Analog and digital signals - differences, advantages and disadvantages. Digital and analog signal: what are the similarities and differences, advantages and disadvantages? Wireless communication channels

Usually we don't care how the telephone line works (but not when we have to shout at the top of our lungs into the phone: “Please repeat, I can’t hear anything!”).

Telephone companies provide a variety of different services to customers. It is not so easy to understand the price lists of these services - what is actually offered, and how much you should pay for which service. We won't talk about pricing in this article, but we will try to find out what the differences are between the most commonly offered telephone products and services.

ANALOG LINES, DIGITAL LINES

Firstly, lines can be analog and digital. The analog signal changes continuously; it always has a specific value, representing, for example, the volume and pitch of the voice being transmitted, or the color and brightness of a certain area of ​​the image. Digital signals only have discrete values. As a rule, the signal is either on or off, either it is present or it is not. In other words, its value is either 1 or 0.

Analog telephone lines have been used in telephony since time immemorial. Even fifty-year-old telephones can most likely be connected to a local loop - the line between the home telephone socket and the central telephone exchange. (A telephone central office is not a shiny skyscraper in the center of a city; the average subscriber loop is no more than 2.5 miles (four kilometers) long, so a "telephone central office" is usually located in some nondescript building nearby.)

During a telephone conversation, the microphone built into the handset converts speech into an analog signal, which is transmitted to the central telephone exchange, from where it goes either to another subscriber loop or to other switching devices if the called number is outside the coverage area of ​​this exchange. When dialing a number, the telephone generates in-band signals transmitted over the same main channel, indicating who the call is intended for.

Over the course of their existence, telephone companies have accumulated extensive experience in voice transmission. It has been established that the frequency range from 300 to 3100 Hz is generally sufficient to perform this task. Let us recall that hi-fi audio systems are capable of reproducing sound without distortion in the frequency range of 20-20000 Hz, which means that the telephone range is usually only enough for the subscriber to recognize the caller by voice (for other applications this range is likely to be too much narrow - for transmitting music, for example, telephone communication is completely unsuitable). Telephone companies provide a smooth decline in the amplitude-frequency response at high and low frequencies using an analog telephone channel of 4000 Hz.

The central telephone exchange, as a rule, digitizes the signal intended for further transmission over the telephone network. With the exception of Gilbeth County (Arkansas) and Rat Fork (Wyoming), all American telephone networks transmit signals between central stations digitally. Although many companies use digital private branch exchanges and data transmission facilities, and all ISDN facilities are based on digital encoding, subscriber loops are still the “last bastion” of analog communications. This is explained by the fact that most telephones in private homes do not have means of signal digitization and cannot work with lines with a bandwidth of over 4000 Hz.

WHAT IS 4000 Hz ENOUGH FOR?

A modem is a device that converts computer digital signals into analog signals with frequencies within the bandwidth of a telephone line. The maximum throughput of a link is directly related to the bandwidth. More precisely, the amount of throughput (in bits/sec) is determined by the bandwidth and the signal-to-noise ratio tolerance. Currently, the maximum throughput of modems - 33.6 Kbps - is already close to this limit. Users of 28.8 Kbps modems are well aware that noisy analog lines rarely provide their full throughput, which is often much lower. Compression, caching and other tricks help to somewhat improve the situation, and yet we are more likely to live to see the invention of a perpetual motion machine than to see the advent of modems with a bandwidth of 50 or at least 40 Kbps on ordinary analog lines.

Telephone companies solve the opposite problem - digitize the analog signal. To transmit the resulting digital signal, channels with a bandwidth of 64 Kbit/s are used (this is the world standard). This channel, called DS0 (digital signal, level zero), is the basic building block from which all other telephone lines are built. For example, you can combine (the correct term is condense) 24 DS0 channels into a DS1 channel. By renting a T-1 line, the user actually receives a DS1 channel. When calculating the total throughput of DS1, we must remember that after every 192 data bits (that is, 8000 times per second), one synchronization bit is transmitted: a total of 1.544 Mbps (64000 times 24 plus 8000).

LEased lines, switched lines

In addition to the T-1 line, the client can rent leased lines or use regular switching lines. By renting a T-1 circuit or a low-speed data line, such as a dataphone digital service (DDS), from a telephone company, the subscriber is effectively renting a direct connection and, as a result, becomes the sole user of a 1.544 Mbps circuit (T-1 ) or 56 Kbit/s (low-speed line).

