Every day people are faced with the use of electronic devices. Modern life is impossible without them. After all, we are talking about a TV, radio, computer, phone, multicooker and more. Earlier, a few years ago, no one thought about what signal is used in each working device. Now the words "analog", "digital", "discrete" have long been heard. Some of the listed types of signals are of high quality and reliable.
Digital transmission came into use much later than analog. This is due to the fact that such a signal is much easier to maintain, and the equipment at that time was not so improved.
Every person constantly encounters the concept of "discreteness". If we translate this word from Latin, then it will mean "discontinuity". Going deep into science, we can say that a discrete signal is a method of transmitting information, which implies a change in the carrier medium over time. The latter takes on any of all possible values. Now discretion is fading into the background, after the decision was made to produce systems on a chip. They are holistic, and all components interact closely with each other. In discreteness, everything is exactly the opposite - each detail is completed and connected to others through special communication lines.
Signal
A signal is a special code that is transmitted into space by one or more systems. This wording is general.
In the field of information and communication, a signal is a special medium of any data, which is used to transmit messages. It can be created, but not accepted, the last condition is optional. If the signal is a message, then catching it is considered necessary.
The described code is given by a mathematical function. It characterizes all possible parameter changes. In radio engineering theory, this model is considered basic. In it, the analogue of the signal was called noise. It is a function of time that freely interacts with the transmitted code and distorts it.
The article describes the types of signals: discrete, analog and digital. Also briefly given the basic theory on the topic described.
Signal types
There are several signals available. Let's consider what types are.
- According to the physical medium of the data carrier, an electrical signal, optical, acoustic and electromagnetic, is separated. There are several more species, but they are little known.
- According to the method of assignment, signals are divided into regular and irregular. The first are deterministic methods of data transfer, which are specified by an analytical function. Random ones are formulated due to the theory of probability, and they also take any values \u200b\u200bat different intervals.
- Depending on the functions that describe all the parameters of the signal, data transmission methods can be analog, discrete, digital (a method that is level-quantized). They are used to power many electrical appliances.
The reader is now familiar with all types of signal transmission. It will not be difficult for any person to understand them, the main thing is to think a little and remember the school physics course.
What is the signal processed for?
The signal is processed in order to transmit and receive information that is encrypted in it. Once it is extracted, it can be used in a variety of ways. In some situations, it is reformatted.
There is another reason for processing all signals. It consists in a slight compression of frequencies (so as not to damage the information). After that, it is formatted and transmitted at slow speeds.
In analog and digital signals, special techniques are used. In particular, filtering, convolution, correlation. They are necessary to restore the signal if it is damaged or has noise.
Creation and formation
Often, analog-to-digital (ADC) is needed to generate signals, and most often they are both used only in a situation with the use of DSP technologies. In other cases, only the use of a DAC is suitable.
When creating physical analog codes with the further use of digital methods, they rely on the information received, which is transmitted from special devices.
Dynamic range
It is calculated as the difference between the higher and lower loudness levels, which are expressed in decibels. It completely depends on the piece and performance characteristics. We are talking about both music tracks and ordinary dialogues between people. If we take, for example, an announcer who reads the news, then his dynamic range fluctuates around 25-30 dB. And while reading a piece, it can grow up to 50 dB.
Analog signal
An analog signal is a time-continuous way of transmitting data. Its disadvantage is the presence of noise, which sometimes leads to a complete loss of information. Very often situations arise that it is impossible to determine where the important data is in the code, and where the usual distortions are.
It is because of this that digital signal processing has become very popular and is gradually replacing analog.
Digital signal
The digital signal is special, it is described through discrete functions. Its amplitude can take on a certain value from those already set. While the analog signal is capable of arriving with a huge amount of noise, then the digital one filters out most of the received noise.
In addition, this type of data transfer transfers information without unnecessary semantic load. Several codes can be sent through one physical channel at once.
Types of digital signal do not exist, since it stands out as a separate and independent method of data transmission. It is a binary stream. In our time, this signal is considered the most popular. This is due to ease of use.
