A similar signal. Digital and analog signal: what are the similarities and differences, advantages and disadvantages? Digital and discrete signals

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 teaching aids 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 described in the simplest words.

8.1. Analog and digital signals

First you need to figure out how analog circuitry differs from digital in general. 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 surrounding 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, roars of animals, sounds of nature, etc. - also carried out in analog form.
To understand this question even better, let's draw 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 intervals of "checking", 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 via wires or radio waves, interference does not significantly affect such a signal. Therefore, all modern mobile connection, television and radio broadcasting - 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. Truth, digital circuits prefer to work with numbers in a special way, but we'll talk about that in the next lesson.

Now we can 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.

An analog signal is a data signal in which each of the representing parameters is described by a function of time and a continuous set of possible values.

There are two signal spaces - the space L (continuous signals), and the space l (L is small) - the space of sequences. The space l (L is small) is the space of Fourier coefficients (a countable set of numbers that define a continuous function on a finite interval of the domain of definition), the space L is the space of continuous (analog) signals over the domain of definition. Under certain conditions, the space L is uniquely mapped into the space l (for example, the first two Kotelnikov discretization theorems).

Analog signals are described as continuous functions of time, so an analog signal is sometimes referred to as a continuous signal. Analog signals are opposed to discrete (quantized, digital). Examples of continuous spaces and corresponding physical quantities:

direct: electrical voltage

circumference: the position of the rotor, wheel, gears, analog clock hands, or the phase of the carrier signal

segment: position of the piston, control lever, liquid thermometer or electrical signal, limited in amplitude various multidimensional spaces: color, quadrature modulated signal.

The properties of analog signals are largely the opposite of those of quantized or digital signals.

The absence of clearly distinguishable from each other discrete signal levels leads to the impossibility of using the concept of information to describe it in the form as it is understood in digital technologies. The "amount of information" contained in one sample will be limited only by the dynamic range of the measuring instrument.

No redundancy. From the continuity of the value space, it follows that any interference introduced into the signal is indistinguishable from the signal itself and, therefore, the original amplitude cannot be restored. In reality, filtering is possible, for example, by frequency methods, if any additional information about the properties of this signal is known (in particular, the frequency band).

Application:

Analog signals are often used to represent continuously changing physical quantities. For example, an analog electrical signal taken from a thermocouple carries information about temperature changes, a signal from a microphone - about rapid pressure changes in a sound wave, etc.

2.2 Digital signal

A digital signal is a data signal in which each of the representing parameters is described by a discrete time function and a finite set of possible values.

Signals are discrete electrical or light pulses. With this method, the entire capacity of the communication channel is used to transmit one signal. The digital signal uses the entire bandwidth of the cable. Bandwidth is the difference between the maximum and minimum frequency that can be transmitted over the cable. Each device on such networks sends data in both directions, and some can simultaneously receive and transmit. Baseband systems transmit data as a digital signal of a single frequency.

A discrete digital signal is more difficult to transmit over long distances than an analog signal, therefore it is pre-modulated on the transmitter side, and demodulated on the information receiver side. The use of algorithms for checking and restoring digital information in digital systems can significantly increase the reliability of information transmission.

Comment. It should be borne in mind that a real digital signal is analog by its physical nature. Due to noise and changes in the parameters of transmission lines, it has fluctuations in amplitude, phase / frequency (jitter), polarization. But this analog signal (pulse and discrete) is endowed with the properties of a number. As a result, it becomes possible to use numerical methods for its processing (computer processing).

Digital electronics are now more and more crowding out the traditional analog. Leading companies that produce a wide variety of electronic equipment are increasingly declaring a complete transition to digital technology.

Advances in technology for the production of electronic microcircuits ensured the rapid development of digital technology and devices. The use of digital methods of signal processing and transmission can significantly improve the quality of communication lines. Digital methods of processing and switching signals in telephony allow several times to reduce the weight and size characteristics of switching devices, increase the reliability of communication, and introduce additional functionality.

The emergence of high-speed microprocessors, microcircuits random access memory large volumes, small-sized information storage devices on hard drives of large volumes made it possible to create fairly inexpensive universal personal electronic computers (computers), which have found very wide application in everyday life and in production.

Digital technology is indispensable in remote signaling and telecontrol systems used in automated production, control of remote objects, for example, spaceships, gas pumping stations, etc. Digital technology has also taken a strong place in electrical and radio measuring systems. Modern devices for recording and reproducing signals are also unthinkable without the use of digital devices. Digital devices are widely used to control household appliances.

