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(zslcn周生烈编译摘注评) http://en./wiki/Digital_modulation#Digital_modulation_methods This page was last modified on 27 November 2011 at 21:50 Modulation 调制 From Wikipedia, the free encyclopedia (Redirected from Digital modulation) For musical change of key, see Modulation (music). For other uses, see Modulation (disambiguation).
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used. 在电子学和电信中,调制是一种过程,其使用调制信号,来改变高频周期波形的一个或多个属性。高频周期波形被称为载波。调制信号通常包含需要传输的信息。其做法类似于音乐家调整乐器的乐音(一种周期波形),是通过改变其音量、节奏、和音调来进行的。一种周期波形有三个关键参数，它们是其振幅（“音量”）、相位（“节奏”）、和频率（“音调”）。任何这些属性,都可以依照一个低频信号进行修改,以获得被调制的载波信号。通常用于载波信号的,是一个高频正弦波；也可以使用一个方波脉冲序列。 In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband. 在电信中，调制是一个传递消息信号的过程。例如一个数字位流信号或模拟音频信号,居于另一个可以在物理上被发送的信号内。一个正弦波形的调制,被用来将一个基带消息信号,变换成一个通带信号，例如低频音频信号变换成射频信号（RF信号）。在无线电通信中，例如有线电视系统或公共电话交换网络，电信号只能在指定的 较低（非零）限和较高限截止频率 的有限通带频谱上传输。调制一个正弦载波,使它可以保持被传输信号内容的频率,尽可能接近通带的中心频率（通常是载波频率）。 A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (from "modulator–demodulator"). 实现调制的设备称为调制器，实现调制逆操作的设备称为解调器（有时称检测器,或简为demod）。一种可以做两个操作的设备称为调制解调器或modem（来自于“modulator调制器–demodulator解调器”）。
1. [edit] Aim 调制的目的 The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band. 数字调制的目的,是要在模拟带通信道上,或在一个有限的无线电频段上,传输数字位流。例如在公共电话交换网（那里有一个带通滤波器,用来限制频率范围在300和3400赫兹之间）。 The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel at a different frequency, for example over a limited radio frequency band or a cable TV network channel. 模拟调制的目的,是为了,例如在一个受限的无线电频段,或有线电视网络信道上,能以适当的不同频率,传送一个模拟基带(或低通)信号，例如音频信号或电视信号。 Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels (several different carrier frequencies). 模拟和数字调制,使频分复用（FDM）变得容易，这样,就可以在同一共享的物理介质上,使用分开的带通信道（几个不同的载波频率）,同时传输几个低通信号。 The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial bus or a wired local area network. 数字基带调制方法,又称线路编码,其目的是要在基带信道上,传输数字位流；基带信道,通常是一条非滤波器式的铜线，如一条串行总线或一个有线局域网。 The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system. 脉冲调制方法的目的，是要传输窄带模拟信号，例如,在宽带的基带信道上的一个电话呼叫，或者,在某些方案中,作为另一个数字传输系统上的位流。 In music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform. See for example frequency modulation synthesis or ring modulation synthesis. 在音乐合成器中,调制可用于 采用几个振荡器扩展的泛音频谱,来揉合波形。在这种情况下，载波频率通常与调制波形的音阶相同,或比调制波形低得多。参见,例如频率调制合成，或环形调制合成。 2. [edit] Analog modulation methods [编辑]模拟的调制方法
A low-frequency message signal (top) may be carried by an AM or FM radio wave. 一个低频消息信号（见顶部）可以由AM或FM无线电波来承载。 In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques are:[1] 在模拟调制中，连续地进行调制,以对模拟信息信号作出响应。常用的模拟调制技术是：[1] · Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) 调幅（AM）（这里载波信号的振幅,是根据调制信号的即时幅度而改变） ·· Double-sideband modulation (DSB) 双边带调制（DSB） ··· Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band) 带载波的双边带调制（DSB-WC）（用在调幅无线电广播波段上） ··· Double-sideband suppressed-carrier transmission (DSB-SC) 双边带抑制载波传输（DSB-SC） ··· Double-sideband reduced carrier transmission (DSB-RC) 双边带简化载波传输（DSB-RC） ·· Single-sideband modulation (SSB, or SSB-AM) 单边带调制（SSB，或SSB - AM） ··· SSB with carrier (SSB-WC) 带载波的单边带（SSB-WC） ··· SSB suppressed carrier modulation (SSB-SC) 抑制载波的单边带调制（SSB - SC） ·· Vestigial sideband modulation (VSB, or VSB-AM) 残留边带调制（VSB,或VSB-AM） ·· Quadrature amplitude modulation (QAM) 正交幅度调制（QAM） · Angle modulation 角度调制 ·· Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) 调频(FM)(这里,载波信号的频率是根据调制信号的即时振幅而改变) ·· Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) 相位调制(PM)(这里,载波信号的相移是根据调制信号的即时振幅而改变) 3. [edit] Digital modulation methods [编辑]数字的调制方法 In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet). 