Although frame relay technology involves switching individual frames, the corresponding services are offered to the user in the form of virtual communication channels between fixed endpoints. From a network architecture point of view, frame relay should be considered more of a dedicated line than a dial-up line; It is also important that the price of such a service for the same bandwidth is significantly lower.

Switching services (an example of this would be the service of a regular residential telephone) are services purchased from the telephone company. Upon request, the subscriber is provided with a connection via a network of public switches to any node of the telephone network. Unlike the situation with leased lines, in this case the fee is charged for the connection time or the actual volume of traffic and depends largely on the frequency and volume of network use. Digital communications services can be provided based on X.25, Switched 56, ISDN Basic Rate Interface (BRI), ISDN Primary Rate Interface (PRI), Switched Multimegabit Data Service (SMDS) and ATM protocols. Some organizations, such as universities, railroads, or municipalities, create private networks using their own switches and leased lines, and sometimes even their own lines.

If the line you receive from the telephone company is digital, then the exchange of data between the telephone network and the terminal equipment (which is the telephone company term for equipment such as computers, fax machines, video phones, and digital telephones) does not require digital-to-analog conversion. and therefore, there is no need for a modem. However, in this case, the use of the telephone network imposes certain requirements on the subscriber. In particular, you should ensure that the local loop is terminated correctly, that traffic is carried correctly, and that the telephone company's diagnostics are supported.

The line supporting the ISDN BRI protocol must be connected to a device called NT1 (network termination 1). In addition to terminating the line and supporting diagnostic procedures, the NT1 device coordinates a two-wire subscriber loop with a four-wire digital terminal equipment system. When using T-1 or DDS leased digital lines or digital communications services, a channel service unit (CSU) should be used as line load. The CSU works as a terminator, ensures correct line load and processes diagnostic commands. The client's terminal equipment interacts with a data service unit (DSU), which converts digital signals to a standard form and transmits them to the CSU. Structurally, CSU and DSU are often combined into one module called CSU/DSU. The DSU can be built into a router or multiplexer. Thus, in this case (although modems are not needed here), the installation of certain interface devices will be required.

MEDIA FOR TELEPHONE COMMUNICATIONS

Most analog subscriber loops can provide a throughput of 33.6 Kbps only under very favorable conditions. On the other hand, the same twisted pair cable connecting the office to the central office could easily be used to run ISDN BRI, giving 128 Kbps data throughput and another 16 Kbps for management and configuration. What's the matter? The signal transmitted over analog telephone lines is filtered to suppress all frequencies above 4 kHz. When using digital lines, such filtering is not required, so the bandwidth of the twisted pair is significantly wider, and consequently, the throughput increases.

Leased lines with a capacity of 56 and 64 Kbps are two-wire or four-wire digital lines (in the latter case, one pair is used for transmission and the other for reception). The same lines are suitable as a carrier for providing digital communication services, for example, frame relay or Switched 56. Four-wire lines or even optical cables are often used as a carrier for T-1, as well as ISDN PRI and frame relay. T-3 lines are sometimes coaxial cable, but more often they are based on optical cable.

Although ISDN continues to receive the most attention as a means of high-speed signal transmission over long distances, newer means of last-mile (ie, local loop) communications have recently emerged. PairGain and AT&T Paradyne offer products based on Bellcore's high-bit-rate digital subscriber loop (HDSL) technology. These products allow you to equalize the capabilities of all existing subscriber loops; By installing HDSL devices at both ends of the line, you can get DS1 throughput (1.544 Mbit/s) on almost all existing subscriber loops. (HDSL up to 3.7 km long can be used on subscriber loops without repeaters in the case of standard 24-gauge wires. For regular T-1 lines to work, repeaters must be installed every kilometer and a half). An alternative to HDSL in achieving last-mile DS1 throughput is to either use optical cable (which is quite expensive) or install multiple repeaters on each line (not as expensive as fiber optic technology, but still not cheap). In addition, in this case, the costs of the telephone company, and therefore the client, to maintain the line in working condition increase significantly.

But even HDSL is not the last word in technology in the field of increasing capacity in the last mile. HDSL's successor, asymmetrical digital subscriber line (ASDL) technology, is expected to provide 6 Mbps throughput in one direction; the throughput of the other is significantly lower - something around 64 Kbps. Ideally, or at least in the absence of a monopoly - assuming that the cost of the service to the customer is approximately equal to its cost to the telephone company - a large share of customers could use ISDN PRI (or other T-1 based services) at a cost , comparable to the current ISDN BRI price.

Today, however, ISDN proponents likely have little to worry about; In most cases, telephone companies will choose to increase line capacity and pocket all the profits without reducing the cost of service to the customer. It is not at all obvious that tariffs for services should be based on common sense.

