摘要:

Normal fault-locating procedures become impractical in certain conditions, either for purely technical reasons or on account of the loss of service time involved. This paper commences by summarizing a theoretical investigation of the possibilities of applying pulse and frequency-modulation methods to this problem on wide-band coaxial telephone cables. A practical fault-locator employing d.c. pulses is then described. The problem is first contrasted with that of radar, and the factors controlling the choice of the transmitted waveform and those limiting the accuracy of location attainable are then discussed. The fundamental requirements of a frequency-modulation system are examined in some detail, and it is concluded that, although an f.m. instrument would be attractive in certain circumstances, the practical advantage lies with the pulse type of fault-locator owing to the clarity and reliability of its indications when more than one fault is present. The d.c.-pulse instrument described has been in use for some time, and faults on coaxial cables have been located within 1% of their true distances at ranges up to 10 miles.

DOI:10.1049/ji-3-2.1946.0067

出版商:IEE    

年代:1946

数据来源: IET

摘要:

The wide choice of entertainment provided by the broadcast receiver is perhaps its most important feature, and the development of tuning devices to simplify the exercise of that choice has played an outstanding part in radio receiver design. Both station selection and ready identification of a programme found by random searching are involved. The paper opens with a brief historical review of tuning-device evolution from the introduction of single-knob tuning to the immediate pre-war designs. Listening tests are used to establish the degree of mistuning to cause observable deterioration of quality, and to suggest a target design tolerance for tuning errors. A consideration of frequency stability on the broadcast bands leads to an analysis of design limitations and establishes pre-set tuning on medium and long waves and band-spread tuning on short waves as important problems. The design of pre-set tuning devices is reviewed, together with their power drive and remote control, and the development of band-spread tuning is described, together with the associated receiver circuits. Appendices give the measured frequency response and harmonic-content characteristics of the receiver used in the listening tests, together with a mathematical analysis of these characteristics as derived from the intermediate-frequency response.

DOI:10.1049/ji-3-2.1946.0070

出版商:IEE    

年代:1946

数据来源: IET

摘要:

Hitherto communication theory was based on two alternative methods of signal analysis. One is the description of the signal as a function of time; the other is Fourier analysis. Both are idealizations, as the first method operates with sharply defined instants of time, the second with infinite wave-trains of rigorously defined frequencies. But our everyday experiences—especially our auditory sensations—insist on a description in terms ofbothtime and frequency. In the present paper this point of view is developed in quantitative language. Signals are represented in two dimensions, with time and frequency as co-ordinates. Such two-dimensional representations can be called “information diagrams,” as areas in them are proportional to the number of independent data which they can convey. This is a consequence of the fact that the frequency of a signal which is not of infinite duration can be defined only with a certain inaccuracy, which is inversely proportional to the duration, and vice versa. This “uncertainty relation” suggests a new method of description, intermediate between the two extremes of time analysis and spectral analysis. There are certain “elementary signals” which occupy the smallest possible area in the information diagram. They are harmonic oscillations modulated by a “probability pulse.” Each elementary signal can be considered as conveying exactly one datum, or one “quantum of information.” Any signal can be expanded in terms of these by a process which includes time analysis and Fourier analysis as extreme cases.These new methods of analysis, which involve some of the mathematical apparatus of quantum theory, are illustrated by application to some problems of transmission theory, such as direct generation of single sidebands, signals transmitted in minimum time through limited frequency channels, frequency modulation and time-division multiplex telephony.

DOI:10.1049/ji-3-2.1946.0074

出版商:IEE    

年代:1946

数据来源: IET

摘要:

The methods developed in Part 1 are applied to the analysis of hearing sensations, in particular to experiments by Shower and Biddulph, and by Bürck, Kotowski and Lichte on the discrimination of frequency and time by the human ear. It is shown that experiments of widely different character lead to well-defined threshold “areas of discrimination” in the information diagram. At the best, in the interval 60–1 000 c/s the human ear can discriminate very nearly every second datum of information; i.e. the ear is almost as perfect as any instrument can be which is not responsive to phase. Over the whole auditory range the efficiency is much less than 50%, as the discrimination falls off sharply at higher frequencies.The threshold area of discrimination appears to be independent of the duration of the signals between about 20 and 250 millisec. This remarkably wide interval cannot be explained by any mechanism in the inner ear, but may be explained by a new hypothetical effect in nerve conduction, i.e. the mutual influence of adjacent nerve fibres.

DOI:10.1049/ji-3-2.1946.0075

出版商:IEE    

年代:1946

数据来源: IET

摘要:

It is suggested that it may be possible to transmit speech and music in much narrower wavebands than was hitherto thought necessary, not by clipping the ends of the waveband, but by condensing the information. Two possibilities of more economical transmission are discussed. Both have in common that the original waveband is compressed in transmission and re-expanded to the original width in reception. In the first or “kinematical” method a temporary or permanent record is scanned by moving slits or their equivalents, which replace one another in continuous succession before a “window.” Mathematical analysis is simplest if the transmission of the window is graded according to a probability function. A simple harmonic oscillation is reproduced as a group of spectral lines with frequencies which have an approximately constant ratio to the original frequency. The average departure from the law of proportional conversion is in inverse ratio to the time interval in which the record passes before the window. Experiments carried out with simple apparatus indicate that speech can be compressed into a frequency band of 800 or even 500 c/s without losing much of its intelligibility. There are various possibilities for utilizing frequency compression in telephony by means of the “kinematical” method.In a second method the compression and expansion are carried out electrically, without mechanical motion. This method consists essentially in using non-sinusoidal carriers, such as repeated probability pulses, and local oscillators producing waves of the same type. It is shown that one variety of the electrical method is mathematically equivalent to the kinematical method of frequency conversion.

DOI:10.1049/ji-3-2.1946.0076

出版商:IEE    

年代:1946

数据来源: IET