在数字调制中，模拟载波信号由离散信号调制。数字调制方法可以认为是数字-模拟转换，相应的解调或检测则是模拟-数字转换。载波信号中的变化,是根据一个有限数M（调制字母)的替代信符,来选择的。
Schematic of 4 baud (8 bit/s) data link containing arbitraily chosen values. 包含任意选择值的4波特（8位/秒）数据链示意图。 A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems. 一个简单的例子：一条电话线路可以设计成用于传输可听声音,例如音调,而不是数字位(0和1)。然而,计算机可以在电话线上,籍助于调制解调器来传递。其中,表示音调的数字比特序列称为信符。如果有四个替代信符（对应于一种可以产生四种不同的音调的乐器，每次一个），第一个信符可以表示为位序列00,第二个01，第三个10,第四个11。如果调制解调器播放的旋律由每秒1000个音调组成，则其信符速率,或波特率,是1000信符/秒。在本例中,由于每个音调（即符号）表示为由两个数字比特组成的一个消息，比特速率就是符号速率的两倍，即每秒2000比特。对照DSL调制解调器,这类似于拨号调制解调器的所使用的技术。 According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission. 根据数字信号的一种定义，被调制信号是数字信号；而根据另一种定义，调制是数字-模拟转换的一种形式。大多数教科书 将数字调制方案 认为是数字传输的一种形式，同义于数据传输；认为它是模拟传输的 则极少。(其实这两者是统一的：后者是本质，前者是用途。) 3.1 [edit] Fundamental digital modulation methods数字调制的基本方法 The most fundamental digital modulation techniques are based on keying: 最基本的数字调制技术基于键控： · In the case of PSK (phase-shift keying), a finite number of phases are used. 在PSK（相移键控）的情况下，使用有限数量的相位。 · In the case of FSK (frequency-shift keying), a finite number of frequencies are used. 在FSK（频移键控）的情况下，使用有限数量的频率。 · In the case of ASK (amplitude-shift keying), a finite number of amplitudes are used. 在ASK（幅移键控）的情况下，使用有限数量的幅度。 · In the case of QAM (quadrature amplitude modulation), a finite number of at least two phases, and at least two amplitudes are used. 在QAM(正交调幅)的情况下,使用有限数量的相位和幅度,它们各自至少两个。 In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK. 在QAM中，一个同相信号(I信号,例如是余弦波形)和正交相位信号(Q信号,例如是正弦波),采用有限数目的幅度,分别调制，并加在一起(向量和)。它可以看作为一个双通道的系统，每个通道都使用ASK。合成的信号相当于(合成载波的)PSK和ASK的结合。 In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase, frequency or amplitude. 在所有的上述方法中,每个相位、频率、或幅度,被指定在一种唯一模式的二进制位中。通常,由每个相位、频率、和幅度,与/或 组成比特位数相等的编码。信符则是 由特定的相位、频率、和幅度 构成的具体编码。 If the alphabet consists of M = 2N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is fS symbols/second (or baud), the data rate is NfS bit/second. 如果字母M 由M = 2N 种替代信符组成，每种信符表示为一个由N比特位组成的消息。设信符速率（又称波特率）是fS信符/秒（或波特），则数据传输速率是N·fS 比特/秒。 For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate. 例如，一个字母M 由16种替代信符组成：M = 24=16，则每种信符表示为4比特位。因此，数据传输率是波特率的四倍。 In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol. 在PSK，ASK或QAM的情况下,那里调制信号的载波频率是恒定的,用直角坐标的星座图 表示调制字母 通常是很方便的。在星座图中,对于每个信符,x轴处显示为I信号的幅度，y轴处显示为Q信号的幅度（也就是,把已调制载波，看作为星座图中的向量，并将它分解为相互正交的两个向量分量I和Q；I与调制前的载波同相，Q则与之正交。于是，相位和幅度均已调制的载波,分解成只有幅度调制的载波分量I和Q）。 3.2 [edit] Modulator and detector principles of operation [编辑] 调制器和检测器的操作原理 PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I+jQ (where j is the imaginary unit). The resulting so called equivalent lowpass signal or equivalent baseband signal is a complex-valued representation of the real-valued modulated physical signal (the so called passband signal or RF signal). 