Digital signal application
What makes a digital electrical signal different from others? The fact that he is capable of performing complete regeneration in the repeater. When a signal that has the slightest interference enters the communication equipment, it immediately changes its form to digital. This allows, for example, the TV tower to form a signal again, but without the noise effect.
In the event that the code arrives with large distortions, then, unfortunately, it cannot be restored. If we take analog communication in comparison, then in a similar situation the repeater can extract part of the data, spending a lot of energy.
When discussing cellular communications of different formats, it is almost impossible to talk on a digital line with strong distortion, since words or whole phrases are not heard. In this case, analog communication is more effective, because you can continue to conduct a dialogue.
It is because of such problems that the digital signal is generated by repeaters very often in order to reduce the break in the communication line.
Discrete signal
Now every person uses a mobile phone or some kind of "dialer" on his computer. One of the tasks of instruments or software is to transmit a signal, in this case a voice stream. To carry a continuous wave, a channel is required that has a higher-level capacity. That is why the decision was made to use a discrete signal. He does not create the wave itself, but its digital form. Why then? Because the transmission comes from technology (for example, a phone or a computer). What are the advantages of this type of information transfer? With its help, the total amount of transmitted data is reduced, and batch sending is also easier to organize.
The concept of "discretization" has long been consistently used in computer technology. Thanks to such a signal, not continuous information is transmitted, which is completely encoded with special characters and letters, but data collected in special blocks. They are separate and complete particles. This encoding method has long been relegated to the background, but has not completely disappeared. With it, you can easily transfer small pieces of information.
Comparison of digital and analog signals
When buying equipment, hardly anyone thinks about what types of signals are used in this or that device, and even more so about their environment and nature. But sometimes you still have to deal with concepts.
It has long been clear that analog technologies are losing demand, because their use is irrational. Instead, digital communication comes. You need to understand what is at stake and what humanity is rejecting.
In short, an analog signal is a way of transmitting information, which implies the description of data by continuous functions of time. In fact, speaking specifically, the amplitude of the oscillations can be equal to any value within certain limits.
Digital signal processing is described by discrete functions of time. In other words, the vibration amplitude of this method is equal to strictly specified values.
Moving from theory to practice, it must be said that the analog signal is characterized by interference. With digital, there are no such problems, because it successfully "smooths out" them. Due to new technologies, this method of data transmission is able to restore all the original information on its own without the intervention of a scientist.
Talking about television, we can already say with confidence: analog transmission has long outlived its usefulness. Most consumers are switching to a digital signal. The disadvantage of the latter is that if any device is capable of receiving analog transmission, then a more modern method is only a special technique. Although the demand for the outdated method has long fallen, these types of signals are still not able to completely disappear from everyday life.
Digital circuitry is the most important discipline that is studied in all higher and secondary educational institutions that train specialists in electronics. A real radio amateur should also be well versed in this matter. But most of the books and tutorials are written in a language that is very difficult to understand, and it will be difficult for a novice electronics engineer (possibly a schoolchild) to master new information. A series of new training materials from Master Kit is designed to fill this gap: in our articles, complex concepts are told in the simplest words.
8.1. Analog and digital signals
First you need to figure out how analog circuitry generally differs from digital. And the main difference is in the signals with which these circuits work.
All signals can be divided into two main types: analog and digital.
Analog signals
Analog signals are most familiar to us. We can say that the entire natural world around us is analog. Our sight and hearing, as well as all other sense organs, perceive the incoming information in an analog form, that is, continuously in time. Transmission of sound information - human speech, sounds of musical instruments, roar of animals, sounds of nature, etc. - also carried out in analog form.
To understand this issue even better, let's draw an analog signal (Fig. 1.):
Fig. 1. Analog signal
We see that the analog signal is continuous in time and in amplitude. For any moment in time, you can determine the exact value of the amplitude of the analog signal.
Digital signals
Let's analyze the signal amplitude not constantly, but discretely, at fixed intervals. For example, once a second, or more often: ten times a second. How often we do this is called the sampling rate: once per second - 1 Hz, a thousand times per second - 1000 Hz or 1 kHz.
