Signals are characterized by their duration, spectral width and dynamic range. The volume of the signal is used as a generalized characteristic. The duration of the signal determines the time of its existence, the width of the spectrum is the frequency range in which the main energy of the signal is concentrated. The dynamic range characterizes the ratio of the highest instantaneous signal power Pmax to the lowest permissible value of which is determined by the interference power.

An important characteristic of signals is also the base. Signals are called narrowband (simple) if and broadband (complex) if

The elementary signals obtained at the output of the UPS when using the -positional code can be divided into the following groups:

signals that ensure maximum noise immunity with respect to fluctuation noise in deterministic channels. The energy of these signals is most often the same: for a the scalar product for orthogonal signals, for biorthogonal signals, for which the value of m is always even, any of the m signals always corresponds to one opposite signal, and the remaining signals are orthogonal; non-orthogonal signals for which the condition is met

An example of signals that provide maximum noise immunity with a deterministic non-distorting channel and additive white noise are phase modulated signals and bipolar signals direct current. Orthogonal signals include binary frequency modulation (FM) signals if the frequencies of segments of harmonic signals are multiples of the modulation frequency. Biorthogonal signals are used in double phase modulation when Non-orthogonal signals are used in phase modulation when the shifts between the individual signals are, for example, 0°, 120° and 240°.

Many problems of analysis and synthesis of real signals are simplified due to the fact that these signals, usually complex in form, can be represented in the form of simple signals. This is convenient for subsequent analysis of their passage through certain circuits. For example, a certain signal can be represented as a set of orthogonal components (elementary signals):

and in countless ways. Recording (6.1) is called a generalized Fourier series. The interval shows the duration of the signal. Since the system of orthogonal functions used in the decomposition is known in advance, the signal is determined by a set of weighting coefficients for these functions.

Such sets of numbers are called signal spectra. The signal spectrum, presented as a sum of spectral components (6.1), is called discrete.

If a discrete set of basis functions is not enough to represent a signal, and an uncountable set of basis functions differing in the value of a continuously changing parameter p is required, then the signal is represented in the form of an integral

which is called the generalized Fourier integral. The spectrum of such a signal is characterized by a function of a continuous variable (3 and is called continuous.

Considering the passage of each spectrum component through linear circuit with given characteristics, the signal at the output of the circuit is also obtained in the form (6.1) or (6.2) with weighting coefficients or, in the general case, different from or and depending on the characteristics of the circuit under consideration.

In addition to analysis in the theory of PDS, it is necessary to solve problems of signal synthesis. They can be of two types: structural synthesis - determination of the shape of signals that meet specified requirements; parametric synthesis - determination of parameters of signals of a known shape. If in the process of synthesis it is necessary to ensure the extremum of one or another functional (or function), which characterizes the quality of the synthesis, then the synthesis is called optimal.

In practice, signal systems of rectangular and sinusoidal shapes are widely used. Rectangular signals differ from each other in amplitude, duration, number and location of rectangular pulses in a unit interval. Elementary sinusoidal signals are segments of sinusoidal oscillations that differ from each other in amplitude, frequency and phase.

When studying the generalized theory of signals, the following questions are considered.

1. Basic characteristics and methods of analyzing signals used in radio engineering to transmit information.

2. The main types of signal transformations in the process of building channels.

3. Methods for constructing and methods for analyzing radio circuits through which operations are performed on the signal.

Radio engineering signals can be defined as signals that are used in radio engineering. According to their purpose, radio signals are divided into signals:

radio broadcasting,

television,

telegraph,

radar,

radio navigation,

telemetry, etc.

All radio signals are modulated. When generating modulated signals, primary low-frequency signals (analog, discrete, digital) are used.

Analog signal repeats the law of change in the transmitted message.

Discrete signal – the message source transmits information at certain time intervals (for example, about the weather), in addition, a discrete source can be obtained as a result of time sampling of an analog signal.

Digital signal is the display of a message in digital form. Example: we encode a text message in digital signal.

All message characters can be encoded into binary, hexadecimal and other codes. Encoding is carried out automatically using an encoder. Thus, the code symbols are converted into standard signals.

The advantage of digital data transmission is its high noise immunity. The reverse conversion is carried out using a digital-to-analog converter.

Mathematical models of signals

When studying the general properties of signals, one usually abstracts from their physical nature and purpose, replacing them with a mathematical model.