通常使用QAM工作原理来形成和检测PSK和ASK；有时还有FSK。I和Q信号，可以组合成一个复数值信号I+jQ（其中j是虚数单位）。其最终的所谓等效低通信号或 等效基带信号,是已被调制物理信号实数值的复数表示（所谓的通带信号或 射频信号）。 These are the general steps used by the modulator to transmit data: 调制器用来传送数据的一般步骤是： 1. Group the incoming data bits into codewords, one for each symbol that will be transmitted. 将收入的数据位分组成要发送的代码字,每个信符一个。 2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values. 将代码字安排到载波的各种属性中去，例如：正交载波I和Q信号的幅度(相当于低通信号),或频率,或相位值。 3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing.调整脉冲整形或其他一些滤波过程,以限制带宽,形成等效低通的信号的频谱。以便于通常使用的数字信号处理。 4. Perform digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP).执行I和Q信号的数字-模拟转换（DAC）(如今,上述一切通常都是使用DSP(数字信号处理)来实现的)。 5. Generate a high-frequency sine wave carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine wave form with the I and Q signals, resulting in that the equivalent low pass signal is frequency shifted into a modulated passband signal or RF signal. Sometimes this is achieved using DSP technology, for example direct digital synthesis using a waveform table, instead of analog signal processing. In that case the above DAC step should be done after this step. 产生一个高频正弦波载波，或许还有一个余弦正交分量；完成调制,例如,用I和Q信号乘以正弦和余弦波形，导致等效低通信号频移到一个被调制的带通信号或射频信号。有时,这是使用DSP技术来达到的，例如,使用波形表直接数字合成，而不是模拟信号处理。在这种情况下，上述的数模转换DAC步骤,应该在这一步完成之后再做。 6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum 放大并进行模拟带通滤波，以避免谐波失真和频谱重复循环。 At the receiver side, the demodulator typically performs: 在接收端，解调器通常执行： 1. Bandpass filtering. 对天线接收的射频信号进行带通滤波。 2. Automatic gain control, AGC (to compensate for attenuation, for example fading). 自动增益控制（AGC补偿衰减，例如衰落）。 3. Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sinewave and cosine wave frequency (see the superheterodyne receiver principle). 通过将射频信号乘以本地振荡器正弦波和余弦波频率（见超外差收信机原理），使射频信号频移到等效基带I和Q信号，或中频（IF）信号。 4. Sampling and analog-to-digital conversion (ADC) (Sometimes before or instead of the above point, for example by means of undersampling). 采样和模拟-数字转换(ADC)(有时在上一步之前做,或代替上一步,例如借助于欠采样)。 5. Equalization filtering, for example a matched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid intersymbol interference and symbol distortion. 均衡滤波，例如匹配滤波器，对多径传播、时间展延、相位失真、和频率选择性衰落进行补偿,以避免码信符间干扰和信符失真。 6. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal. 检出I和Q信号的幅度,或中频信号的频率或相位。 7. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.将幅度、频率、或相位,量化到最接近的允许信符值。 8. Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).将量化的幅度、频率、或相位,安插到代码字上(位组)。 9. Parallel-to-serial conversion of the codewords into a bit stream. 通过并行到串行的转换,将代码字转换成比特位流。 10. Pass the resultant bit stream on for further processing such as removal of any error-correcting codes. 将最终得到的比特位流作进一步处理，如清除所有的纠错码。 As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations. 通常,对于所有数字通信系统,调制器和解调器的设计必须同时完成。这对于数字调制方案是可能的，因为发信器-收信机对,早已有了数据如何编码,以及在通信系统中是如何表示的知识。在所有的数字通信系统中，所构建的发信机调制器和收信机解调器,它们执行互逆操作。 Non-coherent modulation methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier wave. In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite is coherent modulation. 