It is very likely that digital devices will dominate the electronics market in the future.

First, let's give some basic definitions..

Signal Is any physical quantity (for example, temperature, air pressure, light intensity, current strength, etc.) that changes over time. It is thanks to this change in time that the signal can carry some kind of information.

Electrical signal Is an electrical quantity (for example, voltage, current, power) that changes over time. All electronics mainly work with electrical signals, although more and more light signals have been used recently, which represent the intensity of light that changes over time.

Analog signal Is a signal that can take any values ​​within certain limits (for example, the voltage can vary smoothly from zero to ten volts). Devices that only accept analog signals are called analog devices.

Digital signal Is a signal that can only take two values ​​(sometimes three values). Moreover, some deviations from these values ​​are allowed (Fig. 1.1). For example, the voltage can take two values: from 0 to 0.5 V (zero level) or from 2.5 to 5 V (one level). Devices that work exclusively with digital signals are called digital devices.

In nature, almost all signals are analog, that is, they change continuously within certain limits. That is why the first electronic devices were analog. They converted physical quantities into a voltage or current proportional to them, performed some operations on them, and then performed inverse transformations into physical quantities. For example, a human voice (air vibrations) is converted into electrical vibrations using a microphone, then these electrical signals are amplified by an electronic amplifier and, with the help of a speaker system, are again converted into air vibrations, into a louder sound.

Rice. 1.1. Electrical signals: analog (left) and digital (right).

All operations performed by electronic devices on signals can be conditionally divided into three large groups:

Processing (or transformation);

Broadcast;

Storage.

In all these cases, useful signals are distorted by parasitic signals - noise, interference, interference. In addition, when processing signals (for example, when amplifying, filtering), their shape is also distorted due to imperfection, imperfection of electronic devices. And when transmitted over long distances and during storage, the signals are also weakened.

Rice. 1.2. Distortion by noise and interference of an analog signal (left) and a digital signal (right).

In the case of analog signals, all this significantly degrades the useful signal, since all its values ​​are allowed (Fig. 1.2). Therefore, each transformation, each intermediate storage, each transmission over cable or air, degrades the analog signal, sometimes up to its complete destruction. We must also take into account that all the noise, interference and pickup fundamentally cannot be accurately calculated, therefore, it is absolutely impossible to accurately describe the behavior of any analog devices. In addition, over time, the parameters of all analog devices change due to aging of the elements, so the characteristics of these devices do not remain constant.

Unlike analog ones, digital signals, which have only two permitted values, are much better protected from noise, interference and interference. Small deviations from the permitted values ​​do not distort in any way digital signal, since there are always zones of permissible deviations (Fig. 1.2). That is why digital signals allow much more complex and multi-stage processing, much longer lossless storage and much better transmission than analog ones. In addition, the behavior of digital devices can always be accurately calculated and predicted. Digital devices are much less susceptible to aging, since a small change in their parameters does not affect their functioning in any way. In addition, digital devices are easier to design and debug. It is clear that all these advantages provide the rapid development of digital electronics.

However, digital signals also have a major drawback. The fact is that at each of its permitted levels the digital signal must remain at least for some minimum time interval, otherwise it will be impossible to recognize it. And an analog signal can take any of its values ​​for an infinitely small time. It can be said in another way: the analog signal is defined in continuous time (that is, at any moment in time), and the digital signal - in discrete time (that is, only at selected moments in time). Therefore, the maximum achievable speed of analog devices is always fundamentally higher than digital devices. Analog devices can handle faster changing signals than digital ones. The speed of information processing and transmission by an analog device can always be made higher than the speed of its processing and transmission by a digital device.

In addition, the digital signal transmits information only by two levels and by changing one of its levels to another, and the analog signal also transmits information with each current value of its level, that is, it is more capacious in terms of information transfer. Therefore, to transfer the amount of useful information contained in one analog signal, most often it is necessary to use several digital signals (usually from 4 to 16).

In addition, as already noted, in nature all signals are analog-analog, that is, to convert them into digital signals and for reverse conversion, the use of special equipment (analog-to-digital and digital-to-analog converters) is required. So nothing is given for free, and the fees for the benefits of digital devices can sometimes turn out to be unacceptably high.

I talked about digital signals. Why are these digital signals so good? As strange as it may sound, digital signals are analog by nature, since they are transmitted by changing the value of voltage or current, but transmit signals with previously specified levels. At their core, they are discrete signals. What does the word “discrete” mean? Discrete means consisting of separate parts, separate, discontinuous. Digital signals are just discrete signals, since they have only TWO STATES: "Active" and "inactive" - ​​"voltage / current on" and "no voltage / current".

