Types of signals: analog, digital, discrete. What is digital TV and television, how to choose the right one

Signals are information codes that people use to convey messages to information system. The signal can be given, but it is not necessary to receive it. 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. No noise useful information and distorts the signal by interacting with it.

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

Signal types

Signals by physical environment Information carriers 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 using 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. In addition, digital ones 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 has analog view. 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. Wherein amplified signal may itself cause interference to 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

The digital signal is represented by the sequence 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 in the digital signal for each temperature value there will be a a certain amount of voltage pulses. 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 value to the maximum is done sharply. 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 reconstruct a digital signal with a very high noise level, whereas from analog type if there is a large distortion, it is still possible to “fish out” the information. 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 outputs it exact copy and can be used as desired 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 mobile communications to "digit".

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's happening in mobile phones that transmit information via digital channels

The sound entering the microphone is exposed 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 about bandwidth channels, 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 base station and then reaches another subscriber, who 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.

Very often we hear such definitions as “digital” or “discrete” signal; what is its difference from “analog”?

The essence of the difference is that the analog signal is continuous in time (blue line), while the digital signal consists of a limited set of coordinates (red dots). If we reduce everything to coordinates, then any segment analog signal consists of an infinite number of coordinates.

U digital signal coordinates along the horizontal axis are located at regular intervals, in accordance with the sampling frequency. In the common Audio-CD format this is 44100 points per second. The vertical accuracy of the coordinate height corresponds to the bit depth of the digital signal; for 8 bits it is 256 levels, for 16 bits = 65536 and for 24 bits = 16777216 levels. The higher the bit depth (number of levels), the closer the vertical coordinates are to the original wave.

Analogue sources are: vinyl and audio cassettes. Digital sources are: CD-Audio, DVD-Audio, SA-CD (DSD) and files in WAVE and DSD formats (including derivatives of APE, Flac, Mp3, Ogg, etc.).

Advantages and disadvantages of analog signal

The advantage of an analog signal is that it is in analog form that we perceive sound with our ears. And although our auditory system translates the perceived sound stream into digital view and transmits it in this form to the brain; science and technology have not yet reached the point of being able to connect players and other sound sources directly in this form. Similar studies are currently being actively conducted for people with disabilities, and we enjoy exclusively analog sound.

The disadvantage of an analog signal is the ability to store, transmit and replicate the signal. When recording to magnetic tape or vinyl, the quality of the signal will depend on the properties of the tape or vinyl. Over time, the tape demagnetizes and the quality of the recorded signal deteriorates. Each read gradually destroys the media, and rewriting introduces additional distortions, where additional deviations are added by the next media (tape or vinyl), reading, writing and signal transmission devices.

Making a copy of an analog signal is the same as copying a photograph by taking a photograph of it again.

Advantages and disadvantages of digital signal

The advantages of a digital signal include accuracy when copying and transmitting an audio stream, where the original is no different from the copy.

The main disadvantage is that the digital signal is an intermediate stage and the accuracy of the final analog signal will depend on how detailed and accurately the sound wave is described by coordinates. It is quite logical that the more points there are and the more accurate the coordinates are, the more accurate the wave will be. But there is still no consensus on what number of coordinates and the accuracy of the data is sufficient to say that the digital representation of the signal is sufficient to accurately restore the analog signal, indistinguishable from the original by our ears.

In terms of data volumes, the capacity of a regular analog audio cassette is only about 700-1.1 MB, while a regular CD holds 700 MB. This gives an idea of ​​the need for carriers large capacity. And this gives rise to a separate war of compromises with different requirements by the number of describing points and by the accuracy of the coordinates.

Today, it is considered quite sufficient to represent a sound wave with a sampling frequency of 44.1 kHz and a bit depth of 16 bits. At a sampling rate of 44.1 kHz, it is possible to reconstruct a signal up to 22 kHz. As psychoacoustic studies show, a further increase in the sampling frequency is not noticeable, but an increase in the bit depth gives a subjective improvement.

