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Showing papers by "Northumbria University published in 1989"


Book ChapterDOI
TL;DR: In this paper, a performance analysis has been presented for digital PPM transmitted over an optical fiber channel and detected using both optimum and sub-optimum pre-detection filters.
Abstract: A performance analysis has been presented for digital PPM transmitted over an optical fibre channel and detected using both optimum and sub-optimum pre-detection filters. Receiver sensitivity calculations, carried out at a bit-rate of 140Mbit/s and a wavelength of 1.3 μm, show that the optimum digital PPM system considered offers an 8.6 dB improvement over a typical PCM system. The sub-optimum pre-detection filters considered were a matched filter, an optimised 3-pole filter and a third-order Butterworth filter. These led to sensitivity degradations of 0.4 dB, 0.9 dB and 1.1 dB respectively. This clearly illustrates that receiver complexity can be simplified without large reductions in sensitivity. In particular, the well known and simple Butterworth filter can be employed with only 1.1 dB degradation in sensitivity. The timing requirements for digital optical fibre PPM have been analysed. An original spectral characterisation of the PPM format using its cyclostationary properties has been presented. The characterisation was used to evaluate the inherent systematic jitter associated with the extracted slot clock. An optimisation of the extracted slot clock timing variance and system wrong slot errors (due to imperfect slot synchronisation) was shown to be feasible in terms of the PLL bandwidth and the PPM order. Frame synchronisation was analysed for an original class of frame synchronisers that utilises natural sequences. The extracted frame clock timing variance was evaluated and the probability of wrong slot errors due to the non-ideal frame clock was assessed. The frame clock timing variance and wrong slot errors were shown to be minimisable provided that the proper number of natural sequences is tracked and the appropriate PLL bandwidth is utilised. The analysis has provided a performance evaluation of the optical fibre PPM system in the presence of inherent systematic slot and frame jitter.

27 citations


Book
01 Jan 1989

23 citations



Journal ArticleDOI
TL;DR: In smaller firms, external auditors have a larger role in the prevention and detection of computer fraud than in larger firms, and internal auditors feel that they have a role to play, but there is a shortage of staff with the requisite skills.
Abstract: This study examines the involvement and attitudes of internal auditors to the prevention and detection of computer fraud. This approach differs from previous research which has concentrated on learning from frauds which have occurred. The main enquiry was by means of a questionnaire sent to members of the Institute of Internal Auditors. Verification and additional information was forthcoming by visiting some respondents.

2 citations


Book ChapterDOI
01 Jan 1989
TL;DR: This paper looks at the role of systems methodologies when implementing changes in manufacturing systems, and the technologies required to solve these problems are now quite well established.
Abstract: This paper looks at the role of systems methodologies when implementing changes in manufacturing systems. Two recent developments have given rise to our interest in this area. On the one hand, manufacturing industry, especially in the UK, has had to face over the past few years stiff competition from overseas, both in respect of the quality of its products and its ability to deliver them at the right time and at the right price (Schon- berger, 1986; Voss, 1986, 1987). In order to meet these new pressures many companies have had to make considerable changes to their manufacturing systems and the way in which they were managed, usually through the introduction of some form of computer-assisted technology (Ingersoll Engineers, 1985; Goldratt and Cox, 1986). On the other hand, the technologies required to solve these problems are now quite well established. Indeed, there are numerous vendors of systems, such as Computer-aided Design and Manufacture (CAD/CAM), Flexible Manufacturing Systems (FMS), and Manufacturing Resources Planning (MRP2), to name a few, who claim that their standard product, or some modification of it, will solve a given class of problems in any organisation. In other words, we have reached a stage where known “solutions” are looking for “problems”.