Tham khảo tài liệu 'tiếng anh chuyên ngành học viện công nghệ bưu chính viễn thông phần 4', ngoại ngữ, anh văn thương mại phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Unit 4. Optical Fibres 3. READING 2 Read the following text carefully. PULSE CODE MODULATION PCM The process of choosing measuring points on the analogue speech curve is called sampling. The measurement values are called samples. When sampling we take the first step towards a digital representation of the speech signal as the chosen sampling instants give us the time coordinates of the measuring points. The amplitudes of the samples can assume each value in the amplitude range of the speech signal. When measuring the sample amplitudes we have to round off for practical reasons. In the rounding-off process or the quantizing process all sample amplitudes between two marks on the scale will be given the same quantized value. The number of quantized samples is discrete as we have only a discrete number of marks on our scale. Each quantized sample is then represented by the number of the scale mark . we know now the coordinates on the amplitude axis of the samples. The processes of sampling and quantizing yield a digital representation of the original speech signal but not in a form best suited to transmission over a line of radio path. This process is known as encoding. Most often the sample values are encoded to binary form so that each sample value is represented by a group of binary elements. Typically a quantized can assume one of 256 values. In binary form the sample will be represented by a group of 8 elements. This group is called a PCM word. For transmission purposes the binary values 0 and 1 can be taken as corresponding to the absence and presence of an electrical pulse. Figure 1. Pulse code modulation block On the transmission line the pulses in the PCM words will become gradually more distorted. However as long as it is possible to distinguish between the absence and the presence of a pulse no information loss has occurred. If the pulse train is regenerated . badly distorted 48 Unit 4. Optical Fibres pulses are replaced by fresh pulses at suitable .