Mathematical model – the selected method of mathematical description of the signal, reflecting the most essential properties of the signal. Based on a mathematical model, it is possible to classify signals in order to determine their common properties and fundamental differences.

Radio signals are usually divided into two classes:

deterministic signals,

random signals.

Deterministic signal is a signal whose value at any time is a known quantity or can be calculated in advance.

Random signal is a signal whose instantaneous value is a random variable (for example, a sound signal).

Mathematical models of deterministic signals

Deterministic signals are divided into two classes:

periodic,

non-periodic.

Let s ( t ) – deterministic signal. Periodic signals are described by a periodic function of time:

and repeat after a period T . Approximately t >> T . The remaining signals are non-periodic.

A pulse is a signal whose value is different from zero for a limited time interval (pulse duration ).

However, when describing a mathematical model, functions defined over an infinite time interval are used. The concept of effective (practical) pulse duration is introduced:

.

Exponential momentum.

For example: defining the effective duration of an exponential pulse as the time interval during which the signal value decreases by a factor of 10. Determine the effective pulse duration for the pattern:

Energy characteristics of the signal . Instantaneous power is the signal power at a resistance of 1 ohm:

.

For a non-periodic signal, we introduce the concept of energy at a resistance of 1 Ohm:

.

For a periodic signal, the concept of average power is introduced:

The dynamic range of a signal is defined as the ratio of the maximum P ( t ) to that minimum P ( t ) , which allows you to ensure a given transmission quality (usually expressed in dB):

.

The calm speech of a speaker has a dynamic range of approximately 25...30 dB, for a symphony orchestra up to 90 dB. Selecting a value P min related to the level of interference:
.

5.1 Communication system

A communication system is understood as a set of devices and environments that ensure the transmission of messages from the sender to the recipient. In general, a generalized communication system is represented by a block diagram.

Figure 1 – Generalized communication system

Transmitter is a device that detects and generates a communication signal. A receiver is a device that converts a received communication signal and restores the original message. The impact of interference on the useful signal is manifested in the fact that the received message at the receiver output is not identical to the transmitted one.

A communication channel is understood as a set of technical devices, providing independent transmission of this message over a common communication line in the form of corresponding communication signals. A communication signal is an electrical disturbance that uniquely displays a message.

Communication signals are very diverse in form and represent time-varying voltage or current.

When solving practical problems in communication theory, a signal is characterized by a volume equal to the product of its three characteristics: signal duration, spectrum width and excess of the average signal power over interference. In this case . If these characteristics are expanded parallel to the axes of the Cartesian system, then the volume of a parallelepiped will be obtained. Therefore, the product is called the volume of the signal.

The duration of the signal determines the time interval of its existence.

The width of the signal spectrum is the frequency interval in which the limited frequency spectrum of the signal is located, i.e. .

The communication channel, by its physical nature, is able to effectively transmit only signals whose spectrum lies in a limited frequency band with an acceptable range of power changes.

In addition, the communication channel is provided to the sender of the message for a very specific time. Consequently, by analogy with a signal in communication theory, the concept of channel capacity was introduced, which is defined: ; .

A necessary condition for transmitting a signal with a volume over a communication channel whose capacity is equal to , is or . The physical characteristics of the signal can be changed, but a decrease in one of them is accompanied by an increase in the other.

5.2.2 Bandwidth and transmission speed

Bandwidth is the maximum possible speed of information transfer. The maximum throughput depends on the channel bandwidth as well as on the ratio and is determined by the formula . This is Shannon's formula, which is valid for any communication system in the presence of fluctuation interference.

5.2.3 Channel frequency response

The frequency response of a communication channel is the dependence of residual attenuation on frequency. Residual attenuation is the difference in levels at the input and output of a communication channel. If at the beginning of the line there is power , and at its end - , then the attenuation in non-peres:

.

Similarly for voltages and currents:

; .