非相干调制方法,并不需要收信机有一个 与发送端载波波形相位同步 的参考时钟信号。在这种情况下，调制信符（而不是位、字符、或数据包）是异步传送的。而相干调制则与此相反。 3.3 [edit] List of common digital modulation techniques [编辑]常见数字调制技术表单 The most common digital modulation techniques are: 最常见的数字调制技术有： · Phase-shift keying (PSK): 相移键控（PSK）： ·· Binary PSK (BPSK), using M=2 symbols 二进制PSK（BPSK），使用M = 2种信符 ·· Quadrature PSK (QPSK), using M=4 symbols 正交PSK（QPSK），使用M = 4种符号 ·· 8PSK, using M=8 symbols 8PSK，使用M = 8种信符 ·· 16PSK, using M=16 symbols 16PSK，使用M = 16种信符 ·· Differential PSK (DPSK) 差分PSK（DPSK） ·· Differential QPSK (DQPSK) 差分QPSK（DQPSK） ·· Offset QPSK (OQPSK) 偏移QPSK（OQPSK） ·· π/4–QPSK · Frequency-shift keying (FSK): 频移键控（FSK）： ·· Audio frequency-shift keying (AFSK) 音频频移键控（AFSK） ·· Multi-frequency shift keying (M-ary FSK or MFSK) 多频移键控（M进制FSK或MFSK） ·· Dual-tone multi-frequency (DTMF) 双音多频（DTMF） ·· Continuous-phase frequency-shift keying (CPFSK) 连续相位频移键控（CPFSK） · Amplitude-shift keying (ASK) 幅移键控（ASK） · On-off keying (OOK), the most common ASK form 开关键控（OOK），最常见的ASK形式 ·· M-ary vestigial sideband modulation, for example 8VSB M进制残留边带调制，例如8VSB · Quadrature amplitude modulation (QAM) - a combination of PSK and ASK: 正交调幅（QAM） - PSK和ASK组合： ·· Polar modulation like QAM a combination of PSK and ASK.[citation needed] 极性调制 类似于QAM的一个组合PSK和ASK [需要引证] · Continuous phase modulation (CPM) methods: 连续相位调制（CPM）的方法： ·· Minimum-shift keying (MSK) 最小相移键控（MSK） ·· Gaussian minimum-shift keying (GMSK) 高斯最小相移键控（GMSK） · Orthogonal frequency-division multiplexing (OFDM) modulation: 正交频分复用（OFDM）调制： ·· discrete multitone (DMT) - including adaptive modulation and bit-loading. 离散多音（DMT） - 包括自适应调制和比特加载。 · Wavelet modulation 小波调制 · Trellis coded modulation (TCM), also known as trellis modulation 网格编码调制（TCM），又称格子调制 · Spread-spectrum techniques: 扩谱技术： ·· Direct-sequence spread spectrum (DSSS) 直接序列扩频（DSSS） ·· Chirp spread spectrum (CSS) according to IEEE 802.15.4a CSS uses pseudo-stochastic coding 线性调频扩频（CSS）根据IEEE 802.15.4a , CSS使用伪随机编码 ·· Frequency-hopping spread spectrum (FHSS) applies a special scheme for channel release 跳频扩频（FHSS）为信道发布安排一个指定的计划 MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling). MSK和GMSK是连续相位调制(CPM)的特例。的确,MSK是CPM子族(称之为连续相位频移键控CPFSK) 的一个特例。CPFSK是由一个矩形频率脉冲(即一个线性增加相位的脉冲)所定义；其脉冲宽度为一个信符时间间隔(全响应信令)。 OFDM is based on the idea of frequency-division multiplexing (FDM), but is utilized as a digital modulation scheme. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated sub-carriers are summed to form an OFDM signal. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the orthogonal frequency-division multiple access (OFDMA) and multi-carrier code division multiple access (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users. OFDM基于频分复用（FDM）的概念，但作为一种数字调制方案来使用。比特位流被分成几个并行的数据流，每个被传送的子数据流,使用某种常规的数字调制方案,调制在它自己的子载波上；然后,各被调制的子载波加在一起，以形成OFDM信号。OFDM被认为是一种调制技术,而不是复用技术，因为它使用一串所谓的OFDM符号序列,在一个通信通道上,传送一个比特位流。在正交频分多址接入(OFDMA),和多载波码分多址接入(MC-CDMA)方案中，通过将不同的子载波或扩频码,分配给不同的用户,以允许多个用户共享同一物理介质，这样,OFDM就可以扩展成多用户信道接入方法。 Of the two kinds of RF power amplifier, switching amplifiers (Class C amplifiers) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA, but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers. 