Table 1. Types of telephone services

Digital and analog signal: comparative characteristics

Is a walkie-talkie a popular means of communication?

• Instantly exchange messages
• Talk to several people at the same time
• Durable in use and work in any conditions

Analogue walkie-talkies: advantages and disadvantages

• The sound is transmitted unencoded, which is very popular with most users
• A huge range of different models and a selection of all kinds of accessories
• Ease of operation and user understanding of frequency usage

• You can only have one conversation at a time on one channel
• The need for a transmitter and receiver specifically tuned to the same frequency
• Inability to use programs designed for business

Digital walkie-talkies: advantages and disadvantages

• Excellent noise reduction
• Excellent sound quality at any distance
• Possibility to conduct several conversations simultaneously on one channel
• Possibility of sending short messages
• High channel density
• Signals are received by standard antennas
• Digital processing reduces background noise
• Availability of software
• The digital platform allows you to use both analog and digital radios simultaneously
• You can track the movement of interlocutors on the same network

• High price
• Long training for use
• RF noise interferes with the digital signal, an error may occur

From all of the above, we can conclude that digital radio stations differ from analogue ones by having higher operational and functional characteristics. The main advantage of digital devices is higher signal stability in the presence of interference. That's why they become popular.

Any signal, analog or digital, is an electromagnetic oscillation that propagates at a certain frequency, depending on what signal is transmitted, the device receiving this signal translates it into text, graphic or audio information that is convenient for perception by the user or the device itself. For example, a television or radio signal, a tower or radio station can transmit both an analog and, at the moment, a digital signal. The receiving device, receiving this signal, converts it into image or sound, supplementing it with text information (modern radio receivers).

Sound is transmitted in analog form and, through the receiving device, is converted into electromagnetic vibrations, and as already mentioned, vibrations propagate at a certain frequency. The higher the frequency of the sound, the higher the vibrations will be, which means the output sound will be louder. In general terms, an analog signal propagates continuously, while a digital signal propagates intermittently (discretely).

Since the analog signal propagates constantly, the oscillations are summed up and a carrier frequency appears at the output, which in this case is the main one and the receiver is tuned to it. In the receiver itself, this frequency is separated from other vibrations, which are already converted into sound. The obvious disadvantages of transmission using an analog signal include a large amount of interference, low security of the transmitted signal, as well as a large amount of transmitted information, some of which is superfluous.

If we talk about a digital signal, where data is transmitted discretely, it is worth highlighting its obvious advantages:

• high level of protection of transmitted information due to its encryption;
• ease of digital signal reception;
• absence of extraneous “noise”;
• digital broadcasting can provide a huge number of channels;
• high quality of transmission - the digital signal provides filtering of received data;

To convert an analog signal to a digital signal and vice versa, special devices are used - an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). The ADC is installed in the transmitter, the DAC is installed in the receiver and converts the discrete signal to analog.

Regarding security, why is a digital signal more secure than an analog signal? The digital signal is transmitted in encrypted form and the device that receives the signal must have a code to decrypt the signal. It is also worth noting that the ADC can also transmit the digital address of the receiver; if the signal is intercepted, it will be impossible to completely decrypt it, since part of the code is missing - this approach is widely used in mobile communications.

To summarize, the main difference between an analog and digital signal is the structure of the transmitted signal. Analog signals are a continuous stream of oscillations with varying amplitude and frequency. A digital signal consists of discrete oscillations, the values ​​of which depend on the transmitting medium.

Signals are information codes that people use to convey messages in an information system. The signal can be given, but it is not required to be received. Whereas a message can only be considered a signal (or a set of signals) that was received and decoded by the recipient (analog and digital signal).

One of the first methods of transmitting information without the participation of people or other living beings were signal fires. When danger arose, fires were lit sequentially from one post to another. Next, we will consider the method of transmitting information using electromagnetic signals and will dwell in detail on the topic analog and digital signal.

Any signal can be represented as a function that describes changes in its characteristics. This representation is convenient for studying radio engineering devices and systems. In addition to the signal in radio engineering, there is also noise, which is its alternative. Noise does not carry useful information and distorts the signal by interacting with it.

The concept itself makes it possible to abstract from specific physical quantities when considering phenomena related to the encoding and decoding of information. The mathematical model of the signal in research allows one to rely on the parameters of the time function.

Signal types

Signals based on the physical environment of the information carrier are divided into electrical, optical, acoustic and electromagnetic.