The signal can be characterized by various parameters. Generally speaking, there are a lot of such parameters, but for problems that have to be solved in practice, only a small number of them are significant. For example, when choosing a device to control technological process may require knowledge of signal dispersion; if the signal is used for control, its power is essential, and so on. Three main signal parameters that are essential for transmitting information over the channel are considered. The first important parameter is the signal transmission time T s. The second characteristic that has to be taken into account is power P with signal transmitted over a channel with a certain level of interference Pz. The higher the value P with compared with Pz, the lower the likelihood of an erroneous reception. Thus, the relation of interest is P s /P z . It is convenient to use the logarithm of this ratio, called the excess of signal over noise:

Third important parameter is the frequency spectrum Fx. These three parameters allow you to represent any signal in three-dimensional space with coordinates L, T, F in the form of a parallelepiped with volume T x F x L x. This product is called the volume of the signal and is denoted by V x

An information channel can also be characterized by three corresponding parameters: time of use of the channel T k, the bandwidth of the frequencies transmitted by the channel F k, and the dynamic range of the channel Dk characterizing its ability to transmit different signal levels.

Magnitude

called channel capacity.

Undistorted transmission of signals is possible only if the signal volume “fits” into the channel capacity.

Consequently, the general condition for matching the signal with the information transmission channel is determined by the relation

However, the relation expresses a necessary but not sufficient condition for matching the signal with the channel. A sufficient condition is agreement on all parameters:

For an information channel, the following concepts are used: information input speed, information transmission speed and channel capacity.

Under speed of information input (information flow) I(X) understand the average amount of information entered from the message source into the information channel per unit of time. This characteristic of the message source is determined only by the statistical properties of the messages.

Information transfer rate I(Z,Y) – the average amount of information transmitted over the channel per unit of time. It depends on the statistical properties of the transmitted signal and on the properties of the channel.

Bandwidth C is the highest theoretically achievable information transfer rate for a given channel. This is a characteristic of the channel and does not depend on the signal statistics.

In order to use the information channel most effectively, it is necessary to take measures to ensure that the information transmission speed is as close as possible to the channel capacity. At the same time, the speed of information input should not exceed the channel capacity, otherwise not all information will be transmitted over the channel.

This is the main condition for dynamic coordination of the message source and the information channel.

One of the main issues in the theory of information transmission is determining the dependence of information transmission speed and capacity on channel parameters and characteristics of signals and interference. These questions were first deeply studied by K. Shannon.

“Multichannel communication on railway. d. transport"

Lecture notes

for studentsVcourse

SPI specialization

1. General information about telecommunications systems and networks. 2

1.1. Basic concepts and definitions. 2

1.2. Primary and secondary networks. 3

1.3. Classification and prospects for the development of SMEs.. 4

2. Parameters of typical primary signals. 6

2.1. Generalized system of parameters of the primary signal. 6

2.2. Basic parameters of typical primary signals. 9

2.2.1. Telephone signal. 9

2.3.3. Fax signal. 12

2.3.4. Signal discrete information(SDI) 12

2.3.5. TV signal. 12

3. Principles of time multiplexing of signals. 13

3.1. General principles formation of the main digital channel. 13

3.2. Temporary combining of analog signals. 13

. 14

. 15

3.3. Combining digital streams. 18

3.3.1. Character-by-character synchronous concatenation. 18

3.3.2. Combining asynchronous digital streams. 21

3.3.3 Speed ​​matching procedure. 23

4. Plesiochronic digital hierarchy. 27

4.1. Standards of the plesiochronic hierarchy. 27

4.2. Grouping with two-way speed matching. 31

4.2.1. Temporal grouping of the secondary digital signal. 31

4.2.2. Time multiplexing of tertiary and quaternary digital signal. 32

4.3. Grouping with one-way speed matching. 34

5. E1 TRANSMISSION SYSTEM. 38

5.1. Physical layer E1. 38

5.1.1 Line coding. 39

5.1.2 Signal levels electrical parameters interface, pulse shape. 41

5.2. Channel level E1. 43

5.2.1. Cyclic and supercyclic structure of E1. 43

5.2.2. Transmission error control procedures. Use of redundant CRC-4 code. 45

5.3. Network layer E1. 47

5.4. Structure of E1 transmission systems. 49

6. Synchronous digital hierarchy. 51

6.1. Comparison of SDH and PDH.. 51

6.2. Features of constructing a synchronous hierarchy. 52

6.3. Assembly of STM-N.. 54 modules

6.4. Rules for the formation of the STM-1 transport module. 55

6.5. The process of forming the STM-1 module from the flow of E1 tribes. 57

6. 6. Purpose of headings and indexes. 61

6.7. Features of the technical implementation of synchronous multiplexers. 62

6. 8. Parity methods. 64

6. 9. Reservation. 65

1. General information about telecommunications systems and networks

1.1. Basic concepts and definitions

Multi-channel transmission systems are large and complex technical systems, which embody the most modern knowledge and technologies obtained in various fields of science and technology. To provide a compact yet comprehensive description of these systems, it is necessary to use generally accepted (preferably internationally agreed upon) terms and definitions of the various objects, processes and devices related to this field.

