电子信息与通信专业外语(2)

（二）Theoretical Considerations of PCM

Any practical communication system must be a compromise between the requirements for high quality and low cost. The frequency spectrum will be limited by the bandwidth which can be made available and there must be varying degrees of cumulative distortion and noise. PCM cannot eliminate the transmission degradations but it does reduce and simplify them by confining them essentially to considerations of digital error rate and timing or phase jitter. This state of affairs is attained by acceptance of certain impairments introduced at the terminal analogue/digital and digital/analogue conversion processes. It is necessary to start by an examination of these impairments and the steps which have to be taken to hold them to a subjectively satisfactory level. It must be emphasized that the subjective aspcets are very important and vary from one type of information to another, for instance as between speech and television. This section examines the problem mainly in relation to speech.

It is impossible in one short section to treat the theory in any depth. What will be attempted here is to indicate the nature of the issues involved and the ways in which

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an acceptable code of practice is being established.

The essential elements involved in the terminal processes are: sampling , quantizing , companding and coding .The last is inescapably involved with the desired characteristics of digital transmission which will be dealt with further in subsequent sections.Some reference is needed here, however, because it is also involved with the practical implementation of quantising and companding .

Most of that which follows is relevant to the basic process of digitising a single voice channel and the need for this will probably arise. It should be remembered, however, that nearly all systems in use to date are multiplex systems and these processes of sampling, quantizing , companding and their receiving counterparts are conducted sequentially by a common unit serving a number of voice channels. This sequential operation by a common unit is then the essential basis of the multiplexing process.

(1)Sampling

PCM represents in many ways the culmination of a long search for effective methods of employing time division multiplexing in the handling of analogue information. The basis of all such approaches is the idea of periodic sampling and PCM shares with all the earlier attempts the inherent limitations.

The first limitation is the relationship of the bandwidth or more accurately the upper limit of the spectrum to the sampling rate. The theory that a wave form limited to an upper frequency f may be completely conveyed by sampling at a rate of 2f1 is obviously as it stands theoretical idealism. It presupposes infinitely short sampling periods, a signal of infinte duration and perfect filters. The first of these issues is relatively unimportant and the distortion due to a sampling period of finite but practically feasible duration, for example 2% or 3% of the sampling rate, is quite negligible.

The other considerations are of practical important .A signal such as speech is in its essential nature a succession of short duration periods of transmitted power with little or no sustained repetition, and filters are necessarily imperfect, especially if they are to be simple and cheap.

Determination of a practical compromise for a sampling rate to handle voice signals over the 300~3400 Hz spectrum now firmly established for telephony involves, therefore, some examination of the limitations of practical methods of sampling and the counterpart of demodulating and filtering, together with some consideration to subjective issues concerning the tolerance of voice communication to particular classes of distortion.

A value of 8000 samples per second has received world-wide agreement and is now one of the solidly established parameters.

In PCM the subsequent operations of quantising and coding occupy a finite time and it is normal to use a sample-and-hold arrangement to retain a record of the short duration sample for the period in which it is necessary to operate upon it.

With direct coupling to relatively long lines, in other words with interoffice

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junctions or trunks, any analogue modulation of pulses such as in pulse amplitude modulation (p. a. m) must present acute crosstalk problems demanding wide bandwidth or sophisticated equalising or both .This has been one of the main obstacles to the use of the p. a. m. on any scale in transmission. Within the restricted physical dimensions of a PCM terminal the problems are manageable. Careful design and engineering is still needed but very high standards of accuracy in the transfer of a sample magnitude have been achieved.

Ideal sampling and sampling switches would operated in + and — directions on an AC wave form. There are considerable complexities in this ideal approach, including the imperfect linearity of any semiconductor devices at very low voltage levels. In practice it is usual therefore to work in one sense only superimposing on the AC wave form a standing DC component of a magnitude equal to or greater than the peak AC voltage that is desired to handle. This is referred to as a pedestal.

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