How DACs build a wave

A DAC is a digital-to-analog converter, an element that converts digital sound into analog. We will look superficially at the basic principles. If the comments indicate an interest in considering a number of points in more detail, a separate material will be released.

Multibit DACs

Very often, a wave is represented as steps, which is due to the architecture of the first generation of multi-bit R-2R DACs, which operate similarly to a relay switch.

The DAC input receives the value of the next vertical coordinate and at each clock cycle it switches the current (voltage) level to the appropriate level until the next change.

Although it is believed that the human ear can hear no higher than 20 kHz, and according to Nyquist theory it is possible to restore the signal to 22 kHz, the quality of this signal after restoration remains a question. In the high-frequency region, the resulting “stepped” waveform is usually far from the original one. The easiest way out of the situation is to increase the sampling rate when recording, but this leads to a significant and undesirable increase in file size.

An alternative is to artificially increase the sampling rate when playing back into the DAC by adding intermediate values. Those. we imagine a continuous wave path (gray dotted line) smoothly connecting the original coordinates (red dots) and add intermediate points on this line (dark purple).

When increasing the sampling frequency, it is usually necessary to increase the bit depth so that the coordinates are closer to the approximated wave.

Thanks to intermediate coordinates, it is possible to reduce the “steps” and build a wave closer to the original.

When you see a boost function from 44.1 to 192 kHz in a player or external DAC, it is a function of adding intermediate coordinates, not restoring or creating sound in the region above 20 kHz.

Initially, these were separate SRC chips before the DAC, which then migrated directly to the DAC chips themselves. Today you can find solutions where such a chip is added to modern DACs; this is done in order to provide an alternative to the built-in algorithms in the DAC and sometimes get even more best sound(as for example it is done in Hidizs AP100).

The main industry rejection of multibit DACs occurred due to the inability to further technological development quality indicators with current production technologies and more high cost against “pulse” DACs with comparable characteristics. However, in Hi-End products, preference is often given to old multi-bit DACs rather than new solutions with technically better characteristics.

Switching DACs

In the late 70s it became widespread Alternative option DACs based on “pulse” architecture – “delta-sigma”. Pulse DAC technology has become possible emergence ultra-fast switches and allowed the use of high carrier frequencies.

The signal amplitude is the average value of the pulse amplitudes (pulses of equal amplitude are shown in green, and the resulting sound wave is shown in white).

For example, a sequence of eight cycles of five pulses will give an average amplitude (1+1+1+0+0+1+1+0)/8=0.625. The higher the carrier frequency, the more pulses are subject to smoothing and the result is more exact value amplitudes. This made it possible to present the audio stream in one-bit form with a wide dynamic range.

Averaging can be done with a regular analog filter, and if such a set of pulses is applied directly to the speaker, then at the output we will get sound, and ultra high frequencies will not be reproduced due to the high inertia of the emitter. PWM amplifiers work on this principle in class D, where the energy density of pulses is created not by their number, but by the duration of each pulse (which is easier to implement, but cannot be described with a simple binary code).

A multibit DAC can be thought of as a printer capable of applying color using Pantone inks. Delta Sigma is jet printer with a limited set of colors, but due to the ability to apply very small dots (compared to an antler printer), due to the different density of dots per unit surface it produces more shades.

In an image, we usually do not see individual dots due to the low resolution of the eye, but only the average tone. Likewise, the ear does not hear impulses individually.

Ultimately, with current technologies in pulsed DACs, it is possible to obtain a wave close to what should theoretically be obtained when approximating intermediate coordinates.

It should be noted that after the advent of the delta-sigma DAC, the relevance of drawing a “digital wave” in steps disappeared, because This is how modern DACs do not build a wave in steps. It is correct to construct a discrete signal with dots connected by a smooth line.

Are switching DACs ideal?

But in practice, not everything is rosy, and there are a number of problems and limitations.