Information is a collection of information about any events, phenomena or objects in the world around us. To transmit or store information, various signs (symbols) are used, which are a unique form of representing information. Such signs can be words and phrases of human speech in a particular language, letters and words of written speech, gestures and drawings, mathematical and musical symbols, etc. A set of signs that display this or that information is called a message.

The message may be electrical or non-electrical in nature. In most cases, messages of a non-electrical nature are of interest. The source and recipient of messages are separated by some medium in which the source generates disturbances. It is these disturbances that messages are displayed and perceived by the recipient. For example, during a conversation, the source of messages is the human vocal apparatus, the message is air pressure changing in space and time - acoustic waves, and the recipient is the human ear.

The process of transmitting (transporting) a message from a source to a recipient in accordance with accepted rules is called communication. In this case, any material carrier of the message is used (paper, magnetic tape, etc.) and/or a physical process that displays (carries) the transmitted message. The latter is called a signal. The type of signal is determined by the nature of the physical process of information transmission. A signal is called electrical if the physical process is a transmission electric current(voltage), sound - if the transmission of acoustic vibrations is used, etc.

The set of means that ensure the transmission of messages from the source to the recipient forms a communication channel.

The transmission of messages through electrical signals is called telecommunication, respectively, the communication channel that ensures such transmission is a telecommunication channel.

To transmit any messages of a non-electrical nature over a telecommunication channel, they must undergo certain transformations, which are performed by primary message converters (PMTs). PPS is a device that generates a primary electrical signal (PES) at the transmission point - an electromagnetic oscillation, the change in parameters of which corresponds to a message of a non-electrical nature. Examples of PES are telephone, telegraph, television, audio broadcasting and other signals. Typical PPS include a microphone, a photodiode, a television transmitting camera, etc.

The primary electrical signal can be transmitted directly through a physical circuit containing a pair of metal conductors, but, as a rule, the PES undergoes additional transformations. For example, for transmission over a fiber-optic communication line, the TES is converted into a certain type of optical signal, for directional transmission in open space - into a high-frequency radio signal, etc. On the receiving side, inverse conversions are carried out and the TES is restored again. Next, it goes to an inverse message converter (IMC), a device that converts the electrical signal into a message of a non-electrical nature. Typical OPS are a loudspeaker, LED, TV picture tube, etc.

Various types of telecommunications are classified either by the type of transmitted PES (for example, telephone, videotelephone, telegraph, facsimile, television, etc.), or by the type of transmission line (satellite, fiber-optic, radio relay, etc.), if the channel telecommunications is universal.

A telecommunication system is a collection of technical means and propagation media that support the transmission of telecommunication signals. Wired and wireless lines (or radio lines) are used as the propagation medium.

Wired lines are lines in which electromagnetic signals propagate in space along a continuous guiding medium. Wired ones include metal overhead and cable lines, waveguides, and light guides. In radio links, messages are transmitted via radio waves in open space. This type of communication provides a greater range, is suitable for mobile sources and recipients of messages, but is more susceptible to external interference.

1.2. Primary and secondary networks

The concepts of “primary and secondary networks” were one of the main ones in the terminology of the Interconnected Communications Network (ICN) of Russia (and before that - in the terminology of the EASC) and determined the architecture of its construction.

The primary network is understood as a set of standard physical circuits, standard transmission channels and network paths formed on the basis of network nodes, network stations, terminal devices of the primary network and transmission lines connecting them.

A secondary network is defined as a set of lines and channels of a secondary network, formed on the basis of a primary network, stations and switching nodes or stations and switching nodes, designed to organize communication between two specific points or more. The boundaries of the secondary network are its junctions with subscriber terminal devices. Depending on the main type of telecommunication, the secondary network was called telephone, telegraph, data transmission, television program distribution network, newspaper transmission, etc. Based on territorial characteristics, secondary networks were divided into intercity and zonal (intrazonal and local).

