3.2 Pulse Amplitude Modulation--the spectrum of flat-top PAM signal Low-pass filter: There is some high-frequency loss in the recovered analog waveform due to the filtering effect, this loss can be reduced by decreasing t or by using some additional gain the high frequency in the low-pass filter transfer function This filter is called equalization filter. The pulse width t is also called the aperture since t/ determines the gain of the recovered analog signal, which is small if t is small relative to t 16
16 3.2 Pulse Amplitude Modulation—the spectrum of flat-top PAM signal • Low-pass filter: There is some high-frequency loss in the recovered analog waveform due to the filtering effect, this loss can be reduced by decreasing τ or by using some additional gain the high frequency in the low-pass filter transfer function. This filter is called equalization filter. The pulse width τ is also called the aperture since τ/Ts determines the gain of the recovered analog signal, which is small if τ is small relative to Ts
3.2 Pulse Amplitude Modulation( key results) The transmission of either naturally or instantaneously sampled PAM over a channel requires a very wide frequency response because of the narrow pulse width, which imposes stringent requirements on the magnitude and phase response of the channel. The bandwidth required is much larger than that of the original analog signal, and the noise performance of the PAM SyStem can never be better than that achieved by tansmitting the analog signal directly; It provide a means for converting an analog signal to a PCM Signal; and provide a means for breaking a signal into time slots that multiple pam signals carrying transmit all of the information over a single channel o information from different sources can be interleaved 17
17 3.2 Pulse Amplitude Modulation( key results) • The transmission of either naturally or instantaneously sampled PAM over a channel requires a very wide frequency response because of the narrow pulse width, which imposes stringent requirements on the magnitude and phase response of the channel. • The bandwidth required is much larger than that of the original analog signal, and the noise performance of the PAM system can never be better than that achieved by tansmitting the analog signal directly; • It provide a means for converting an analog signal to a PCM signal; and provide a means for breaking a signal into time slots that multiple PAM signals carrying information from different sources can be interleaved to transmit all of the information over a single channel
3.3 Pulse code modulation Definition: Pulse code modulation (PCm)is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream Quantizing: instead of using the exact sampled value of the analog waveform o(kts, the sample is replaced by the closest allowed value where there are m=2n allowed value, each corresponding to one of the code words Other popular types of AD conversion, including delta modulation DM) and differential pulse code modulation DPCM)will be discussed 18
18 3.3 Pulse code modulation • Definition: Pulse code modulation(PCM) is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream. • Quantizing: instead of using the exact sampled value of the analog waveform ω(kTs ) , the sample is replaced by the closest allowed value where there are M=2n allowed value, each corresponding to one of the code words. • Other popular types of AD conversion, including delta modulation (DM) and differential pulse code modulation (DPCM) will be discussed
3.3 Pulse code modulation--the advantages of PCM Relatively inexpensive digital circuitry may be used extensively in the system PCM Signals derived from all types of analog source (audio, video) may be merged with data signals and transmitted over a common high-speed digital communication system, this merging is called time division multiplexing In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy Pcm waveform The noise performance of a digital system can be superior to that of an analog system. 19
19 3.3 Pulse code modulation—the advantages of PCM • Relatively inexpensive digital circuitry may be used extensively in the system • PCM signals derived from all types of analog source (audio, video) may be merged with data signals and transmitted over a common high-speed digital communication system, this merging is called timedivision multiplexing • In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy PCM waveform. • The noise performance of a digital system can be superior to that of an analog system
3.3 Pulse code modulation (Sampling, Quantizing, and Encoding) PCM signal is generated by carrying out three basic operations: sampling, Quantizing and encoding Sampling operation generates a fat-top pam signal signal in Low-pass analog Instantaneous PAM, Quantized No PAl ized Analog Band limited Flat-top Quanti PCM sampler Encoder of leveIs=M Bandwidth=B and hold Signal PCM transmitter(analog-to-digital conversion): Channel(transmission path) Telephon Teleph Telephone line Regenerative line Regenerative Regenerative! repeater repeater repeater PCM receiver(digital-to-analog conversion) N Regenerative PCM Quantized Low-pass Analog signal Decoder PAM、 (reconstruction) circuit Fig3-7 PCM transmission system 20
20 3.3 Pulse code modulation (Sampling,Quantizing, and Encoding) • PCM signal is generated by carrying out three basic operations: sampling, Quantizing and Encoding; • Sampling operation generates a flat-top PAM signal • Fig.3-7 PCM transmission system Signal in Analog Low-pass filter Bandwidth=B Band limited analog Instantaneous sampler and hold Flat-top PAM Quantized No. of levels=M Quantized PAM Encoder PCM Signal PCM transmitter (analog-to-digital conversion) Telephone line Regenerative repeater Telephone line Regenerative repeater Regenerative repeater Telephone line Channel (transmission path) Regenerative circuit PCM Decoder Quantized PAM Low-pass (reconstruction) filter PCM receiver (digital-to-analog conversion) Analog signal out