The main advantage of digital signals is that they are easier to transmit and process. For transmission, voltage is most often used. Therefore, two states are accepted: the voltage is close to zero (less than 10% of the voltage value) and the voltage is close to the supply voltage (more than 65% of the value). For example, when the supply voltage of the circuit is 5 Volts, we get a signal with a voltage of 0.5 Volts - "zero", but if 4.1 Volts - "one".

Sequential method of transferring information

There are simply two wires, an electrical signal source and an electrical signal receiver, that cling to those wires.

This is a PHYSICAL LEVEL.

As we said, we can only transmit two signals over these two wires: Voltage / current and no voltage / current. What methods of transferring information can we implement?

The easiest way - there is a signal (the light is on) - this is ONE, there is no signal (the light is off) - this is ZERO

If you use your brains, you can come up with a few more different combinations. For example, take a wide impulse as one, and a narrow one as zero:

Or even take the leading edge and cutoff of the impulse as unity and zero. Below is the picture, if you have forgotten what the front and edge of the pulse are.

And here is the practical implementation:

Yes, you can at least think of various combinations, if “receiver” and “sender” agree on reception and transmission... Here I have given just the most popular digital signal transmission methods. That is, all these methods are PROTOCOLS. And, as I said, you can think of a lot of them.

Data exchange rate

Imagine a picture ... Students, there is a lecture ... The teacher dictates the lecture, and the students write it down

But if the teacher dictates the lecture very quickly and in addition this lecture is in physics or mathematical analysis, then as a result we get:

Why did this happen?

From the point of view of digital data transmission, we can say that the speed of data exchange between the "Sender" and "Recipient" is different. Therefore, there may be a real situation when the "Recipient" (student) is not able to receive data from the "Sender" (teacher) due to a mismatch in the data transfer rate: the transmission rate can be higher or lower than the one to which the receiver (student) is configured ...

This problem in different standards of serial data transmission is solved in different ways:

• preliminary agreement on the speed of data transfer (agree with the teacher to dictate the lecture more slowly or slightly faster);
• before the transfer of information, the "Sender" transmits some service information, using which the "Recipient" adjusts to the "Sender" (Teacher: "Whoever does not write this lecture in full, he will not receive credit")

Most often, the first method is used: the required data exchange rate is set in advance in communication devices. For this, a clock generator is used, which generates pulses to synchronize all nodes of the device, as well as to synchronize the communication process between devices.

Flow control

It is also possible that the “Recipient” (student) is not ready to receive the data transmitted by the “Sender” (teacher) for any reason: busyness, malfunction, etc.

This problem is solved by various methods:

1) At the protocol level... For example, it is stipulated in the exchange protocol: after the “Sender” sends the service signal “start of data transmission” for a certain time, the “Receiver” is obliged to confirm the acceptance of this signal by transmitting a special service signal “ready to receive”. This method called "software flow control" - "Soft"

2) At the physical level- additional communication channels are used, through which the "Sender" BEFORE the transfer of information asks the "Recipient" about his readiness to receive). This method is called "hardware flow control" - "Hard";

Both methods are very common. Sometimes they are used simultaneously: both at the physical level and at the level of the exchange protocol.

When transmitting information, it is important synchronize the operation of the transmitter and receiver... The method of setting the communication mode between devices is called "synchronization". Only in this case the "Recipient" can correctly (reliably) receive the message sent by the "Sender".

Communication modes

Simplex communication.

In this case, the Receiver can only receive signals from the sender and cannot influence him in any way. This is mainly television or radio. We can only watch or listen to them.

Half duplex communication.

In this mode, both the sender and the receiver can transmit signals to each other alternately if the channel is free. A great example of half-duplex communication is a walkie-talkie. If both subscribers chirp each into their walkie-talkie at the same time, then no one will hear anyone.

- First, first. I am the second. How can you hear?

- I hear you okay, hang up!

The signal can only be sent by the sender, in this case the receiver will receive it. Or the signal can be sent by the receiver, in which case the sender receives it. That is, both the sender and the receiver have equal rights to access the channel (communication line). If both of them simultaneously transmit a signal to the line, then, as I said, nothing will come of it.

Duplex communication.

In this mode, both reception and transmission of a signal can be carried out in two directions at once. simultaneously... A vivid example of this is a conversation on a mobile or home phone, or a Skype conversation.

