° per wavelength along it, and a uniform amplitude distribution. This paper describes how gain improvement and side-lobe reduction can be obtained by arranging for a greater phase change than this, and at the same time tapering the amplitude distribution.

DOI:10.1049/ji-3-2.1947.0080

出版商:IEE    

年代:1947

数据来源: IET

摘要:

The number is displayed on the screen of a cathode-ray tube, the characters being directly written by movement of the spot. The effect of a steady display is created by a high speed repetitive traverse over the outlines required, a black-out being effected between the characters and on the fly-back.The spot control is achieved by a combination of alternating figure-producing potentials and steady spacing or layout potentials. The former are provided by a common “figure generator” which uses a continuously rotating multi-unit variable condenser to modulate a 500-kc/s supply, generated by a built-in oscillator. The outputs are rectified to give the X and Y component potentials of the integers 1, 2… 0 at an audio frequency.A layout selector connects the tube in sequence to the required figure potentials and adds the steady potentials to locate each in its appropriate position in the frame. The application to “calling line identification” is described in an Appendix; other applications and developments are outlined.

DOI:10.1049/ji-3-2.1947.0083

出版商:IEE    

年代:1947

数据来源: IET