TIẾNG ANH CHUYÊN NGÀNH HỌC VIỆN CÔNG NGHỆ BƯU CHÍNH VIỄN THÔNG phần 4

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, i.e. 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. Sampling Quantizing Encoding Reconstruction Decoding Regeneration Regeneration Regeneration 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, i.e. badly distorted 48 Unit 4. Optical Fibres pulses are replaced by fresh pulses at suitable intervals, the information pulses can be transmitted long distances with practically no distortion at all. This is one of the advantages of digital transmission over analogue transmission: the information is contained in the existence or not of a pulse rather than in the form of the pulse. On the receiving side, the PCM words are decoded, i.e. they are translated back to quantized samples. The analogue speech signal is then reconstructed by interpolation between the quantized samples. There is a small difference between the analogue speech signal on the receiving side due to the rounding off of the speech samples. This difference is known as quantizing distortion. 3.1. Main phrases - rounding off = quantizing process: quá trình lượng tử hóa - the same quantized value: cùng một giá trị lượng tử 3.2. Find the information to complete the sentences. Example: The small difference between the analogue speech signal on the receiving side due to the rounding off of the speech samples is known as: quantizing distortion. 1. The measurement values are called: ... 2. The process of choosing measuring points on the analogue speech curve: ... 3. Typically, a quantized can assume: ... 4. The PCM words are decoded, that means they are: ... 5. The processes of sampling and quantizing brings: ... 6. In binary form, the sample will be represented by: ... 7. In the quantizing process, all sample amplitudes between two marks on the scale: ... 8. Each quantized sample is: ... 9. On the receiving side, the PCM words are: ... 10. One of the advantages of digital transmission is: ... 4. WRITING 4.1. Use the Figure 2 to order the sentences below to form a text describing a simple FDM system. 49 Unit 4. Optical Fibres Figure 2. An FDM system A. The second and the third channels cannot be transmitted directly over the line. B. At the receiving end, filters separate the three channels and finally demodulators restore channels 2 and 3 to their original frequency position. C. To illustrate the principle of a frequency division multiplex (FDM) system, consider the simple case of transmission of 3 telephone channels, of bandwidth 300- 3400 Hz, over a common line. D. Now, the three channels can be passed over the common line, boosted at intervals as necessary. E. First, they are passed through a modulator which frequency-shifts them to frequency bands 4300 to 7400 Hz and 8300 to 11400 Hz respectively. F. The first of these channels can be transmitted directly over the common line and occupies the bandwidth 300- 3400 Hz. 4.2. Complete the description of a TDM system, using Figure 3 as a basis. 50 Unit 4. Optical Fibres Figure 2. A TDM system To …………………(1) the principle of a ……………… (2) system, consider the simple case of transmission of one ………………..(3) signal, of bandwidth 300- 4800 Hz, over a ……….........….(4) line. The audio signal cannot be transmitted directly over the line. …………….(5) it is passed ………………. (6) a low pass filter which restricts the upper frequency to 3400 Hz. ………….....….(7), a clock pulse is synchronized with the audio signal at the ….....………(8) gate so that the output is a PAM waveform. The PAM waveform is then converted to a quantized PCM waveform by the encoder. The waveform can now be ……………….(9) directly over the PCM line, regenerated at intervals as necessary. At the receiving end, a decoder converts the PCM back to PAM which finally is passed through a filter to restore the ………..……..(10) audio signal. III. TÓM TẮT 1. Cấu tạo, các ưu điểm của sợi quang, nguyên tắc họat động của sợi quang. 2. Cấu trúc mô tả một quá trình. 3. Mô tả một hệ thống FDM. 4. PCM: lấy mẫu, lượng tử hoá, mã hoá, giải mã... VOCABULARY apparatus avalanche photodiode n dụng cụ, thiết bị n điot quang kiểu thác 51 Unit 4. Optical Fibres bandwidth n booster n cladding n closely-defined wavelength n copper cable n core n crossing place n crosstalk n device n duct space n electrical isolation n filter n frequency division multiplex (FDM) n glass pipe n graded-index multimode n guided electromagnetic mode n immune to/against adj. interference n light ray n light-emitting diode (LED) n opaque adj. optical fibre n original frequency position n photodetector n pin photodiode n pulse of light n pulse spreading n reflection n stepped-index monomode n stepped-index multimode n stray signal n to fade = to attenuate v to give off v độ rộng băng tần bộ khuyếch đại vỏ bọc bước sóng xác định gần nhau cáp đồng lõi, ruột điểm nối ngang xuyên âm thiết bị không gian trong ống cáp cách điện bộ lọc ghép kênh phân chia theo tần số ống thuỷ tinh sợi đa mode chiết xuất biến đổi đều mode điện từ được định hướng tránh, chống được nhiễu tia sáng điot phát quang mờ, đục ... - tailieumienphi.vn