An analog signal is a function of a continuous argument (time). If the graph is periodically interrupted, as happens in a sequence of pulses, for example, they already talk about a certain discreteness of the burst.

The history of the appearance of the term

Computer Engineering

If you read it carefully, nowhere is it written where the definition came from - analog. In the West, the term has been used since the forties by computer professionals. It was during the Second World War that the first computer systems, called digital, appeared. And to distinguish, I had to come up with new epithets.

To the world household appliances the concept of analog entered only in the early 80s, when the first Intel processors, and the world was played with toys on the ZX-Spectrum, an emulator for devices today can be obtained on the Internet. The gameplay required extraordinary perseverance, skill and excellent reaction. Along with the kids, they collected boxes and beat enemy aliens and adults. Modern games are much inferior to the first birds that captured the minds of the players for a while.

Sound recording and telephony

By the early 80s, pop music in electronic processing began to appear. The musical telegraph was presented to the public in 1876, but did not gain recognition. Popular music is liked by the audience in the broadest sense of the word. The telegraph was able to issue a single note, transmit it over a distance, where it was reproduced by a speaker of a special design. Although the Beatles used an electronic organ in the creation of Sergeant Pepper, the synthesizer came into use in the late 70s. A truly popular and digital instrument became already in the mid-80s: let's remember Modern Talking. Previously used synthesizers on analog circuits, starting with Novachord in 1939.

So, an ordinary citizen did not need to distinguish between analog and digital technologies until the latter became firmly in use. The word analog has been in the public domain since the early 1980s. As for the origin of the term, it is traditionally believed that the index was borrowed from telephony, later migrated to sound recording. The analog vibrations are fed directly to the speaker, and the voice is immediately heard. The signal is similar to human speech, becoming an electrical analogue.

If you apply a digital signal to the speaker, an indescribable cacophony of notes of different keys will be heard. This "speech" is familiar to anyone who has loaded programs and games from magnetic tape into computer memory. It doesn’t sound like a human, because it’s digital. As for the discrete signal, in the simplest systems, it is fed directly to a speaker that serves as an integrator. The success or failure of an enterprise depends entirely on the right parameters.

At the same time, the term figured in sound recording, where music and voice went directly from a microphone to tape. Magnetic recording has become an analogue of real artists. Vinyl records are like musicians and are still considered the best medium for any composition. Although they show a limited lifespan. CDs nowadays often contain digital audio that can be decoded by a decoder. According to Wikipedia, a new era began in 1975 (en.wikipedia.org/wiki/History_of_sound_recording).

Electrical measurements

In an analog signal, there is a proportionality between voltage or current and the response on the reproducing device. The term will then be considered derived from the Greek analogos. What does proportional mean. However, the comparison is similar to the above: the signal is similar to the voice reproduced by the speakers.

In addition, in technology, another term is used to denote analog signals - continuous. Which corresponds to the above definition.

general information

Signal energy

As follows from the definition, an analog signal has infinite energy, not limited in time. Therefore, its parameters are averaged. For example, 220 V present in the outlet is called RMS for this reason. Therefore, the effective (averaged over a certain interval) values ​​are used. It is already clear that there is a 50 Hz analog signal in the socket.

When it comes to discreteness, finite values ​​are used. For example, when buying a stun gun, you need to make sure that the impact energy does not exceed a particular value measured in joules. Otherwise, there will be trouble with use or during inspection. Since, starting from a specific energy value, the stun gun is used only by special forces, with an established upper limit. Others are illegal in principle and can be fatal when used.

The pulse energy is found by multiplying the current and voltage by the duration. And this shows the finiteness of the parameter for discrete signals. In technology, there are also digital sequences. It differs from a discrete digital signal by rigidly set parameters:

1. Duration.
2. Amplitude.
3. The presence of two specified states: 0 and 1.
4. Machine bits 0 and 1 are added to pre-agreed and understandable words for the participants (assembly language).

Mutual signal conversion

An additional definition of an analog signal is its apparent randomness, the absence of visible rules, or similarity with some natural processes. For example, a sine wave can describe the rotation of the earth around the sun. This is an analog signal. In circuit and signal theory, a sinusoid is represented by a rotating amplitude vector. And the phase of the current and voltage is different - these are two different vectors, giving rise to reactive processes. What is observed in inductors and capacitors.

