DOI:10.1049/pi-3.1953.0001

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes first the excessive redundancy in television signals, using the term in the sense of communication theory. The information in a picture is largely contained in the edges and boundaries, but in practice these occupy a very small part of the total area. The unequal probabilities of signal elements lying in boundary and smooth surface regions are considered, together with the transition probabilities between adjacent elements. Compression is shown to be possible by making such probabilities more nearly equal, using a system of recoding.Such recoding may be achieved by a variable-velocity scanning system if the scan velocity is made high for all regions of the picture having low picture-detail and low for regions having high picture-detail. Picture-detail is defined precisely and measured values are given from observations on working programme material.Finally, compression is shown to be theoretically attainable in both the vertical and the horizontal directions. Correlation between successive line-scan waveforms is interpreted in terms of transition probabilities, and again a redundancy is seen to exist. Suggestions are made for reducing this by storage of every successive frame and scanning alternately in the horizontal and vertical directions, in both cases using a variable-velocity system. Finally, the paper should not be read as describing an existing working system, but purely as a discussion of possibilities.

DOI:10.1049/pi-3.1953.0002

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The ultimate bandwidth and stability of Pound-stabilized microwave oscillators are investigated. It is shown that, if high Q-factor cavity resonators are used and steps are taken to eliminate modulation at the mains frequency, a bandwidth of 1 part in 1010and a stability of 1 part in 108per hour can be obtained. The residual drift seems to be largely associated with changes in the crystal diode used as the modulator in the control circuit. Some applications of stable microwave oscillators are discussed and the results of measurements of the spatial variations of the refractive index of the atmosphere are given. Neighbouring air pockets of about 1 litre in volume can, under disturbed conditions, have refractive indices differing by as much as 1 part in 105, and variations of 1 part in 106are fairly general, even inside the laboratory.

DOI:10.1049/pi-3.1953.0003

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes a shunted-T circuit provided with a voltage divider, consisting of two resistors or two capacitors, at the input, connected in series across the source.The shunt is connected across the source, whilst the T-circuit is connected across the lower-voltage part of the voltage divider. In this manner the voltages of the shunt and the T-circuit are made different, although they are provided by the same source. The balance condition depends, therefore, not only upon the values of the impedances in the circuit, but also upon the values of the components of the voltage divider.The circuit therefore has a multiplying ratio, or ratio arms, and thus has a much wider range of measurement for a given set of standards. Furthermore, by means of measurement with an additional standard in the circuit, the angular frequency can be made to disappear from the balance equations.Tests have been made over a wide range of resistance, inductance and capacitance, with frequencies varying between 1000 c/s and 50 Mc/s. The accuracy varied between ±0.020% and about ±1%, depending upon the frequency and the standards used.

DOI:10.1049/pi-3.1953.0004

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The errors in phase-difference measurement which arise when using the Decca Navigator system, operating at frequencies near 100 kc/s, in the immediate neighbourhood of obstacles such as trees and telegraph wires, have been examined; the results of the measurements are described and discussed. Vertical obstacles show some uniformity of behaviour, but long horizontal conductors produce widely varying effects. A tentative theory of the phase errors due to a single tree is given in an Appendix, and fairly good agreement with observations is obtained.In Section 2 of the paper, errors obtained at good sites on the base line extensions of the red and green lattices are shown graphically. The effects of the near-field components and of the finite conductivity of the ground are apparent. Good agreement is obtained between the mean of the observations and a composite theoretical curve, based on the use of Norton's plane-earth theory and Bremmer's curved-earth theory of ground-wave propagation. Rapid fluctuations of error about the mean curve are observed, indicating that the phase velocity of the ground wave may show variations from point to point of up to 1% of the mean value.

DOI:10.1049/pi-3.1953.0005

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0008

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0009

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0010

出版商:IEE    

年代:1953

数据来源: IET