Signals represent messagesphysical quantity, such as electrical signal can pass throughrange、frequency、phaseTo represent different messages.This electrical signal hasanalog signalanddigital signalTwo types.The signal is a tool to carry messages and a carrier of messages.Broadly speaking, it includes optical signals, acoustic signals and electrical signals.According to actual use, signals includeTV signal, broadcast signalRadar signal，communication signals Etc;According to the time characteristicsDeterministic signalAnd random signals.
Signal is carriernewsIs the carrier of messages.Broadly speaking, it includes optical signals, acoustic signals and electrical signals.For example, the ancient people used lightingthe Beacon TowerRolling caused byWolf smoke, to send the message of enemy invasion to the distant army, which belongs to light signal;When we talk,acoustic waveDelivered to othersEarsTo make others understand ourintention, which belongs to acoustic signal;
All kinds of travel in spaceradio wavesThe current in the telephone network extending in all directions can be used to express variousnews, this is an electrical signal.Through the light, soundelectrical signalOnly after receiving can you know the message that the other party wants to express.
There are many ways to classify signals. Signals are classified according to mathematical relationship, value characteristics, energy power, processing analysis, and timefunctionThe characteristics and whether the value is a real number can be divided into deterministic signal and non deterministic signal (also calledrandom signal）、Continuous signalandDiscrete signal(i.e. analog signal and digital signal), energy signal and power signal, time domain signal and frequency domain signaltime-limited signal andFrequency limit signal, real signal andComplex signalEtc.
Analog and digital signals
Analog signal means that the signal waveform changes by simulating the change of information. Its main feature is that the amplitude is continuous and can take an infinite number of values;In time, it can be continuous or discontinuous.As shown in Figure 2.
Digital signal is not only discrete in time, but also in amplitudedispersedOnly a limited number of signals can be taken.Such as telegraph signal,Pulse code modulation(PCM, Pulse Code Modulation) signals are digital signals.Binary signal is a kind of digital signal, which represents different information by different combinations of "1" and "0".
Communication systems are divided into analog communication systems and digital communication systems according to whether they transmit analog signals or digital signals.If analog signals are sent to the transmission system, this communication mode is analog communication.Most of the telephone, radio and television systems used today are analog communications.
If the analog signal passes throughsampling、quantification、codeAfter being converted into digital signals and then transmitted, this communication mode is digital communication.
Compared with analog communication, digital communication occupies channelfrequency bandIt is wide, but it has the advantages of strong anti-interference ability, no noise accumulation, easy storage, processing and exchange, strong confidentiality, easy large-scale integration, miniaturization, etc., and is increasingly widely used.
Analog signals and digital signals can be converted to each other: analog signals are generally quantized into digital signals by PCM pulse code modulation, that is, different amplitudes of analog signals correspond to different binary values. For example, 8-bit coding can quantize analog signals to 2 ^ 8=256 orders of magnitude. In practice, 24 bit or 30 bit coding is often used;Digital signals are generally converted into analog signals by phase shifting the carrier.computer、Computer LANUsed with MANBinaryDigital signals, which are actually transmitted in the computer wide area network, include both binary digital signals and analog signals converted from digital signals.But digital signal is more promising.
Digital baseband signal
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The digital signal shall be converted into a code type suitable for channel transmission before being sent to the channel for transmission.Digital baseband signalIs an unmodulated electrical signal.The following describes the code types of some commonly used binary digital baseband signals.
Unipolar nonreturn
As shown in Figure 3 (a), in this binary code, positive pulse (or negative pulse) is used to represent information "1", and low level (usually zero level) is used to represent "0"SymbolThe level remains unchanged during the period.
Bipolar irreversibility
As shown in Figure 3 (b),BipolarNon return to zero code uses positive pulse to represent "1", and negative pulse to represent "0".In the whole symbolTBDuring this period, the level also remains unchanged.
Unipolar reversion
As shown in Figure 3 (c), the difference between this code and unipolar non return to zero code lies in the pulse duration of "1" codeτless thanTB。
Is the pulse duty cycle, and the duty cycle of the commonly used RZ code is 50%.
Bipolar reversion
As shown in Figure 3 (d), its duty cycle is generally 50%.
Differential code
Differential code is not usedpulseThe value itself represents the "1" code or the "0" code, but the value of the symbol is represented by the change of the polarity of the front and rear pulses, that is, the change of the level of the adjacent front and rear symbols represents "1", and the change of the level of the adjacent front and rear symbols represents "0", which can also be reversed.The differential code can be unipolar or bipolar.Figure 3 (e) shows a bipolar differential code.