For clarity, let's draw graphs of the analog (top) and digital (bottom) signals (Fig. 2):
Fig. 2. Analog signal (top) and digital copy (bottom)
We see that in every instantaneous period of time it is possible to find out the instantaneous digital value of the signal amplitude. What happens to the signal (according to what law it changes, what is its amplitude) between the "check" intervals, we do not know, this information is lost to us. The less often we check the signal level (the lower the sampling rate), the less information we have about the signal. Of course, the opposite is also true: the higher the sampling rate, the better the quality of the signal representation. In the limit, increasing the sampling rate to infinity, we get practically the same analog signal.
Does this mean that the analog signal is better than the digital one anyway? In theory, perhaps yes. But in practice, modern analog-to-digital converters (ADCs) operate at such a high sampling rate (up to several million samples per second), they describe an analog signal in digital form so qualitatively that the human senses (eyes, ears) can no longer feel the difference between original signal and its digital model. A digital signal has a very significant advantage: it is easier to transmit it via wires or radio waves, interference does not significantly affect such a signal. Therefore, all modern mobile communications, television and radio broadcasting are digital.
The lower graph in Fig. 2 can be easily represented in another form - as a long sequence of a pair of numbers: time / amplitude. And numbers are exactly what digital circuits need. True, digital circuits prefer to work with numbers in a special representation, but we will talk about this in the next lesson.
We can now draw important conclusions:
The digital signal is discrete, it can be determined only for certain points in time;
- the higher the sampling rate, the better the accuracy of the digital signal representation.
The difference between analog and digital communication.
When dealing with radio communication, very often you have to come across terms such as "Analog signal" and "Digital signal"... For specialists in these words there is no mystery, but for people who are ignorant, the difference between "digital" and "analogue" may be completely unknown. And yet there is a very significant difference.
So. Radio communication is always the transmission of information (voice, SMS, telesignalization) between two subscribers, a signal source, a transmitter (radio station, repeater, base station) and a receiver.
When we talk about a signal, we usually mean electromagnetic oscillations, which induce an EMF and cause fluctuations in the current in the receiver antenna. Further, the receiving device - converts the received vibrations back into an audio frequency signal and outputs to the speaker.
In any case, the transmitter signal can be represented in both digital and analog form. After all, for example, sound itself is an analog signal. At the radio station, the sound received by the microphone is converted into the already mentioned electromagnetic vibrations. The higher the sound frequency, the higher the frequency of oscillations at the output, and the louder the speaker speaks, the greater the amplitude. 
The resulting electromagnetic vibrations, or waves, propagate through space using a transmitting antenna. So that the air is not clogged with low-frequency interference, and so that different radio stations have the opportunity to work in parallel without interfering with each other, the vibrations resulting from the effect of sound are summed up, that is, "superimposed" on other vibrations that have a constant frequency. The last frequency is usually called "carrier", and it is to its perception that we tune our radio receiver in order to "catch" the analog signal of the radio station.
In the receiver, the opposite process takes place: the carrier frequency is separated, and the electromagnetic oscillations received by the antenna are converted into sound oscillations, and the information that the person who transmitted the message wanted to convey is heard from the speaker.
In the process of transmitting an audio signal from a radio station to a receiver, third-party interference may occur, the frequency and amplitude may change, which, of course, will affect the sounds emitted by the radio receiver. Finally, both the transmitter and the receiver themselves introduce some error during signal conversion. Therefore, the sound reproduced by an analog radio receiver always has some distortion. The voice can be perfectly reproduced despite the changes, but the background will be hiss or even some wheezing caused by interference. The less confident the reception, the louder and more distinct these extraneous noise effects will be. 
In addition, the terrestrial analog signal has a very weak degree of protection against unauthorized access. For public radio stations, this, of course, does not matter. But during the use of the first mobile phones, there was one unpleasant moment associated with the fact that almost any outside radio receiver could easily be tuned to the desired wavelength to eavesdrop on your telephone conversation.