有两种射频功率放大器,其中开关放大器(丙类放大器)成本低，比相同输出功率的线性放大器所消耗的电池功率少。但是,它们只用在相对恒定的幅度调制信号上，如角调制（FSK或PSK）和CDMA，而不用于QAM和OFDM。然而，即使开关放大器完全不适合用于正常的QAM星座上，QAM调制器的原理还是用来驱动这些用于调频和其他波形的开关放大器；有时QAM解调器所接收的信号,是由这些开关放大器输出的信号。 4. [edit] Digital baseband modulation or line coding [编辑] 数字基带调制或线路编码 Main article: Line code 主条目：线路代码 The term digital baseband modulation (or digital baseband transmission) is synonymous to line codes. These are methods to transfer a digital bit stream over an analog baseband channel (a.k.a. lowpass channel) using a pulse train, i.e. a discrete number of signal levels, by directly modulating the voltage or current on a cable. Common examples are unipolar, non-return-to-zero (NRZ), Manchester and alternate mark inversion (AMI) codings.[2] 术语数字基带调制（或数字基带传输）与线路编码同义。这些方法,都是在模拟基带信道(又称低通信道)上,使用一个数字脉冲序列,来传输一个数据比特位流；也就是,通过直接调制电缆上的电压或电流,形成信号电平的一个离散数。常见的例子是单极编码，不归零编码（NRZ），曼彻斯特编码和交替反转（AMI）编码。[2]
5. [edit] Pulse modulation methods [编辑]脉冲调制方法 Pulse modulation schemes aim at transferring a narrowband analog signal over an analog baseband channel as a two-level signal by modulating a pulse wave. Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e. as a quantized discrete-time signal) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example some line code. These are not modulation schemes in the conventional sense since they are not channel coding schemes, but should be considered as source coding schemes, and in some cases analog-to-digital conversion techniques. 脉冲调制方案的目的,是在模拟基带信道上,通过调制脉冲波形,使成为一个两级别的信号,来传输窄带模拟信号。有些脉冲调制方案,还允许窄带模拟信号 作为数字信号(即,作为量化的 时间离散的信号) 以固定的比特速率来传送；它可以在基本数字传输系统中传输,例如,某种线路编码。然而,这些都不是传统意义上的调制方案，因为他们不是信道编码方案，而应当认为是信源编码方案；某些情况下,是一些模拟--数字转换技术。 Analog-over-analog methods: 在模拟信号上实现模拟调制的方法： · Pulse-amplitude modulation (PAM) 脉冲幅度调制（PAM） · Pulse-width modulation (PWM) ·脉冲宽度调制（PWM） · Pulse-position modulation (PPM) 脉冲位置调制（PPM） Analog-over-digital methods: 在数字信号上实现模拟调制的方法： · Pulse-code modulation (PCM) ·脉冲编码调制（PCM） ·· Differential PCM (DPCM) 差分PCM（DPCM） ·· Adaptive DPCM (ADPCM) 自适应DPCM（ADPCM） · Delta modulation (DM or Δ-modulation) 增量调制（DM或Δ调制） · Delta-sigma modulation (∑Δ) delta-sigma调制（ΣΔ） · Continuously variable slope delta modulation (CVSDM), also called Adaptive-delta modulation (ADM) 连续可变斜率增量调制（CVSDM），也称为自适应增量调制（ADM） · Pulse-density modulation (PDM) 脉冲密度调制（PDM） 6. [edit] Miscellaneous modulation techniques [编辑]其它调制技术 · The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation. 在无线电频率处使用开关键控来传送莫尔斯电码,称之为连续波(CW)操作。 · Adaptive modulation 自适应调制 · Space modulation A method whereby signals are modulated within airspace, such as that used in Instrument landing systems. 空间调制 一种在空域内实现调制信号的方法，如用于仪表着陆系统的那种。 7. [edit] See also 参见
· Modem · Codec 8. [edit] References 参考
1。 undu Sudakshina (2010). Analog and Digital Communications. Pearson Education India. pp. 163–184. ISBN 9788131731871. 2. e-Lin Du and M. N. S. Swamy (2010). Wireless Communication Systems: From RF Subsystems to 4G Enabling Technologies. Cambridge University Press. p. 188. ISBN 9780521114035. |
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