14单元-DigitalSignal讲解.ppt

庄阿梅 Digital Signal In addition to being represented（代表，表示） by an analog signal（模拟信号）, data can also be represented by a digital signal（数据信号）. For example, a 1 can be encoded（译码） as a positive voltage（正电压） and a 0 as a zero voltage（零电压）. 数据除了可以用模拟信号表示，还可以用数字信号表示。例如，1可以被译码为正电压，0被译码为0电压。 The three characteristics of periodic（周期的） analog signal(amplitude（振幅）, period, and phase（相位）) can be redefined（重新定义） for a periodic. 周期性的模拟信号的三个特征（振幅、周期、相位）会因为周期而被重新定义。 Most digital signals are aperiodic（非周期性的） and thus period or frequency is not appropriate（合适的）.Two new terms, bit interval（间隔） (instead of period ) and bit rate(instead of frequency ) are used to describe digital signal. 大部分数字信号非周期性的，所以周期和频率的概念并不适用。两个新术语，比特间隔（而非周期）和比特率（而非频率）用于描述数字信号。 The bit time required to send one signal bit. The bit rate（比特率） is the number of bit intervals（比特间隔） per second. This means that the bit rate is the number of bits sent in one second, usually expressed in bps(bits per second). 比特间隔是发送一个比特做需要的时间，而比特率则是一秒内发射的比特间隔数。也就是说比特率是一秒内发送的比特数，通常用bps表示。 A digital signal can be decomposed（分解）into an infinite number of simple sine（正弦） waves called harmonics（谐波）, each with a different amplitude, frequency and phase. 一个信号数字可以被分解为无限多的简单正弦波。这些正弦波叫谐波，每个谐波都具有不同的振幅、频率和相位。 This means that when we send a digital signal along a transmission medium（传输媒介）, we are sending an infinite number of simple signals. 这就意味着当我们通过传输媒介发射一个数字信号时，我们正送出去无限多个简单信号。 To receive an exact replica（复制品） of the digital, all of the frequency components must be faithfully（忠实地） transferred（转换）. If some of the components are not passed through the medium, distortion （失真）of the signal at the receiver is the result. 为了得到精确无误的复制的数据，所有的频率成分都要被忠实地转换。如果有一部分没被介质传输，那么接收器所接收到的信号就会失真。 Since no practical medium(such as a cable) is capable（能够的） of transferring the entire range of frequencies, we always have distortion. 因为没有一个实际的媒介（例如电缆）能够传输整个所有的频率范围，所以我们总有失真的问题。 Although the frequency spectrum（频谱） of a digital signal contains an infinite number of