On the basis of secondary networks, systems are organized that are a set of technical means that carry out telecommunications of a certain type and include the corresponding secondary network and subsystems: numbering, signaling, cost accounting and settlement with subscribers, maintenance and management.

At the present stage, with the advent of new communication services, in addition to telephone, with the advent of a large number of independent providers who supply these services, as well as technologies such as ATM and MPLS and others, the standards of which cover both primary and secondary information transmission networks, the boundaries between primary and secondary networks are constantly being erased.

The rapid development of modern technologies leads to the fact that the regulatory framework is sharply behind the existing situation in networks.

For today, in my opinion, we should focus on the following definitions: we should leave the concept of the primary network as transport network(transmission lines with terminal equipment); secondary network – service network ( telephone communications, data transmission, etc.)

1.3. Classification and development prospects for SMEs

Multichannel transmission systems (MCS) are a set of technical means that provide simultaneous and independent transmission of several signals with the required quality over one transmission line. SMEs are classified according to the following criteria.

1. By type of guiding medium: wired and wireless.

In turn, they distinguish: a) wired over overhead lines - VSP; via cable lines - KSP; via fiber optic lines - VOSP; b) wireless via radio relay transmission lines - RRSP; via satellite links - SSP.

2. By the number of message sources (number of channels N): a) small-channel – N< 12 (обычно по воздушным линиям связи); б) среднеканальные – N= 12 – 60 (обычно КСП по симметричным кабелям или РРСП); в) многоканальные – N >300 (usually CSP over coaxial cables or RRSP, as well as VOSP); d) ultra-multi-channel - N >> 3000 (only VOSP or KSP over “large” coaxial cables, for example the K-3600 system).

To unify SMEs, the number of message sources (channels) is determined by the number of equivalent telephone messages that can be transmitted to SMEs.

3 According to the form of the transmitted signals: a) analog (ASP) - used for transmitting analog electrical signals, which over a finite time interval can take on an infinite number of states (Fig. 1.4, a). An example of such ASP are systems such as V-12, K-1920, etc.; b) discrete - used for transmitting discrete signals that, over a finite time interval, have a finite (discrete, countable) number of states (Fig. 1.4,b); c) digital (DSP) – used for transmitting digital signals that are discrete in time and have two allowed levels “1” and “0” instantaneous values ​​(Fig. 1.4, c). An example of a DSP is equipment such as IKM-30, IKM-1920, etc.

Rice. 1.4 a. Rice. 1.4 b. Rice. 1.4 in.

Main trends in SME development:

1. constant and steady transition from ASP to DSP;

2. priority development of VOSP, especially trunk lines with a large number of channels;

3. increasing the share of BSC;

4. increasing reliability, improving the quality indicators of SMEs.

2. Parameters of typical primary signals

2.1. Generalized system of parameters of the primary signal

Spectral Density Gx(f) random process characterizes the power distribution of individual spectral components of the signal x(t). If the signal x(t) periodic, then the function Gx(f) discrete; if the signal x(t) non-periodic, then the function Gx(f) continuous.

It is impossible to transmit a signal without distortion without transmitting its spectrum. Any reduction in spectrum allowed during transmission leads to signal distortion.

All really existing communication signals are random processes with an infinitely wide spectrum. At the same time, the main energy is concentrated in a relatively narrow frequency band. Since it is impossible to transmit the entire signal spectrum, the communication line transmits that part of the signal spectrum in which the main energy is concentrated, and at the same time the distortions do not exceed permissible values.

Figure 2.1 shows characteristic dependencies Gx(f):

Rice. 2.1. Characteristic dependences of spectral density Gx(f):

a) for the case when the signal spectrum is concentrated mainly in the frequency band Fн< f < Fв, где Fн, Fв – нижние и верхние граничные частоты (рис. 2.1 а);

If Fв/Fн >> 1, then the signal is considered broadband; at Fв/Fн ≈ 1 – narrowband.

b) when 0< f < Fв т. е. Fн = 0 (рис. 2.1, б);

c) when the signal has an infinitely wide and uniform spectrum, this option is convenient mathematical model and corresponds to a conditional signal called “white noise” (Fig. 2.1, c).

Signal spectrum width equal to the maximum difference FВ and minimum FН frequencies of the transmitted spectrum ΔF=FВ – FН is one of its most important characteristics.

