计算机网络-桂林电子科技大学初步.ppt

Typical Companding Functions Magnitude: 量级 Delta (增量) Modulation Analog input is approximated by a staircase function Move up or down one level (?) at each sample interval Binary behavior Function moves up or down at each sample interval Delta Modulation - example Delta Modulation - Operation Delta Modulation - Performance Good voice reproduction PCM - 128 levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM Data compression can improve on this e.g. Interframe coding techniques for video Analog Data, Analog Signals Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency division multiplexing (chapter 8) Types of modulation Amplitude Frequency Phase Analog Modulation 作业 P125： 4.2 4.5 * NRZ Differential Encoding Data represented by changes rather than levels More reliable detection of transition rather than level In complex transmission layouts it is easy to lose sense of polarity NRZ pros and cons Pros (优点) Easy to engineer Make good use of bandwidth Cons (缺点) dc component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission Multilevel Binary Use more than two levels Bipolar-AMI(交替传号反码) zero represented by no line signal one represented by positive or negative pulse one pulses alternate (交替) in polarity No loss of sync (同步) if a long string of ones (zeros still a problem) No net dc component(净直流分量) Lower bandwidth Easy error detection Bipolar-AMI and Pseudoternary Pseudoternary (伪三进码) One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI Trade-Off for Multilevel Binary Not as efficient as NRZ Each signal element only represents one bit In a 3 level system could represent log23 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Requires approx. 3dB more signal power for same probability of bit error Biphase (双相位码) Manchester Transition in middle