Because Since the overwhelming number of records are stored in a multi-bit signal, conversion to a pulse signal using the “bit to bit” principle requires an unnecessarily high carrier frequency, which modern DACs do not support.

The main function of modern pulse DACs is to convert a multi-bit signal into a single-bit signal with a relatively low carrier frequency with data decimation. Basically, it is these algorithms that determine the final sound quality of pulse DACs.

To reduce the problem of high carrier frequency, the audio stream is divided into several one-bit streams, where each stream is responsible for its bit group, which is equivalent to a multiple of the carrier frequency of the number of streams. Such DACs are called multibit delta-sigma.

Today, pulse DACs have received a second wind in high-speed microcircuits general purpose in products from NAD and Chord due to the ability to flexibly program conversion algorithms.

DSD format

After the widespread use of delta-sigma DACs, it was quite logical for the recording format to appear binary code directly delta-sigma encoding. This format is called DSD (Direct Stream Digital).

The format was not widely used for several reasons. Editing files in this format turned out to be unnecessarily limited: you cannot mix streams, adjust volume, or apply equalization. This means that without loss of quality, you can only archive analog recordings and produce two-microphone recording of live performances without further processing. In a word, you can’t really make money.

In the fight against piracy, SA-CD format discs were not (and are still not) supported by computers, which makes it impossible to make copies of them. No copies – no wide audience. It was possible to play DSD audio content only from a separate SA-CD player from a proprietary disc. If for the PCM format there is an SPDIF standard for digital data transfer from a source to a separate DAC, then for the DSD format there is no standard and the first pirated copies of SA-CD discs were digitized with analog outputs SA-CD players (although the situation seems stupid, in fact some recordings were released only on SA-CD, or the same recording on Audio-CD was deliberately made poorly to promote SA-CD).

The turning point came with the release game consoles SONY, where the SA-CD disc was automatically copied to HDD consoles. Fans of the DSD format took advantage of this. The appearance of pirated recordings stimulated the market to release separate DACs for playing DSD streams. Most external DACs with DSD support today support USB data transfer using the DoP format as a separate encoding of the digital signal via SPDIF.

Carrier frequencies for DSD are relatively small, 2.8 and 5.6 MHz, but this audio stream does not require any conversion with data thinning and is quite competitive with formats high resolution, such as DVD-Audio.

There is no clear answer to the question of which is better, DSP or PCM. It all depends on the quality of implementation of a particular DAC and the talent of the sound engineer when recording final file.

General conclusion

Analog sound is what we hear and perceive as the world around us with our eyes. Digital audio, this is a set of coordinates that describe a sound wave, and which we cannot hear directly without conversion to an analog signal.

An analog signal recorded directly onto an audio cassette or vinyl cannot be re-recorded without loss of quality, while a wave in digital representation can be copied bit for bit.

Digital recording formats are a constant trade-off between the amount of coordinate accuracy versus file size, and any digital signal is only an approximation of the original analog signal. However, at the same time different level digital signal recording and playback technologies and media storage technologies for analog signal provide greater benefits to the digital representation of the signal, similarly digital camera versus a film camera.

More focused on working with digital broadcasting. It is considered more progressive and advanced. However, to understand what its advantage is, you need to study the terminology more deeply.

Analog TV

Analog television is built on the basis of an analog signal. It runs continuously, which is not an indicator of quality. After all, if there is a signal, the whole picture and sound suffers. Among the advantages of an analog signal is the fact that it is easily picked up by ordinary broadcast. But despite its obvious advantages, the analog signal today is considered obsolete and is rarely used anywhere. The disadvantages of analog TV are poor-quality signal, lack of secure connection etc.

Televisions operating with an analog signal can be found in the provinces. Digital TV is not profitable for small towns. And people get used to it and don’t want to change their traditions.

In addition, an analog signal cannot reveal the full potential of modern video equipment: plasma and liquid crystal televisions.