The signal power averaged over the time interval T → ∞ is called the average long-term power Рх. Wed If T is finite, for example 1 minute or 1 hour, then we get the average minute or average hourly power. Finally, at T → 0 we obtain the instantaneous value of the signal power Рх at the moment t0.

Since x(t) – random process, then strictly theoretically, at certain moments in time, spikes in the signal x(t) and, accordingly, the instantaneous value of the power Px(t) (averaged over a small interval ΔT) can be very large. Typically, the maximum signal power is taken to be the value Px max = Xmax2, which the instantaneous value Px can only exceed with a very low probability ε. Typically ε = 0.01 or 0.001.

Signal crest factor is the ratio of its maximum power Pmax, defined above, to the average long-term Pav, expressed in logarithmic units (decibels):

.

For most signals, Kp does not exceed 13–18 dB.

During the transmission process, the signal x(t) for one reason or another (sometimes conscious) is distorted, resulting in the recipient receiving a signal x’(t) ≠ x(t). The signal reproduction error x(t) is estimated by the error power Pε, defined as

The recipient does not notice signal distortion if Pε does not exceed a certain permissible (threshold) value Pε max. Dynamic range refers to the amount

, dB,

where Pmax is the maximum possible signal power.

Dynamic range is also defined as the ratio of the maximum (peak) power Rsmax signal to its minimum power Рс min, expressed in logarithmic units. Peak power refers to the signal power exceeded for a certain time. Dynamic range of a signal using the decimal logarithm system

The dynamic range of speech signals is 35 – 40 dB.

In real conditions, communication signals are transmitted over transmission lines subject to various types of interference. Therefore, the most important thing is not the absolute value of the signal power, but its ratio to the interference power. From these considerations, a special value is usually considered and normalized - the security of a signal from one or another type of interference.

Under security refers to the difference between signal and noise levels at a given point in the communication channel:

Source information performance is determined by the ratio of the amount of information IΣ transmitted using the PES to the recipient (receiver) during time tΣ to the value of the interval tΣ:

As tΣ → ∞, the value of I determines the average information productivity of the source; if tΣ is small, then I characterizes instantaneous information productivity.

Let's find the amount of information for a discrete signal source that has L allowed states (levels) (Fig. 2.2).

On the interval ti< t< ti+1 сигнал принимает i-th level(i Є ) with probability pi..jpg" width="195" height="43">

Then the performance of the discrete source will be equal to

where Tp is the duration of an elementary message (Fig. 2.2), FT = 1/Tp is the frequency of repetition of messages ( clock frequency).

Example. Let the probability of accepting the i-th level be the same for everyone i Є ,

Substituting the value of pi we find

If the signal has two allowed levels (“0” and “1”), i.e. L = 2, and p0 = p1 = 0.5, then we obtain for a digital signal

That is, the information performance of the binary signal source coincides with its clock frequency. For example, the information performance of a main digital channel (BDC) source whose clock frequency is 64 kHz will be 64 kBit/s.

For analog signal

where the values ​​of FВ, Рср and Рε max were determined above; D* and Kn* are the dynamic range and crest factor of the signal, respectively, expressed in times (not in dicibels).

If we can accept that D*/K* >> 1, then from the previous formula we have

Here D and Kp are substituted in decibels, FB - in hertz.

2.2. Basic parameters of typical primary signals

2.2.1. Telephone signal

The average spectral density (synonym - energy spectrum) of the speech signal received at the output of the telephone microphone is shown in Fig. 2.3.

The spectrum is concentrated mainly in the range from 0.3 to 3.4 kHz. This is due, first of all, to the parameters of the primary subscriber converters - microphone and telephone. The maximum of the spectrum corresponds to the frequency F0, which for male and female voices varies from 300 to 500 Hz.

The distribution density of subscriber levels at the input of multi-channel transmission systems is approximately described by the normal law (Fig. 2.4).

Depending on at what point in the system this distribution is measured, the function W(p) will shift in parallel along the p level axis. Its maximum corresponds to the рср level for some average subscriber at this point. As a rule, the function W(p) reduced to the system input (usually the point of the zero relative level of the TNOU) is indicated:

The spread of levels relative to рср does not depend on the measurement point and is characterized by dispersion σр, which is equal to 4.5 ... 5.5 dB. For the normal law, the “three sigma” rule is valid, according to which the maximum subscriber level pmax with a probability of 99.9% is equal to pmax< (рср + Зσр).