To protect against this, the so-called "toning" of the signal or, in other words, the CTCSS (Continuous Tone-Coded Squelch System) system is used, a continuous tone coded noise reduction system or a "friend / foe" signal identification system, designed to separate users operating in the same frequency range, into groups. Users (correspondents) from the same group can hear each other thanks to the identification code. Explaining clearly, the principle of operation of this system is as follows. Together with the transmitted information, an additional signal (or in another tone) is also sent on the air. The receiver, in addition to the carrier, recognizes this tone with the appropriate setting and receives the signal. If the tone is not tuned in the radio-receiver, then the signal is not received. There are quite a few encryption standards that differ for different manufacturers.
Analog broadcasting has such disadvantages. Because of them, for example, television promises to become fully digital in a relatively short time. 
Digital communications and broadcasting are considered more immune to interference and external influences. The thing is that when using "digital" the analog signal from the microphone at the transmitting station is encrypted into a digital code. No, of course, the flow of numbers and numbers does not spread into the surrounding space. Just the sound of a certain frequency and volume is assigned a code from radio pulses. The duration and frequency of the pulses are predefined - it is the same for both the transmitter and the receiver. The presence of an impulse corresponds to one, the absence - to zero. Therefore, this connection is called "digital".
A device that converts an analog signal into a digital code is called analog-to-digital converter (ADC)... And a device installed in the receiver and converting the code into an analog signal corresponding to the voice of your friend in the speaker of a GSM cell phone, called a digital-to-analog converter (DAC).
During digital signal transmission, errors and distortion are virtually eliminated. If the impulse becomes a little stronger, longer, or vice versa, then it will still be recognized by the system as a unit. And zero will remain zero, even if some random weak signal appears in its place. For ADC and DAC there are no other values \u200b\u200blike 0.2 or 0.9 - just zero and one. Therefore, interference to digital communications and broadcasting has little effect.
Moreover, the "digit" is also more protected from unauthorized access. Indeed, in order for the DAC of the device to be able to decrypt the signal, it is necessary that it "knows" the decryption code. The ADC together with the signal can transmit the digital address of the device selected as the receiver. Thus, even if the radio signal is intercepted, it cannot be recognized due to the absence of at least a part of the code. This is especially true for communication.
So, differences between digital and analog signals:
1) An analog signal can be distorted by interference, and a digital signal can either be clogged with interference at all, or arrive without distortion. The digital signal is either exactly there or completely absent (either zero or one).
2) The analog signal is available for perception by all devices operating on the same principle as the transmitter. The digital signal is reliably protected by a code, it is difficult to intercept it if it is not intended for you. 
In addition to purely analog and purely digital stations, there are also radio stations that support both analog and digital modes. They are designed to transition from analog to digital communications.
So, having at your disposal a fleet of analog radio stations, you can gradually switch to a digital communication standard.
For example, initially you built a communication system at the Baikal 30 Radio Stations.
Let me remind you that this is an analog station with 16 channels. 
But time passes, and the station ceases to suit you as a user. Yes, it is reliable, but powerful, and with a good battery up to 2600 mAh. But when the park of radio stations is expanded by more than 100 people, and especially when working in groups, its 16 channels begin to be lacking.
You don't have to immediately run out and buy digital radio stations. Most manufacturers intentionally introduce a model with an analog transmission mode.
That is, you can gradually switch to, for example, Baikal-501 or Vertex-EVX531 while maintaining the existing communication system in working order.
The advantages of such a transition are undeniable.
You get the station working
1) longer (there is less consumption in digital mode.)
2) More functions (group call, lone worker)
3) 32 memory channels.
That is, you actually create initially 2 channel bases. For new purchased stations (digital channels) and a base of assistance channels with existing stations (analogue channels). Gradually, as you purchase equipment, you will reduce the fleet of radio stations of the second bank and increase the first.
Ultimately, you will achieve your goal - to transfer your entire base to a digital communication standard.
Yaesu Fusion DR-1 digital repeater can serve as a good addition and extension to any base. 
It is a dual band (144 / 430MHz) repeater that supports analog FM communication as well as digital protocol at the same time System Fusion
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One of the key features System Fusion
is the function AMS (automatic mode selection)which instantly recognizes whether a signal is received in V / D mode, voice mode or FR data mode by analog FM or digital C4FM, and automatically switches to the corresponding one. Thus, thanks to our digital transceivers FT1DR and FTM-400DRSystem Fusion
To keep in touch with analog FM radio stations, you no longer need to manually switch modes each time.