According to the setting method, the signal can be regular or irregular. A regular signal is represented as a deterministic function of time. An irregular signal in radio engineering is represented by a chaotic function of time and is analyzed by a probabilistic approach.

Signals, depending on the function that describes their parameters, can be analog or discrete. A discrete signal that has been quantized is called a digital signal.

Signal Processing

Analog and digital signals are processed and directed to transmit and receive information encoded in the signal. Once information is extracted, it can be used for various purposes. In special cases, information is formatted.

Analog signals are amplified, filtered, modulated, and demodulated. Digital data can also be subject to compression, detection, etc.

Analog signal

Our senses perceive all information entering them in analog form. For example, if we see a car passing by, we see its movement continuously. If our brain could receive information about its position once every 10 seconds, people would constantly get run over. But we can estimate distance much faster and this distance is clearly defined at each moment of time.

Absolutely the same thing happens with other information, we can evaluate the volume at any moment, feel the pressure our fingers exert on objects, etc. In other words, almost all information that can arise in nature is analogue. The easiest way to transmit such information is with analog signals, which are continuous and defined at any time.

To understand what an analog electrical signal looks like, you can imagine a graph that shows amplitude on the vertical axis and time on the horizontal axis. If we, for example, measure the change in temperature, then a continuous line will appear on the graph, displaying its value at each moment in time. To transmit such a signal using electric current, we need to compare the temperature value with the voltage value. So, for example, 35.342 degrees Celsius can be encoded as a voltage of 3.5342 V.

Analog signals used to be used in all types of communications. To avoid interference, such a signal must be amplified. The higher the noise level, that is, interference, the more the signal must be amplified so that it can be received without distortion. This method of signal processing spends a lot of energy generating heat. In this case, the amplified signal may itself cause interference for other communication channels.

Nowadays, analog signals are still used in television and radio, to convert the input signal in microphones. But in general, this type of signal is being replaced or replaced by digital signals everywhere.

Digital signal

A digital signal is represented by a sequence of digital values. The most commonly used signals today are binary digital signals, as they are used in binary electronics and are easier to encode.

Unlike the previous signal type, a digital signal has two values ​​“1” and “0”. If we remember our example with temperature measurement, then the signal will be generated differently. If the voltage supplied by the analog signal corresponds to the value of the measured temperature, then a certain number of voltage pulses will be supplied in the digital signal for each temperature value. The voltage pulse itself will be equal to “1”, and the absence of voltage will be “0”. The receiving equipment will decode the pulses and restore the original data.

Having imagined what a digital signal will look like on a graph, we will see that the transition from zero to maximum is abrupt. It is this feature that allows the receiving equipment to “see” the signal more clearly. If any interference occurs, it is easier for the receiver to decode the signal than with analog transmission.

However, it is impossible to restore a digital signal with a very high noise level, while it is still possible to “extract” information from an analog type with large distortion. This is due to the cliff effect. The essence of the effect is that digital signals can be transmitted over certain distances, and then simply stop. This effect occurs everywhere and is solved by simply regenerating the signal. Where the signal breaks, you need to insert a repeater or reduce the length of the communication line. The repeater does not amplify the signal, but recognizes its original form and produces an exact copy of it and can be used in any way in the circuit. Such signal repetition methods are actively used in network technologies.

Among other things, analog and digital signals also differ in the ability to encode and encrypt information. This is one of the reasons for the transition of mobile communications to digital.

Analog and digital signal and digital-to-analog conversion

We need to talk a little more about how analog information is transmitted over digital communication channels. Let's use examples again. As already mentioned, sound is an analog signal.

What happens in mobile phones that transmit information via digital channels

Sound entering the microphone undergoes analog-to-digital conversion (ADC). This process consists of 3 steps. Individual signal values ​​are taken at equal intervals of time, a process called sampling. According to Kotelnikov’s theorem on channel capacity, the frequency of taking these values ​​should be twice as high as the highest signal frequency. That is, if our channel has a frequency limit of 4 kHz, then the sampling frequency will be 8 kHz. Next, all selected signal values ​​are rounded or, in other words, quantized. The more levels created, the higher the accuracy of the reconstructed signal at the receiver. All values ​​are then converted into binary code, which is transmitted to the base station and then reaches the other party, which is the receiver. A digital-to-analog conversion (DAC) procedure takes place in the receiver's phone. This is a reverse procedure, the purpose of which is to obtain a signal at the output that is as identical as possible to the original one. Then the analog signal comes out in the form of sound from the phone speaker.