The ratio of the average signal power Рср to the power of the maximum error Рε, which the ear does not yet feel during a conversation, for all subscribers, as the experiment shows, is

The same can be said about the peak factor of any subscriber signal, which is equal to Kp ≈ 15 - 17 dB.

Then the dynamic range of the signal is

When assessing the information productivity of telephone signal sources by ((performance formula number for an analog source)), it is necessary to take into account that each subscriber speaks on average half the time allotted for dialogue with another subscriber. In addition, a significant proportion of time is spent on pauses, thinking over answers, etc. Due to these factors, the productivity of the message source decreases on average by 3 - 4 times, which is taken into account by the activity coefficient τа = Z-1 Then using the formula for the information productivity of an analog source signal, get it

2.2.2. Audio broadcast signal

Sound sources when transmitting sound broadcasting (SB) programs are usually musical instruments and the voice of a person. High-quality broadband microphones and loudspeakers are used as primary pollutant signal converters, capable in principle of transmitting the entire spectrum of sounds that the human ear can hear. The frequency spectrum of the broadcast signal is located in the frequency band from 15 dHz. However, depending on the requirements for playback quality, the frequency band may be limited:

for higher class transmission - FH = 0.02 kHz, FB = 15 kHz;

in the first class - FH = 0.05 kHz, FB = 10 kHz;

in the second class - FH = 0.1 kHz, FB = 6 kHz.

As a rule, international and republican radio programs are transmitted via international highways in the 1st class, local pollutant distribution networks usually provide transmission quality in the 2nd class, the equipment of studios and recording houses is designed to transmit a pollutant signal in the highest class.

The permissible error in the reproduction of the pollutant signal, estimated by the value

101g(Pcp/ Pε), dB, is found through professional expertise using high-quality equipment (primary converters). It is approximately 54 – 56 dB. The crest factor of the pollutant signal is 16 – 18 dB. Accordingly, the dynamic range at the base is D = 70 – 74 dB. We determine the performance of the pollutant signal source:

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When using the Gazeta-2 fax equipment, used to transmit newspaper strips over long-distance communication lines, the highest frequency of the pattern is 180 kHz with a transmission time of one strip of 2.3 .... 2.5 minutes. The image of a newspaper strip is rasterized (linear) with the number of levels L = 2. Then

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The transmission speed is estimated either by the frequency fT = 1/τi, or by the number of elementary symbols per 1 s in baud (1 baud corresponds to the transmission of one symbol per second). According to this parameter, sources of discrete information are divided into low-speed (including telegraph), which have a speed of no more than 200 Baud, medium-speed - from 300 to 1200 Baud, and high-speed - more than 1200 Baud.

2.3.5. TV signal.

In television, as well as in fax communications, the primary signal is generated by the scanning method. The electrical signal, which includes the image signal and control pulses, is called complete TV signal. A broadcast television signal is characterized by D = 40 dB, FB = 6.0 MHz.

3. Principles of time multiplexing of signals

3.1. General principles for forming the main digital channel

As is known, when transitioning from analogue to digital form, the signal undergoes the following transformations (Fig. 3.1.):

Rice. 3.1. Convert analog signal to digital PCM signal

Discretization of individual signals in time, resulting in the formation pulse signal, modeled by amplitude, i.e. AIM signal;

Combining N individual AIM signals into a group AIM signal using the principles of time division of channels;

Quantization of the group AIM signal by level;

Sequential encoding of samples of a group PCM signal, resulting in the formation of a group PCM signal, i.e., a digital signal.

Thus, with a sampling frequency FD=8 kHz (TD=125 μs) and code bit depth m=8, we obtain a transmission rate of the generated PCM signal of 64 kbit/s, which is the speed of the main digital channel (BCC). Conversion of an analog signal to a PCM signal is standardized by ITU-T Recommendation G-711.

3.2. Temporary combining of analog signals

With time multiplexing, signals are transmitted discretely in time. Moreover, between adjacent samples of one signal there are always “time windows” in which there is no transmission of this signal. These “windows” are filled with samples of other signals. Depending on the form in which the sample of each signal is presented, two types of time multiplexing are possible:

a) signal compression in analog-pulse form;

b) signal compression in digital form.