Digital television

Cable TV

Cable TV its name defines only the method of transmitting information, but not the signal. So, for example, a digital or analog signal prepared for transmission can be transmitted in one transmission medium or another: through a copper cable, ether, etc.

Therefore, you should not allocate cable TV to separate species, because it can be both digital and analog.

Before you choose the best TV option for you, be sure to weigh all your options. Evaluate the equipment you have in your home. After all, if you have a tube one, the digital signal will be problematic, but modern LCD TVs have analog connectors. Also take an interest in what opportunities your television providers offer you.

The difference between digital and analog television is most easily demonstrated by the differences between digital and analog sound or photographs. If there is an analog television picture and soundtrack encoded using analog electrical signal, then in digital, accordingly, digital coding is used.

At the end of the 90s in our country there was only analog color television. The broadcast used the French SECAM coding system. Older readers probably remember that on video cassettes one could find films encoded using alternative systems– PAL or NTSC, for comfortable viewing of which you needed a VCR with their support.

To watch digital TV on a computer, all you need is a small USB module and a UHF antenna

The main disadvantages of the analogue are poor protection against interference, as well as a fairly wide band of the radio frequency spectrum required for transmitting one channel.

Therefore, on air we were limited to a maximum of two dozen color channels, and on cable networks an average of 70 (with rare exceptions).

If your TV is not equipped with a receiver digital channels, then you can buy a separate decoder that supports DVB-T standard 2

With an analog signal it is quite difficult to create a service convenient for the user and operator (with the possibility quick connection/ disabling channel packages, etc.). In addition, analog needs high-power transmitters with a large coverage area because Receivers need high level signal, which means frequency spectrum It is used very irrationally in radio broadcasts: in neighboring territories it is impossible to transmit on the same channel; competent frequency planning is required.

A digital signal does not have these disadvantages. The main advantage of digital is that the encoded channel can be compressed using modern algorithms (for example MPEG). Exactly how to encode a signal and how to compress it is determined by the standard. Today in Europe and Russia the main family of standards is DVB - the brainchild of the international consortium DVB Project.

The family includes standards for satellite, terrestrial, cable and mobile television, differing in the degree of compression, noise immunity and other parameters (important, depending on the transmission medium used). However, in the press “digital” in Lately most often called the terrestrial standard (in the case of Russia - DVB-T2). Let's start with it.

From analog to digital on air: Russian version

Given the advantages of digital, the world community began the transition to modern broadcasting standards in the early 2000s. In all countries, this process went on (and continues to go on) simultaneously with the “optimization” of radio broadcasting, which is actively used not only by television people, but also mobile operators, military and other consumers.

Due to compression into 1 analog TV channel, for example, in the DVB-T2 standard, allows you to fit up to 10 digital channels with approximately the same picture quality. In addition, part of the spectrum frees up the aforementioned reduction in transmitter power. Within one country, these processes are regulated by the state, and on a continental scale - by various interstate agreements.

According to one such agreement, Russian border areas must eventually stop broadcasting in analogue format. So the transition from analogue to digital television determines not only the desire for new technologies, but also responsibility to our closest neighbors.

Inexpensive USB adapters for receiving digital channels can be found not only for PCs...
...but also for smartphones and tablets. At the same time, they will be connected via micro-USB

Transition from analogue to digital television began airing in our country in 2009. At that time, the DVB-T standard, which had already been implemented in a number of European countries, was taken as the basis.

You need to understand that television is a chain of interaction between a whole list of intermediaries between the content producer and its consumer, each of whom has a fleet of analog equipment that needs to be replaced. The state project involves updating only part of this chain - distribution and transmission equipment.

In some cases, the state helps television studios with the purchase of new filming equipment.

But viewers have to think about replacing the “receivers” themselves. All these difficulties prevent us from switching to new standard, wherever such transformations are undertaken.