On repeater DR-1X, AMS can be configured to convert an incoming digital C4FM signal to analog FM and retransmit it, thus allowing communication between digital and analog transceivers. AMS it can also be configured to automatically relay the incoming mode to the output, allowing digital and analog users to share the same repeater.
Until now, FM repeaters have only been used for traditional FM communications, and digital repeaters have been used for digital only. However, now simply replacing the conventional analog FM repeater with DR-1X, you can continue to use normal FM communications, and also use a repeater for more advanced digital radio communications System Fusion
... Other peripheral devices such as duplexer and amplifier, etc. can continue to be used as usual.
More detailed characteristics of the equipment can be seen on the website in the products section
Each station has one frequency on which an analogue television signal is broadcast. This can result in static, snow, or halo on the channel. It can also cause fluctuations in color, brightness and sound quality. And, like radio signals, analog transmission is reduced and further away from the source.
In a digital code, you can encode almost any type of transmitted electrical signal (including analog), and it does not matter if it is a picture, video signal, audio signal, or text information, and these types of signals can be transmitted practically simultaneously (in a single digital stream).
A digital signal, by its electrical properties (as in the example with a tone signal), has a higher information transmission capacity than an analog signal. Also, a digital signal can be transmitted over a greater distance than an analog one, and without reducing the quality of the transmitted signal.
This means you enjoy consistently clear images, high quality sound, and static or snowy. Digital transmission requires less bandwidth than analogous analog signal. This allows you to experience quality programming at home. The image value is 4 units of width for every 3 units of height.
Unfortunately, television receivers (televisions) designed to receive analog television will no longer be able to receive a digital terrestrial signal. But in any case, this does not mean that you have to go to the store and buy a new TV capable of receiving digital TV.
In order for you to be able to receive digital broadcasting on a TV that supports only an analogue broadcasting signal, you just need to purchase a so-called digital television broadcasting receiver (or, in other words, a digital broadcasting receiver).
Digital terrestrial receiver (receiver), connected to the TV through the antenna jack or through a low-frequency audio-video cable. In this case, the terrestrial antenna is no longer connected to the antenna jack of the TV, but to the jack of the digital receiver itself. The general diagram of such a connection is shown in Fig. 1.
The general principle of this technique will be as follows:
The digital terrestrial radio signal will be received by the terrestrial antenna, from the antenna this signal will come to the digital receiver, and from the receiver the analog signal will go to your TV. Here, the TV will already be used as a monitor, and switching between TV channels will take place from the remote control of the digital terrestrial receiver (receiver).
Here I think it should be mentioned, and about the reception of sound radio stations.
To receive a digital signal from broadcasting stations, old-style radio receivers (supporting analogue broadcasting) will also no longer work, and you will need a special radio receiver that supports digital radio signal reception.
Benefits of digital terrestrial TV:
* As mentioned earlier, the main and most important advantage of digital terrestrial TV is of course mobility. You can watch your favorite programs not only at home, but also while on the road. Also, possibly in the future, digital terrestrial TV can be viewed on a mobile phone.
* Digital terrestrial TV is the ability to receive images and sound in very good quality.
* According to its electrical properties, or rather, electromagnetic properties, a digital signal can be transmitted over a greater distance than an analogue one, and without reducing the quality of the transmitted signal.
Here, it should also be taken into account that the digital radio signal is more resistant to the electromagnetic interference that surrounds us (interference can come both from nearby electrical and radio devices, and from power lines passing nearby).
* In digital format, significantly more TV channels can be transmitted, while the picture and sound quality will be much better than with analog signal transmission.
* The undoubted advantage of digital broadcasting is, of course, ease of setup, whereas, for example, installing and configuring satellite television requires certain knowledge and skills.
I think this is certainly not the whole list of advantages of digital broadcasting over analogue, but, as they say, wait and see.
Digital television is rapidly gaining popularity in our country, but many people still do not know how it is fundamentally different from the good old analog TV.