It follows from the definition that an analog signal is easily converted into a discrete one. Any switching power supply cuts the input voltage from the outlet into bundles. Therefore, it is engaged in the conversion of an analog signal with a frequency of 50 Hz into discrete ultrasonic bursts. By varying the cutting parameters, the power supply adjusts the output values ​​to the requirements of the electrical load.

The reverse process takes place inside a radio wave receiver with an amplitude detector. After rectifying the signal, pulses of different amplitudes are formed on the diodes. The information is embedded in the envelope of such a signal, the line connecting the tops of the message. The filter is responsible for converting discrete pulses into an analog value. The principle is based on the integration of energy: during the period of voltage presence, the charge of the capacitor increases, then, in the interval between the peaks, the current is formed due to the previously accumulated stock of electrons. The resulting wave is fed to the amplifier low frequencies, later to the speakers, where the result is heard by others.

The digital signal is encoded differently. There, the amplitude of the pulse is embedded in the machine word. It consists of ones and zeros, decoding is required. The operation is handled by electronic devices: graphics adapter, software products... Everyone downloaded K-Lite codecs from the Internet, this is the case. The driver is engaged in decoding the digital signal and converting it for output to the speakers and display.

There is no need to rush into confusion when the adapter is called a 3-D accelerator and vice versa. The first only transforms the supplied signal. For example, there is always an adapter behind the DVI digital input. He is only engaged in converting numbers from ones and zeros to display on the screen matrix. Retrieves information about brightness and RGB pixel values. As for the 3D accelerator, the device in the composition has the right (but is not required) to contain an adapter, but the main task is complex calculations for building three-dimensional images. This technique allows you to unload the central processor and speed up the work of a personal computer.

From analog to digital, the signal is converted to an ADC. This happens in software or inside the microcircuit. Separate systems combine both methods. The procedure begins by taking samples that fit within the specified area. Each, being transformed, becomes a machine word containing a calculated digit. Then the samples are packed with parcels, it becomes possible to send them to other subscribers of a complex system.

The sampling rules are normalized by the Kotelnikov theorem, which shows the maximum sampling frequency. It is forbidden to take the countdown more often, since there is a loss of information. Simplistically, a six-fold excess of the sampling frequency over the upper limit of the signal spectrum is considered sufficient. More headroom is considered an added benefit to ensure good quality... Anyone have seen the indication of the sampling rate of the audio recording. Usually the parameter is higher than 44 kHz. The reason is the peculiarities of human hearing: the upper limit of the spectrum is 10 kHz. Therefore, a sampling rate of 44 kHz is sufficient for mediocre sound reproduction.

The difference between discrete and digital signal

Finally, a person from the outside world usually perceives analog information. If the eye sees a blinking light, peripheral vision captures the surrounding landscape. Consequently, the final effect does not appear to be discrete. Of course, it is possible to try to create a different perception, but this is difficult and will turn out to be entirely artificial. This is the basis for the use of Morse code, which consists of dots and dashes that are easily distinguishable against the background of noise. Discrete strokes of the telegraph key are difficult to confuse with natural signals, even in the presence of strong noise.

Similarly, digital lines have been introduced in the art to eliminate interference. Any video lover tries to get their hands on an encoded copy of a movie at the highest resolution. Digital information can be transmitted over long distances without the slightest distortion. The rules known on both sides for the formation of pre-agreed words become assistants. Sometimes, redundant information is embedded in a digital signal, which makes it possible to correct or notice errors. This eliminates the wrong perception.

Pulse signals

More precisely, discrete signals are set by counts at certain points in time. It is clear that such a sequence is not formed in reality due to the fact that the front and the fall have a finite length. The impulse is not transmitted instantly. Therefore, the spectrum of the sequence is not considered to be discrete. This means that the signal cannot be called that. In practice, two classes are distinguished:

1. Analog impulse signals - the spectrum of which is found by the Fourier transform, therefore, continuous, at least in some areas. The result of the action of voltage or current on a circuit is found in a convolution operation.
2. Discrete pulse signals also show a discrete spectrum, operations with them are carried out through discrete Fourier transforms. Therefore, discrete convolution is also applied.

These clarifications are important for literalists who have read that pulse signals are analog. Discrete ones were named after the features of the spectrum. The term analog is used to differentiate. The epithet continuous is applicable, as already mentioned above, and in connection with the peculiarities of the spectrum.

Clarification: only the spectrum of an infinite sequence of pulses is considered strictly discrete. For a pack, harmonic components are always vague. Such a spectrum resembles a sequence of amplitude modulated pulses.