Analog signal refers to the continuous change ofphysical quantityIt indicates that the amplitude, frequency, or phase of the signal changes continuously with time, ortime intervalThe characteristic quantity representing information can be presented as a signal of any value at any instant.
Analog signals refer to the information expressed by continuously changing physical quantities, such as temperature, humidity, pressure, length, current, voltage, etc. We usually call analog signalsContinuous signalIt can have an infinite number of different values in a certain time range.Digital signals are discrete in valueDiscontinuityThe signal of.
Various physical quantities in actual production and life, such as the image taken by the camera, the pressure, flow rate, speed, humidity, etc. recorded in the workshop control room, are analog signals.Digital signal is formed by sampling, quantizing and coding on the basis of analog signal.To be specific, sampling is to select the input analog signal according to the appropriatetime intervalGet theSample valueQuantization is to use binary code system to represent the values measured at each time after sampling, and coding is to generate quantizationBinary numberArranged together to form an orderpulse sequence 。
analog signalTransmission processFirst, convert the information signal into almost "identical" fluctuationselectrical signal(Therefore, it is called "analog"), and then transmitted through wired or wireless mode. After the electrical signal is receivedReceiving equipmentRestore to information signal.
For nearly a hundred years, whether wired or wirelessradio and televisionFor a long time, analog signals are used to transmit signals.It is said that the analog signal is almost "the same" as the original signal in waveform, and it seems that it should achieve goodcommunication effect However, the fact is on the contrary. In the past, we often encountered the phenomenon that we could not hear clearly when we called;Radio stations are always lacking in effectiveness;There are also flickering snowflakes on TV images.This is because the signal has to go through a lot of processing and transmission in the transmission process, and these devices will inevitably produce some interference;In addition, if it is wired transmission, electrical equipment near the line will also generateelectromagnetic interference ;If it isWireless transmissionIs more "open", and all kinds of interference in the air are irresistible.These interferences are easy to cause signal distortion and also bring some noise.These distortions and additional noise will accumulate with the increase of transmission distance, which will seriously affect the communication quality.In this regard, people have thought of many ways.One is to take various measures to resist interference, such as improving the quality of information processing equipment and minimizing its interference;Another example is to giveTransmission linePlus shielding;Another example is to use FM carrier instead of AM carrier.However, none of these methods can fundamentally solve the problem of interference.Another method is to try to remove the noise in the signal and restore the distorted signal. However, for analog signals, it is difficult to effectively deduce the original undistorted signal from the distorted signal, and some even make it worse.
Analog signals are mainly related todispersedOfdigital signalRelatively continuous signals, analog signals are distributed in all corners of nature, such asChange of temperature, anddigital signalIt is an artificially abstracted signal with discontinuous amplitude values.Electrical analog signals mainly refer to electrical signals with continuous amplitude and phase, which can beAnalog circuitPerform various operations, such as amplification, addition, multiplication, etc.
The conversion of analog signal to digital signal requires four basic steps: signal sampling, signal holding, signal quantization and signal coding.
Sampling is to discrete the continuous signal in time, that is, collect the original analog signal point by point according to a specific time intervalInstantaneous value。In terms of effect,sampling frequency The higher, theDiscrete signalThe closer to the original analog signal, but if the sampling frequency is too highActual circuitThe requirement of is higher, which will also bring a lot of computing and storage.If the sampling frequency is too low, the information will be lost. If it is serious, the information will be distorted and cannot be used.After taking its instantaneous value, it should be kept at the original position for a period of timeSerrated typeThe wave signal is provided to the subsequent signal quantization.
Quantizing the collected discrete signal is to convert the signal with specific amplitude intoAnalog-to-digital converterThe integral multiple of the smallest unit ofQuantitative unit。eachSample valueRepresents the instantaneous amplitude of the analog signal obtained by one sampling.Usually, the unit of quantification is a multiple of 2,Quantization Bits The more, the smaller the quantization error, and the better the quantization result.In the actual quantization process, due to the need for approximate processing, there must be a quantization error. This error occurs at the end of theDigital to analog conversionThe error is usually calledQuantization noise。Generally, the quantization error can be reduced by increasing the quantization bits, but when the signal amplitude is reduced to a certain valuerelevanceIt is even more obvious.
Coding the quantized discrete signal is the last step of converting analog signal to digital signal. It is commonly realized by parallel comparison circuit and successive approximation circuit. With the help of certain circuit, the quantized discrete signal can be converted into corresponding digital signal.[1]
The main disadvantage of analog signals is that they are always subject toNoise(unwanted random variation value in the signal).After the signal is copied many times or transmitted over a long distancerandom noise The impact of may become very significant.In electricity, usingGrounding shield(shield), good contact and use of linesCoaxial cableorTwisted pair, which can alleviate these negative effects to a certain extent.
Noise effectThe signal will be damaged.It is almost impossible to restore the damaged analog signal again, because the amplification of the required signal will amplify the noise signal at the same time.If the difference between the noise frequency and the required signal frequency is large, you can introduceElectronic filterFilter out the noise of specific frequency, but this scheme can only reduce the impact of noise as much as possible.Therefore, under the effect of noise, although the simulationSignal theoryHas infinite resolution, but not necessarily greater thandigital signalMore accurate.
Although the digital signal processing algorithm is relatively complex, the existingDigital Signal Processor This task can be completed quickly.In addition, the gradual popularization of computer and other systems makes the transmission and processing of digital signals more convenient.Devices such as cameras are gradually digitized, although they must initially receive the real world in the form of analog signalsphysical quantityAnd will finally passAnalog digital converterConvert todigital signalTo facilitate computer processing or transmission via the Internet.[2]