Description of analog and digital television
It is easy to guess that analog and digital television are based on analog and digital signals, respectively. Analog continuously, which means that in the event of any outside influence, it becomes vulnerable, which leads to a worse picture and sound quality. The undoubted advantage of an analog signal is the ability to receive it using a simple terrestrial antenna. You can also use the services of a cable TV provider. We can say that the analog signal is already outdated today, since it is significantly inferior to the digital signal in a number of important parameters - quality, safety, etc.
Modern televisions are designed primarily to work with a digital signal, although they also have an analog connector. It's just that the analog signal is not able to reveal the full potential of modern plasma and LCD TVs, only a digital signal can give the best picture quality. It, in contrast to analog, comes in compact "portions" that are separated by pauses, and therefore it is very difficult to influence such a signal. Even when transmitting a digital signal over a very long distance, the picture and sound quality remains at the highest level. Among other things, a digital signal allows you to transmit much more channels than an analog one, therefore, subscribers who connect to digital television receive more than a hundred TV channels of a wide variety of topics.
Comparison of analog and digital television
Alas, analog television today actually has no clear advantages over digital broadcasting, except perhaps the ability to "catch" a signal using a conventional antenna. However, digital television can also be mobile using a digital signal receiver. Considering that regardless of the distance, the digital signal remains tamperproof and protected from interference and guarantees a high level of quality, the advantages of digital television become quite obvious.
TheDifference.ru has determined that the difference between analog and digital television is as follows:
Digital television provides a higher level of signal quality and protection. The analog signal was and remains vulnerable to external influences and cannot provide such a high-quality image.
Digital television is more mobile - today you can receive a digital signal while on the road or far from home.
Analogue television is not capable of providing as many channels as digital television. Due to the peculiarities of the digital signal, when connecting to digital TV, a subscriber can access several hundred different TV channels.
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A signal is defined as a voltage or current that can be transmitted as a message or information. By their nature, all signals are analog, be it DC or AC, digital or pulsed. However, it is common to distinguish between analog and digital signals.
A digital signal is a signal that is processed in a certain way and converted into numbers. Usually these digital signals are associated with real analog signals, but sometimes there is no connection between them. An example is the transmission of data over local area networks (LANs) or other high-speed networks.
In digital signal processing (DSP), an analog signal is converted to binary form by a device called an analog-to-digital converter (ADC). The ADC outputs a binary representation of the analog signal, which is then processed by an arithmetic digital signal processor (DSP). After processing, the information contained in the signal can be converted back to analog form using a digital-to-analog converter (DAC).
Another key concept in signal definition is the fact that a signal always carries some information. This leads us to the key problem of physical analog signal processing - the problem of information retrieval.
Signal processing objectives.
The main purpose of signal processing is the need to obtain the information they contain. This information is usually present in the amplitude of a signal (absolute or relative), in frequency or in spectral composition, in phase or in the relative time dependences of several signals.
Once the desired information has been extracted from the signal, it can be used in a variety of ways. In some cases, it is desirable to reformat the information contained in the signal.
In particular, the signal format change occurs when the audio signal is transmitted in a frequency division multiple access (FDMA) telephone system. In this case, analog techniques are used to place multiple voice channels in the frequency spectrum for transmission through microwave radio relay, coaxial or fiber optic cable.
In the case of digital communications, analog audio information is first converted to digital using an ADC. Digital information representing individual audio channels is time multiplexed (time division multiplexing, TDMA) and transmitted over a serial digital link (as in a PCM system).
Another reason for signal processing is to compress the signal bandwidth (without significant loss of information), followed by formatting and transmission of information at lower rates, which allows you to narrow the required channel bandwidth. High speed modems and adaptive pulse code modulation (ADPCM) systems widely use data redundancy (compression) algorithms, as do digital mobile communication systems, MPEG audio recording systems, and high definition television (HDTV).
Industrial data acquisition and control systems use information from sensors to generate appropriate feedback signals, which in turn directly control the process. Note that these systems require both ADCs and DACs as well as sensors, signal conditioners, and DSPs (or microcontrollers).
In some cases, there is noise in the signal containing information, and the main goal is to recover the signal. Techniques such as filtering, autocorrelation, convolution, etc. are often used to accomplish this task in both the analog and digital domains.