A person talks on the phone every day, watches various TV channels, listens to music, and surfs the Internet. All communications and other information environments are based on the transmission of signals of various types. Many people ask questions about how analog information differs from other types of data, what a digital signal is. The answer to them can be obtained by understanding the definition of various electrical signals and studying their fundamental differences between each other.

Analog signal

An analog signal (continuous) is a natural information signal that has a certain number of parameters that are described by a time function and a continuous set of all possible values.

Human senses capture all information from the environment in analog form. For example, if a person sees a truck passing nearby, then its movement is observed and changes continuously. If the brain received information about the movement of vehicles once every 15 seconds, then people would always fall under its wheels. A person evaluates distance instantly, and at each moment in time it is defined and different.

The same thing happens with other information - people hear sound and evaluate its volume, evaluate the quality of the video signal, and the like. Accordingly, all types of data are analog in nature and are constantly changing.

On a note. Analog and digital signals are involved in transmitting the speech of interlocutors who communicate by telephone; the Internet operates on the basis of the exchange of these signal channels over a network cable. These types of signals are electrical in nature.

An analog signal is described by a mathematical time function similar to a sine wave. If you take measurements, for example, of water temperature, periodically heating and cooling it, then the graph of the function will display a continuous line that reflects its value in each time period.

To avoid interference, such signals must be amplified using special means and devices. If the level of signal interference is high, then it needs to be amplified more. This process is accompanied by large expenditures of energy. An amplified radio signal, for example, can often itself become an interference for other communication channels.

Interesting to know. Analog signals were previously used in all types of communications. However, now it is being replaced everywhere or has already been replaced (mobile communications and the Internet) by more advanced digital signals.

Analog and digital television still coexist, but the digital type of television and radio broadcasting is rapidly replacing the analog method of data transmission due to its significant advantages.

To describe this type of information signal, three main parameters are used:

• frequency;
• wave length;
• amplitude.

Disadvantages of an analog signal

An analog signal has the following properties, which show their difference from the digital version:

1. This type of signal is characterized by redundancy. That is, the analog information in them is not filtered - they carry a lot of unnecessary information data. However, it is possible to pass information through a filter, knowing additional parameters and the nature of the signal, for example, using the frequency method;
2. Safety. He is almost completely helpless against unauthorized intrusions from the outside;
3. Absolute helplessness in the face of various types of interference. If any interference is imposed on the data transmission channel, it will be transmitted unchanged by the signal receiver;
4. There is no specific differentiation of sampling levels - the quality and quantity of transmitted information is not limited in any way.

The above properties are the disadvantages of the analog method of data transmission, on the basis of which we can consider it completely obsolete.

Digital and discrete signals

Digital signals are artificial information signals, presented in the form of regular digital values ​​that describe specific parameters of the transmitted information.

For information. Nowadays, a simple-to-encode bit stream is predominantly used - a binary digital signal. This is the type that can be used in binary electronics.

The difference between the digital type of data transmission and the analog version is that such a signal has a specific number of values. In the case of a bit stream, there are two of them: “0” and “1”.

The transition from zero to maximum in a digital signal is abrupt, allowing the receiving equipment to read it more clearly. If certain noise and interference occurs, it will be easier for the receiver to decode a digital electrical signal than with analog information transmission.

However, digital signals differ from the analog version in one drawback: with a high level of interference, they cannot be restored, but it is possible to extract information from a continuum signal. An example of this would be a telephone conversation between two people, during which entire words and even phrases of one of the interlocutors may disappear.

This effect in the digital environment is called the break effect, which can be localized by reducing the length of the communication line or installing a repeater, which completely copies the original type of signal and transmits it further.

Analog information can be transmitted over digital channels after going through the digitization process with special devices. This process is called analog-to-digital conversion (ADC). This process can also be reversed - digital-to-analog conversion (DAC). An example of a DAC device would be a digital TV receiver.

Digital systems are also distinguished by the ability to encrypt and encode data, which has become an important reason for the digitization of mobile communications and the Internet.

Discrete signal

There is a third type of information – discrete. A signal of this kind is intermittent and changes over time, taking on any of the possible (prescribed in advance) values.

Discrete information transfer is characterized by the fact that changes occur according to three scenarios:

1. The electrical signal changes only in time, remaining continuous (unchanged) in magnitude;
2. It changes only in magnitude, while remaining continuous in time;
3. It can also change simultaneously in both magnitude and time.

Discreteness has found application in batch transmission of large amounts of data in computing systems.

Based on the foregoing, it can be determined that continuity and multiplicity of values ​​are the main differences between analog information and discrete and digital information. Digital data transmission is replacing analog transmission; it is not for nothing that humanity now lives in the digital age.

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