And in our country the transition was even more difficult. At first, a very high pace was taken, but after a few years, “the horses were changed midstream”, saving time on the next evolutionary step: it was decided to introduce a more advanced second generation of the “terrestrial” standard - DVB-T2, which provides placement more digital channels on the frequency band of an analog channel (compared to DVB-T).

It should be noted that the transition does not imply any increase in the resolution of the broadcast image. The project only involves changing the way it is presented, and we should expect HD quality on air only in the distant future (the standard supports HD, but at the state level it was decided not to touch this topic for now).

Today, DVB-T2 transmitters operate almost throughout the country. Somewhere, only 1 multiplex is currently included (a package that takes the place of one analog channel); in other areas the second one was already included. This means that, having the appropriate receiving equipment, from the air you can watch 20 channels for free in decent quality.

Although from the very beginning of the transition it was said that by 2015 our country should completely switch to digital and turn off analogue, for now the issue of turning off was postponed, therefore analog television continues to function.

Photo: manufacturing companies

Any signal, whether analog or digital, is electromagnetic vibrations, propagating with a certain frequency. Depending on what signal is being distributed, the device receiving this signal determines what image to display on the screen, respectively, with sound.

For example, a television tower or radio station can transmit both analog and digital signals. Sound is transmitted in analog form, and through the receiving device is converted into electromagnetic vibrations. As already mentioned, vibrations propagate with a certain frequency. The higher the frequency of the sound, the higher the vibrations, resulting in a louder sounding voice.

In general terms, an analog signal propagates continuously, and a digital signal propagates discretely (intermittently), i.e. the amplitude of oscillations takes on certain values ​​per unit time.

If we continue the example of an analog audio signal, we get a process in which electromagnetic waves are propagated using a transmitter (antenna). Because The analog signal propagates constantly, then the oscillations are summed up, and a carrier frequency appears at the output, which is the fundamental one, i.e. The receiver is tuned to it.

In the receiver itself, this frequency is separated from other vibrations, which are converted into sound.

The disadvantages of transmitting information using an analog signal are obvious:

• Arises a large number of interference;
• More unnecessary information is transmitted;
• Signal Transmission Security

If in radio broadcasting the transmission of information using an analog signal occurs less noticeably, then in television, the issue of switching to digital transmission is extremely important.

The main advantages of a digital signal over an analog signal are:

• Higher level of protection. The security of digital signal transmission is based on the fact that the “digit” is transmitted in encrypted form;
• Ease of signal reception. A digital signal can be received at any distance from your place of residence;
• Digital broadcasting can provide a huge number of channels. It is this opportunity that provides fans of digital television big amount TV channels for watching films and programs;
• The transmission quality is several orders of magnitude higher than with analogue broadcasting. The digital signal provides filtering of received data, and it is also possible to restore the original information.

Accordingly, special devices are used to convert an analog signal into a digital one, and vice versa.

• A device that converts an analog signal to a digital signal is called analog-to-digital converter(ADC);
• A device that converts a digital signal into an analogue one is called digital-to-analog converter(DAC).

Accordingly, the ADC is installed in the transmitter, and the DAC is installed in the receiver and converts the discrete signal into an analogue signal corresponding to the voice.

Why is a digital signal more secure?

The digital signal is transmitted in encrypted form and the digital-analog device must have a code for decryption. The ADC can transmit and digital address receiver Even if the signal is intercepted, it will be impossible to completely decipher it due to the absence of part of the code. This property digital transmission is widely used in mobile communications.

So the main difference between analog and digital signal is different structure transmitted signal. Analog signals are a continuous stream of oscillations with varying amplitude and frequency.

A digital signal is discrete (intermittent) oscillations, the values ​​of which depend on the transmitting medium.

Sometimes consumers have questions about how the signal is transmitted on television.

In television, before transmitting a signal in digital form, the analog signal must be digitized. After this, you need to choose the medium in which the transmission will take place: copper cable, airwaves, fiber optic cable.

For example, many users are sure that cable television is only digital transmission data. This is wrong. Cable television is both an analogue and digital type of signal transmission.