PURPOSES OF SIGNAL PROCESSING- Retrieving signal information (amplitude, phase, frequency, spectral components, timing)
- Signal format conversion (telephony with channel division FDMA, TDMA, CDMA)
- Data compression (modems, cell phones, HDTV television, MPEG compression)
- Feedback signal generation (industrial process control)
- Separation of signal from noise (filtering, autocorrelation, convolution)
- Isolation and storage of a signal in digital form for further processing (FFT)
Signal generation
In most of the above situations (involving the use of DSP technologies), both an ADC and a DAC are needed. However, in some cases, only a DAC is required where analog signals can be directly generated from the DSP and DAC. Video scanned displays are a good example in which a digitally generated signal drives the video image or a RAMDAC (Digital to Analogue Pixel Array Converter) block.
Another example is artificially synthesized music and speech. In fact, the generation of physical analog signals using only digital techniques relies on information previously obtained from sources of such physical analog signals. In display systems, the data on the display must convey relevant information to the operator. When developing sound systems, the statistical properties of the generated sounds are set, which were previously determined using the widespread use of DSP methods (sound source, microphone, preamplifier, ADC, etc.).
Signal processing methods and technologies
Signals can be processed using analog techniques (analog signal processing, or ASP), digital techniques (digital signal processing, or DSP), or a combination of analog and digital techniques (combined signal processing, or MSP). In some cases the choice of methods is clear, in other cases there is no clarity in the choice and the final decision is based on certain considerations.
With regard to DSP, the main difference from traditional computer data analysis is the high speed and efficiency of performing complex digital processing functions such as filtering, analysis using data and data compression in real time.
Combined signal processing means that the system performs both analog and digital processing. Such a system can be implemented as a printed circuit board, hybrid integrated circuit (IC), or a single chip with integrated elements. ADCs and DACs are considered as combined signal processing devices, since each of them implements both analog and digital functions.
Recent advances in VLSI technology enable complex (digital and analog) processing on a single chip. The very nature of DSP implies that these functions can be performed in real time.
Comparison of analog and digital signal processing
Today's engineer is faced with choosing the right combination of analog and digital methods to solve a signal processing problem. It is impossible to process physical analog signals using only digital methods, since all sensors (microphones, thermocouples, piezoelectric crystals, heads of magnetic drives, etc.) are analog devices.
Some types of signals require normalization circuits for further signal processing, both analog and digital. Signal normalization circuits are analog processors that perform functions such as amplification, accumulation (in measuring and preliminary (buffer) amplifiers), signal detection against a background of noise (high-precision common-mode amplifiers, equalizers and linear receivers), dynamic range compression (logarithmic amplifiers, logarithmic DACs and programmable gain amplifiers) and filtering (passive or active).
Several methods for implementing the signal processing are shown in Figure 1. The top area of \u200b\u200bthe figure shows a purely analog approach. The rest of the areas show the DSP implementation. Note that once DSP technology is selected, the next solution should be to locate the ADC in the signal processing path.
ANALOGUE AND DIGITAL SIGNAL PROCESSING
Figure 1. Signal processing methods
In general, since the ADC has been moved closer to the sensor, most of the analog signal processing is now done by the ADC. The increase in ADC capabilities can be expressed in increasing the sampling rate, expanding the dynamic range, increasing the resolution, cutting off input noise, using input filtering and programmable amplifiers (PGA), the presence of on-chip voltage references, etc. All mentioned additions increase the functional level and simplify the system.
With modern DAC and ADC manufacturing technologies available with high sampling rates and resolutions, significant progress has been made in integrating an increasing number of circuits directly into the ADC / DAC.
In the measurement field, for example, there are 24-bit ADCs with built-in programmable amplifiers (PGAs) that allow full-scale 10 mV bridge signals to be digitized directly without further normalization (eg AD773x series).
At voice and audio frequencies, complex encoder-decoding devices - codecs (Analog Front End, AFE) - are common, which have an analog circuit built into the microcircuit that meets the minimum requirements for external normalization components (AD1819B and AD73322).