3.2.1. General principles of analog signal combining

When temporarily combining analog signals (Fig. 3.2), each of the signals of a multi-channel system a1 (t) ÷ an(t) (Fig. 3.3, a, c) is pre-converted from analog form to the AIM-1 or AIM-2 signal.

Rice. 3.2

The formation of AIM signals is carried out using samplers (see Fig. 3.24), which are controlled by the corresponding switching pulses U d 1 ÷ U d n. Since these signals are orthogonal (non-overlapping) in time (see Fig. 3.25, b, d), then the signal samples a d 1 (t) ÷ a d n(t) also do not coincide in time and can be directly combined into a group signal U gr (t) using linear adder 2 (Fig. 3.25, d). Formation of time-shifted pulse sequences U d 1 ÷ U d n carried out using generating equipment (GE) 3. Using the transmitting device of synchronizing signals 4, it also generates a special synchronization signal, which is combined with samples of information signals a1 (t) ÷ an(t) . An elementary transmission cycle in a multi-channel system is built according to the principle: a sample of the 1st channel, 2nd, etc., up to the nth is transmitted, then a clock signal is transmitted; then again samples of the 1st, 2nd channel, etc.

On the receiving side (Fig. 3.4) samplers 11 – 1 n carry out the selection of samples of only “their” channels from the group signal. After channel filter 3 i, i= 1, ...,n the continuous signal is restored ai(t) from sampled a d i(t) ,.

Channel samplers on the transmitting and receiving sides must operate synchronously and in phase. For this purpose, forced synchronization of the receiving part is used. It is performed using a special synchronization signal receiver 2, which extracts a synchronization signal from the group signal and supplies it to the receiving generator equipment 4. For error-free selection of the synchronization signal, the latter is given specific characteristics that distinguish it from information samples. The difference may be amplitude, duration, shape, etc. GO transmission and reception are built almost identically, only the master oscillator on the transmission side operates in autonomous mode, and on the receiving side in forced synchronization mode. The advantages of this temporary seal option are as follows:

1) a common GO is used for all channels;

2) all signals are sampled at the same frequency, which allows the use of the same type of samplers and channel filters;

3) analog-to-digital conversion (level quantization and encoding operations) are performed by one group quantizer and encoder;

4) digital-to-analog conversion on the receiving side is carried out by one I group decoder, which generates a group sampled signal of the form Fig. 3.25, d.

3.2.2. Transmission system PKM-30

This type of temporary compaction is used in primary digital systems transmission type IKM-30. The transmission cycle in these systems is illustrated in Fig. 3.5.

The cycle period Tts is equal to the sampling period of the telephone signal Td = 125 μs (since Fd = 8 kHz).

In the TC interval, they are sequentially transmitted digitally binary code samples of 30 telephone signals and two service digital signals: frame synchronization (CS) and control and interaction signals for automatic telephone exchange (SUV). Each sample is transmitted in its own channel interval (CI), has a code combination duration Tk and consists of m discharges. Discharge duration – Tt. For m = 8 we get

Channel intervals, numbered 0, 1, 2, ..., 31, are used as follows: KI0 - for transmitting the DS signal, KI16 - SUV, intervals KI1÷KI15 and KI17÷ KI31 - for transmitting, respectively, 1 - 15th and 16 – 31 telephone signals. The transmission of the SUV is carried out by organizing a “remote signal channel”, in contrast to most ASPs, where the SUV is transmitted in the same channel as the information signal. In the primary DSP, a sample of the SUV of one subscriber is transmitted in the form of a 3-bit code combination, while one KI16 houses the samples of the SUV of two subscribers. To transmit samples of all 30 subscribers once, it will take time Tsc = Tts (30/2 + 1) = 16 Tts = 2 ms, which is called a multi-frame, while one of the KI16 in the multi-frame is used to transmit a digital signal of multi-frame synchronization (MCS). Using the SDS signal on the receiving side, the encoded samples of the SUV of individual channels are separated. Structural scheme SUV receiver is almost similar to Fig. 3.4.

The main disadvantages of the considered temporary compaction option are the following:

1) as the number of combined signals increases, the time interval between adjacent samples decreases (see Fig. 3.3, d), during which the group encoder (or decoder) must convert to a digital signal (and vice versa), due to which the implementation of these group devices becomes more complicated ;