Chapter 10. Digital Signals 第十章 數位訊號 亞洲大學 資訊工程學系碩士班 呂克明教授 二 ○○ 六年十二月十一日

1. Chapter 10. Digital Signals 第十章 數位訊號 亞洲大學 資訊工程學系碩士班 呂克明教授 二○○六年十二月十一日

2. Chapter 10. Digital Signals(第十章數位訊號) • Introduction (緒言) • Digital Baseband Signals (數位基帶訊號) • Pulse-Code Modulation (脈衝數碼調變) • Time-Division Multiplexing (時分多工) • Bandwidth Requirements (頻寬需求) • Digital Carrier Systems (數位載波系統)

3. Introduction (緒言) • A baseband signal may consist of one or more information signals. • A number of telephony signals in digital form may be combined into one baseband signal by the process known as time-division multiplexing. • Analog signals may be convert into digital signals for the transmission. • Digital signals also originate in the form of computer and other data. • In general, a digital signal is a coded version of the original data or analog signal.

4. Digital Baseband Signals(數位基帶訊號) • Digital signals are coded representations of information. • Keyboard characters are usually encoded in binary digital code. • Analog signals such as speech and video may be converted to a digital form through an analog-to-digital (A/D) converter.(類比至數位轉換器) • A particular form of A/D conversion is employed, known as pulse-code modulation (PCM, 脈衝碼調變技術). • The digital information is transmitted as a waveform.

5. Pulse Code Modulation(脈衝數碼調變) • Speech and video appear naturally as analog signals, and these must be converted to digital form for transmission over a digital link. • The particular form of A/D conversion used is known as pulse-code modulation (PCM, 脈衝碼調變技術). • Sampling frequency: • The sampling frequency must at least twice the highest frequency in the spectrum of the signal being sampled. • With the upper cutoff frequency of the audio filter at 4 kHz, the sampling frequency can be standardized at 8 kHz.

6. Time-Division Multiplexing(時分多工) • A number of signals in binary digital form can be transmitted through a common channel by interleaving the pulses in time as time-division multiplying (TDM). • For speech signals, a separate codec may be used for each voice channel, the outputs from these being combined to form a TDM baseband signal, as shown in Fig. 10.6. • At the baseband level in the receiver, the TDM signal is demultiplexed, the PCM signals being routed to separate codecs for decoding. In satellite systems, the TDM waveform is used to modulate the carrier wave. • The TDM signal format is best described with reference to the widely used Bell T1 system. The signal format is illustrated in Fig. 10.7a. Each PCM word contains 8 bits, and a frame contains 24 PCM channels.

7. Bandwidth Requirements(頻寬需求) • In a satellite transmission system, the baseband signal is modulated onto a carrier for transmission. • A number of filtering stages: Filtering of the signals takes place at a number of stages. • Eliminate sideband: The baseband itself is band-limited by filtering to prevent the generation of excessive sidebands in the modulation process. • Filtering: The modulated signal undergoes bandpass filtering as part of the amplification process in the transmitter. • Delay: With a satellite link, the main channel does introduce a propagation delay. • Noise: It is necessary to limit the noise at the receiver end.

8. Digital Carrier Systems(數位載波系統) • On-off keying (OOK), also known as amplitude-shift keying (ASK, 移幅轉換技術). • The binary signal in this case is unipolar and is used to switch the carrier on and off. (藉由載波有無的編碼方式來傳遞訊息) • Frequency-shift keying (FSK, 移頻轉換技術). • The binary signal is used to frequency modulate the carrier, one frequency being used for a binary 1 and another for a binary 0. • These are also referred to as the mark-space frequencies. (它的頻率是固定某一差值的兩個頻率，不過我們是把其中一個頻率的 ON 或 OFF 當作 1，另一個頻率的 ON 或 OFF 當成 0)

9. Digital Carrier Systems(數位載波系統) (continued) • Binary phase-shift keying (BPSK,二元弦角移相轉換技術). • Polarity changes in the binary signal are used to produce 180 degree changes in the carrier phase. • This may be achieved through the use of double-sideband, suppressed-carrier modulation (DSBSC), with the binary signal as a polar nonreturn-to-zero (NRZ) waveform. • In effect, the carrier amplitude is multiplied by +/- pulsed waveform. • When the binary signal is +1, the carrier sinusoid is unchanged, and when it is -1, the carrier sinusoid is changed in phase by 180 degree. • Binary phase-shift keying is also known at baseband before modulation, to limit the sidebands produced, and as part of the filtering needed for the reduction of intersymbol interference (ISI), as described in Sec. 10.5. • The resulting modulated waveform is sketched in Fig 10.11.

10. Digital Carrier Systems(數位載波系統) (continued) • Differential phase-shift keying (DPSK,差異移相轉換技術). • This is phase-shift keying for a which the phase of the carrier is changed only if the current bit differs from the previous one. • A reference bit must be sent at the start of message, but otherwise the method has the advantage of not requiring a reference carrier at the receiver for demodulation. • Quadrature phase-shift keying (QPSK , 正交弦角移相轉換技術) • This is phase-shift keying for a 4-symbol waveform, adjacent phase shifts being equispaced by 90 degree. • The concept can be extended to more than four levels, when it is denoted as MPSK for M-ary phase shift keying. • Quadrature amplitude modulation (QAM, 正交振幅調幅法) • This is also a multilevel modulation method in which the amplitude and the phase of the carrier are modulated.