There are also video codecs (AFEs) for tasks such as CCD image processing and others (such as the AD9814, AD9816, and AD984X series).
Implementation example
As an example of DSP use, compare analog and digital low pass filters (LPFs), each with a cutoff frequency of 1 kHz.
The digital filter is implemented as a typical digital system, as shown in Figure 2. Notice that there are several implicit assumptions made in the diagram. First, in order to accurately process the signal, it is assumed that the ADC / DAC path has sufficient sampling rate, resolution and dynamic range. Second, in order to complete all its computations within the sampling interval (1 / f s), the DSP device must be fast enough. Thirdly, at the ADC input and DAC output there is still a need for analog filters for limiting and restoring the signal spectrum (anti-aliasing filter and anti-imaging filter), although the requirements for their performance are small. With these assumptions, you can compare digital and analog filters.
Figure 2. Block diagram of a digital filter
The required cutoff frequency for both filters is 1 kHz. The analog conversion is of the first kind of the sixth order (characterized by the presence of ripple in the transmission coefficient in the passband and the absence of ripple outside the passband). Its characteristics are shown in Figure 2. In practice, this filter can be represented by three second-order filters, each of which is built on an operational amplifier and several and capacitors. The sixth-order filter is simple enough to create with advanced CAD systems for filters, but accurate component selection is required to meet the 0.5 dB flatness specification.
The 129-factor digital FIR filter shown in Figure 2 has a flatness of only 0.002 dB in passband, a linear phase response, and a much steeper roll-off. In practice, such characteristics cannot be realized using analog methods. Another obvious advantage of the circuit is that the digital filter does not require selection of components and is not subject to parameter drift, since the filter clock frequency is stabilized by a crystal resonator. A filter with 129 coefficients requires 129 multiply-and-accumulate (MAC) operations to compute the output sample. These calculations must be completed within a 1 / fs sampling interval to ensure real-time performance. In this example, the sampling rate is 10 kHz, so 100 µs is sufficient for processing if there is no significant additional computation required. The ADSP-21xx DSP family can complete the entire multiply-accumulate process (and other functions required to implement the filter) in one instruction cycle. Therefore, a 129-factor filter requires over 129/100 μs \u003d 1.3 million operations per second (MIPS). Existing DSPs are much faster and are therefore not a limiting factor for these applications. The 16-bit fixed-point ADSP-218x series reaches 75MIPS performance. Listing 1 shows the assembly code that implements the filter on the ADSP-21xx DSP processor family. Note that the actual lines of the executable code are marked with arrows; the rest is comments.
Figure 3.Analog and digital filters
Of course, in practice, there are many other factors considered when comparing analog versus digital filters or analog versus digital signal processing techniques in general. Modern signal processing systems combine analog and digital methods to achieve the desired function and take advantage of the best methods, both analog and digital.
ASSEMBLY PROGRAM:
FIR FILTER FOR ADSP-21XX (SINGLE PRECISION)
MODULE fir_sub; (FIR filter subroutine Call parameters of subroutine I0 -\u003e Oldest data in delay line I4 -\u003e Start of filter coefficient table L0 \u003d Filter length (N) L4 \u003d Filter length (N) M1, M5 \u003d 1 CNTR \u003d Filter length - 1 (N-1) Returned values \u200b\u200bMR1 \u200b\u200b\u003d Summation result (rounded and clipped) I0 -\u003e Oldest data in delay line I4 -\u003e Start of filter coefficient table Variable registers MX0, MY0, MR Runtime (N - 1) + 6 cycles \u003d N + 5 cycles All odds are written in 1.15 format) .ENTRY fir; fir: MR \u003d 0, MX0 \u003d DM (I0, M1), MY0 \u003d PM (I4, M5) CNTR \u003d N-1; DO convolution UNTIL CE; convolution: MR \u003d MR + MX0 * MY0 (SS), MX0 \u003d DM (I0, M1), MY0 \u003d PM (I4, M5); MR \u003d MR + MX0 * MY0 (RND); IF MV SAT MR; RTS; .ENDMOD; REAL-TIME SIGNAL PROCESSING
Literature:
Together with the article "Types of signals" read:
