The Postal Museum

Contact closure is a time-dependent process and the establishment of equilibrium can take several hours. In general, with near-instantaneous single-step mechanical loading, it is characterised by an initial delay and by two time constants associated with slipping and with seizure at the surface, respectively. There is an optimum load, or narrow range of load, for least resistance. Larger (and smaller) loads result in increased resistance. The least resistance can, but need not, be associated with film-penetration and metal-metal contact. When a suitably sharp and unlubricated asperity or fragment is pushed into the flat surface of a semi-infinite mass of the same material, it is embedded. The shape of the embedded material is a cap of a sphere with an entry angle in the surface of approximately 45° for ‘soft’ materials.

The relative merits of the zero-forcing, mean-square error and quantized-feedback algorithms are compared for data transmission over the switched telephone network, bearing in mind the nature of the transmission impairments met in this medium. The design and performance of an experimental 4-level vestigial-sideband amplitude-modulated modem operating at a data signalling rate of 4800 bit/s with a hybrid 37-tap equalizer using the mean-square error algorithm for 6 leading taps and quantized feedback for 30 trailing taps is discussed.

‘Adaptation’ is known to affect the results of television quality-grading tests, in the sense that observers become more critical when the average picture quality is high and vice versa. The magnitude of the effect of adaptation for a particular type of impairment is dependent on the value of an ‘adaptation coefficient’ ; for the two types of impairment tested, flat random noise and a single echo of 8 μs delay, the adaptation coefficient was found to be larger in the former case. A method of adjusting test results for a standard ‘conditioning situation’, defined by a grand mean score of 0.5, is described. The magnitudes of the adjustments required are given for values of the grand mean typical of those that might occur in practice.

Quality-grading tests have been made on the effect of video crosstalk on 625-line PAL colour-television signals. With undistorted crosstalk, the worst disturbance was found to be caused by a monochrome pulse-and-bar test signal; subjective impairments of 1 imp and ¼imp are produced by signal/crosstalk ratios of 36dB and 46dB, respectively. When the crosstalk voltage was proportional to frequency, resulting in ‘differentiated’ crosstalk, the worst disturbance was that due to a colour-bar test signal; subjective impairments of 1 imp and ¼imp are produced by signal/crosstalk ratios of 28dB and 34dB, defined in terms of attenuation at the chrominance subcarrier frequency.

Public telecommunication networks are required to transmit various types of information, and any new method of transmission must provide a quality at least as good as the present standards. This paper describes the basis on which the fundamental characteristics were chosen for the new 24-channel short-distance p.c.m. system now being introduced by the British Post Office.Representative overall telephone connections were selected and assessments made of their speech-transmission performance when certain links in them are provided by p.c.m. systems having various sets of fundamental characteristics. Special attention was given to requirements concerning load capacity, quantising distortion (including performance at very low signal levels) and circuit noise. Acceptable values for each quantity were determined and certain compression characteristics examined in the light of these requirementsThe final choice of fundamental characteristics was based mainly on consideration of speech, and the new systems should provide a suitable standard for any conditions of use for telephony that can be envisaged at present. It is believed that other services that the systems may be required to transmit impose less onerous conditions than those for speech, and so it is expected that they will also satisfy requirements for these other services.

A graphical method for obtaining transfer factors for interference problems involving wide-deviation frequency-modulated satellite systems is extended to problems involving interference between line-of-sight radio-relay systems and unloaded and undispersed satellite systems. Interference problems involving single-sideband amplitude-modulated systems are also considered.

A class of medium-attitude communication-satellite systems which may established by multiple-satellite launch techniques is described. As the initial satellites fail they are replaced by satellites in slightly different orbits; the system thus evolves into one in which all satellites follow a common earth track. Multiple-access coverage is in consequence greatly improved.

Group-delay equalisers operating at microwave frequencies may find application in multicharinel-telephony radio-relay systems, e.g. in systems without translation to an intermediate frequency in repeaters. Studies to ascertain the feasibility of such equalisers operating in the 4 Gc/s band are described. Several equalisers have been designed and constructed in standard waveguide WG11, using a pair of identical bandpass ladder networks terminating a waveguide hybrid or a 3dB directional coupler. In all cases except one, the equalising networks had delay characteristics that were arbitrarily assumed; the exception was a network designed to equalise the group-delay performance, over a bandwidth of 20 Mc/s, of a pair of bandpass filters in tandem. Basic equaliser insertion losses of 0.5–2dB have been observed, the figure depending on the complexity of the network. The effect of temperature changes on the equaliser and filters in tandem resulted in the midband frequency of the equaliser shifting in step with the frequency of the filters, so that the group-delay performance remained equalised over a temperature range of 20degC. It is concluded that, although the performance of the equalisers is satisfactory, they are less flexible in operation and are mechanically bulky compared with intermediate-frequency equalisers.