电子信息与通信专业外语

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电子信息与通信专业外语

赵淑清

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PCM AND DIGITAL TRANSMISSION

（一）Introduction

This process of converting an analogue wave form such as that of telephone speech to a digital form inevitably involves an increase in bandwidth or in the frequency spectrum occupied in the medium being used and would appear at first to be a rather pointless complication of the process of conveying voice signals. The technical and economic advantages will be explained in following sections but a brief initial outline may help to put things in perspective.

For many years there have been attempts to realize the economic potentialities of time division multiplexing. It is by no means a new concept and was used in telegraphy before telephony before telephony came on the scene. The devices used were simply synchronized commutators where one at the sending end “sampled” each of a number of individual channels in sequence and transmitted the samples in sequence. The other device at the receiving end received the samples and distributed them in the correct sequence to the corresponding individual channels. Obviously there was a problem of maintaining the commutators in synchronism. This problem remains but modern electronic methods have provided cheap and reliable ways of achieving the required result.

Where, as in telegraphy, the information is digital, i.e. comprises a succession of units of information limited to a very few discrete values, e.g. the binary mark and space of telegraphy, TDM is extremely simple and cheap.

It has long been known that an analogue wave form of limited bandwidth restricted to an upper frequency limit of f1 may be accurately conveyed by transmitting instantaneous samples at a sampling rate slightly in excess of 2f1. If these samples are transmitted as narrow pulses then the pulses corresponding to several channels may be sent consecutively in a regular cycle and it thus becomes feasible to apply TDM to analogue information.

This form of TDM has been used for transmission purposes and also for switching, and various modulation techniques have been employed: for instance pulse amplitude, pulse width and pulse position. In switching, where the samples can be transmitted over wide band highways, reasonable control of signal impairments can be achieved but on junction and trunk routes the inevitable distortion of the pulses in amplitude and phase make it very difficult to control inter-channel interference or crosstalk without prohibitively costly equalising.

It was partly in this quest for a more satisfactory answer to the TDM problem and partly to counter the high noise levels of earlier radio links that the late Alec Reeves over 30 years ago conceived the idea of PCM in which an analogue wave form is sampled at regular intervals by narrow pulses and a numerical “description” of the amplitude of each sample is transmitted in place of the analogue wave form.

Although originally conceived against the background of noisy radio links the first large scale commercial use of PCM has been on cable pairs. The economic

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importance of increasing the capacity of the vast quantities of copper pairs now installed on short-and-medium-haul routes has long been recognized . The relatively poor crosstalk and noise characteristics of those pairs has proved a major obstacle to the introduction of multiplexing by traditional FDM methods . With PCM it has prove practicable as will be explained in the following sections, to use two regular cable pairs to handle from 24 to 32 conversations.

The conversion of analogue information to a binary digital form of coding introduces a new range of opportunities and problems. The key to the attractions of digital transmission resides in the concept of regeneration. In classical analogue communication systems the limiting factor in the establishing of a satisfactory connection between two remote users is the signal/noise ratio. Modern methods have established very satisfactory standards in regard to the limiting of attenuations of the signal, but no analogue amplifier can prevent the inevitable accumulation of noise. Each time the signal is amplified so also is the noise which has been added to it within the pass band involved.

The characteristic of digital transmission is that since the signal has a restricted number of states (typically for binary data only two) then provided the acquired noise on any section does not exceed the level at which ambiguities (errors) will occur in recognizing which state was transmitted, the signal may be regenerated without error and noise is not accumulated. This is true in regard to all essential characteristics of the signal except for a residual form of noise known as phase or timing jitter, i.e. the varying displacement in time of the pulses from their ideal isochronous positions.

This ability of regeneration to avoid almost all noise accumulation results in the signal/noise ratio required on each section to reduce regeneration interpretation errors to a negligible on a complete connection. This means that the wider frequency spectrum requirements of the digital mode are more than counterbalanced (certainly on enclosed media)by the vastly improved noise and interference tolerance .This is an example of exchanging bandwidth for signal/noise ratio.

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