Polarity alternating reversal code
As shown in Figure 3 (f), zero level means "0";"1" is represented by positive and negative alternating pulses.This code type has no DC component.
N-order high-density bipolar (HDBn) codes
HDBnIs a concatenated "0" suppression code.When the concatenation "0" in the binary sequence is not greater thannThe encoding method is the same as AMI code.When the consecutive "0" code exceedsnWhen, everyn+One is connected to "0" as a group, which is replaced by a specific code group, so that HDBnThe consecutive "0" code in the code cannot exceedn.
HDBnHDB3, the third-order high-density bipolar code, is the most widely used in, as shown in Figure 3 (g).When there are more than 3 consecutive "0" codes, every 4 consecutive "0" codes are used as a group, and "000V" or "B'00V" are substituted respectively. Therefore, there will be no more than 3 consecutive "0" codes in the code.In the code, V and B 'are both transmission numbers (i.e. pulses), and V is the breaking point, which breaks the principle of pulse polarity alternation in the code stream, so that the receiver can identify.B 'code is a non destructive point. It does not break the principle of polarity alternation, but is an additional pulse to maintain the balance of positive and negative pulses.
The replacement principle of HDB3 code is as follows:
(a) The "1" code (called signal code, represented by B) and the V code in the original code need to keep their polarity alternating to ensure that there is no DC component in the code sequence.
(b) The V code must be the same polarity as the previous B code, so as to be consistent with the normalAMI codeDistinguish.If this condition is met, use "000V" to replace the "0" group of the four connections;If not, replace it with "B'00V". B ′ and V are of the same polarity.At this time, the combination of B code and B 'code should maintain the rule of alternating polarity of signal code in condition a.
Table 1 shows an example of HDB3 code coding.
When decoding at the receiving end, it is found that there are two adjacent homopolar codes, then the latter is the V code, which changes the V code and its first three bit codes into "0".In this way, the original code stream can be restored.
The code types of AMI and HDB3 have been recommended by CCITT (now ITU-T) inPulse code modulation systemAs the interface code type.
railway signal
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“railway signal ”It is a discipline formed by the railway department to ensure train operation safety and improve transportation efficiency.After the founding of the People's Republic of China, especially since 1958, relevant colleges and universities affiliated to the Ministry of Railways such asBeijing Railway University(NowNorthern Jiaotong University）The department of telecommunications, such as Shanghai Railway University, has set up the specialty of "signaling", which has trained a large number of professionals for China's railway signaling industry.Railway signaling mainly studies how to "centralize" the "station interlocking" and let the station attendant control it uniformly, and how to implement the "blocking" of single track or double track sections to prevent trains from head-on collision or rear collision, because the "signaling" discipline includes three aspects: signaling, centralization and blocking, which is referred to as“Signal set closure”。
The signal is the order to indicate the train operation and shunting operation, which must be strictly executed by the relevant traffic personnel.
Railway signals are divided into visual signals and auditory signals.
Visual signals are divided into daytime, nighttime and day night general signals.
Basic color of visual signal:
Red - parking;
Yellow - pay attention to or reduce the speed;
Green - run at the specified speed.
Audible signal: sound from horn, whistle, sounding block and whistle of locomotive and rail car.
According to the display meaning of stop signal, it can be divided intoAbsolute signalAnd non absolute signals (also calledPermissive signal）Two.Absolute signal refers to the signal display that the train and shunting must obey unconditionally when displaying the signal of stopping operation.When the lights of inbound, outbound, route and passing annunciators are off, unclear or incorrect, they are regarded as stop signals.When the light of approach annunciator goes out, the display is unclear or the display is incorrect, it is deemed that the inbound annunciator is closed.When the passing annunciator with permission signal displays the stop signal, the railway bureau is allowed to specify that the freight train with difficulty in starting after stopping shall not stop in front of the annunciator, and shall run to the next passing annunciator at a maximum speed of 20 km/h, and operate according to the requirements of its display.
Edge Properties
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Fast signal switching time (edge rate) will result inbackflow, crosstalkDamped oscillation(ringing) and reflection.The edge rate of signal and the working frequency of signal are two different concepts. A high edge rate is not necessarily a high frequency.For example, in practical applications, the operating frequency of the system may not be high.However, if the rising rate of the signal is too fast, a large ringing phenomenon will occur, which will also bring about the problem of signal integrity.When the ringing signal reaches the limit value that the device can tolerate, the internalsemiconductorCharacteristics change（Electron migration）The phenomenon of device heating and power consumption increase reduces the reliability of the system, and the faster the edge rate, the greater the power consumption.
