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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING teletext system. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Телевидение Many television channels now transmit a variety of additional information in the form of teletext. How are teletext signals transmitted? How to take them? What are the options for building decoders? The published article answers these questions. The author also talks about software and working with the remote control in various modes, about possible distortions, how to eliminate them, and, finally, how to ensure the reception of teletext systems in old TVs. Teletext is an information system for the mass user, providing the transmission of various information to TV owners in addition to conventional television programs. The literature on this subject available to radio amateurs contains only fragmentary information of a general nature. To fill this gap, we will try to consider in more detail the organizational, technical and operational aspects of the functioning of the teletext system. The development of the principles of operation of such systems, the formation and transmission of signals in them, the designs of transmitting and receiving devices began in the 60s almost simultaneously in England, France and the FRG. The most rational option was proposed by the British Air Force Corporation, and it is currently used as the global WST (World System Teletext) standard. The French Antiope system found only limited use. Information transmitted according to the WST standard can be textual or graphical. It is formed at the television center in the form of pages numbered from 100 to 899 and grouped into so-called magazines. Each of them is devoted to a certain topic, for example, sports, economics, etc. It contains about a hundred pages. The first page contains a table of contents (a list of journal sections). Typically, a section, such as a train schedule, consists of several pages. To receive information from the teletext (TXT) system, the owner of the TV must set it to a program that broadcasts such a program, and, switching to receive TXT signals, call up page 100 with a list of magazines. Then, selecting and calling the desired magazine, view its contents and call the desired section on the TV screen for viewing. This is the general algorithm for obtaining information in the TXT system. There are several options for its implementation. This is LIST mode, requiring all of the above steps to be completed. There are modes with a simplified procedure: FAST (fast, convenient teletext), FLOF (Full Level One Features - one function for all levels, which in free translation means calling all pages with one button), TOP (Table of Pages - a list of pages). In some regions, the less common Antiope, Safari, Spanish teletext. The main differences between these modes are the nature of the links between pages and the way they are searched. In the LIST and TOP modes there is no such connection, the pages are independent and are called by their numbers. True, if one of the pages of a multi-page section is called, a message is displayed along with it about the presence of a continuation and the number of pages in it (they are called subpages). The difference between the LIST and TOP modes is that in the LIST mode, to call up a page, you need to dial its number on the remote control, and in the TOP mode, the menu (list of pages on the screen) is used, on which the cursor is set (controlled from the remote control) opposite the line with the name of the desired journal, section. In FLOF mode, all information is grouped into four topics, and the remote control has four colored buttons to call them. When you click one of them, all the pages of the topic are sequentially displayed on the screen one after another. Page changes can be paused for analysis and then continued or terminated. In FAST mode, page traversal is organized differently. On the first page of each magazine, in addition to the list of sections and page numbers, there are four colored fields with page numbers. Each field corresponds to a button of the same color on the remote control. When pressed, it calls up (without dialing) the page whose number was indicated in the selected field. This page also has fields, but with different numbers. Acting in this way, you can reach the desired section and page in a few steps. However, in any mode, each page can be selected in the same way as in LIST mode - by dialing its number. Despite the abundance of modes for implementing the information retrieval process, each telecenter can use only two methods: LIST and one of the fast modes (FAST, FLOF, TOP). At the same time, the receiving side must be provided with the possibility of multi-mode operation for receiving messages from any television center. The TXT page of the WST standard consists of 25 lines of 40 characters per line. The first line is the title of the page. Lines 2-25 contain TXT information, and in FAST and FLOF modes, line 25 serves as a status line. The header contains the page number of the owner on the screen; number and name of the W page transmitted by the television center at the current moment; date and time of transmission; number and numbers of subpages. The status bar displays colored fields with topic titles (FLOF mode) or page numbers (FAST mode). Any string is transmitted as a series of 45 bytes. Bytes 1-3 are sync. Bytes 4, 5 are the address of the line: the journal number and the line number in the page. Bytes 6-45 of the header are used as follows: page number N is stored in 6, 7; at 8-11 - date and time; at 12-45 - the number and title of the W page, as well as symbolic information displayed in the header (day of the week, etc.). The same bytes in other lines contain the character information of the transmitted text. To improve noise immunity, the eighth bit of each byte is given a value that provides an odd number of ones in the byte. The string address is bit-protected. TXT information, prepared for transmission by a special telecentre service, is stored in digital form in a data bank, from which it is cyclically retrieved and page-by-page entered into a television video signal (PTTV). Page transfers occur during vertical blanking pulses (CHPs). Recall that the THD of the first half-frame (field) of the PTSD occupies the interval from the 623rd line of the previous field to the 23rd line of the first field, and the second field - from the 311th to the 335th line. Some of them are already occupied by equalizing horizontal pulses, SECAM burst signals and TV test signals. Only 12 lines with numbers 6, 16-18, 22, 23, 318, 319, 329-332 are free in each frame. They are where TXT signals are placed. On fig. 1 shows the oscillogram of the PCTV during the transmission of the THD and the position of the TXT signals in it. It has two vertical axes: the level of the instantaneous power p emitted by the transmitter, and the level of the brightness signal Y corresponding to this power. Since domestic broadcasting is carried out using negative modulation, the zero values on these axes are located at different levels, and the axes are directed in different directions.
The TXT string is transmitted in the interval between two horizontal blanking pulses. This interval is 52 μs, and during this time 45 bytes (360 bits) of information must be transferred. Therefore, their transmission speed must be at least 6,923 Mbps. The WST standard assumes that the bit series of the TXT string is transmitted as square wave signals with a pulse and pause duration of 0,144144 µs. A bit with a value of 1 corresponds to a signal with a level of 80% brightness of the DTV, and a bit of 0 - 30% of the brightness. These signals occupy a frequency band of 4...10 MHz, which is beyond the limits of the PDTV spectrum, which is limited in different broadcasting systems with a frequency of 5...6 MHz. To introduce them into the PDTV spectrum, the subcarrier of the teletext signals is shifted to 3,46875 MHz (harmonic 222 line frequency), with the upper sideband suppressed. With one TV line per half frame for TXT signaling, the WST throughput is two TXT lines per frame, or 0,5 s per page. These are the structure and order of encoding the lines of TXT pages in our SECAM-D/K broadcasting system. There are no special burst signals in PAL, and pages can be transmitted faster by using more TV lines. The NTSC system uses a different system for placing TXT signals in the PTTV, and in some countries a different number of lines per page and characters per line are used. The Antiope system uses a different string format. More detailed information about these systems is contained in [1, 2]. In our country, TXT programs are broadcast on ORT, TV Center, NTV, TV-6 programs and on satellite television channels. Each of them forms its own package of journals and defines their content in its own way. Thus, ORT broadcasts a package called "Russian teletext service on 1 TV channel TELEINF" from five magazines: news and sports, economics and finance, goods and services, leisure, kaleidoscope. The package contains pages numbered from 100 to 512. On page 100, the table of contents of the package is given: the names of the journals and the numbers of their first pages. Page 101 indicates the frequency of updating information in the package: news - twice a day; weather, finance, sports, TV programs - daily; other information - two or three times a week. The package is organized in FAST mode, but there are color fields only on the first pages of sections. The enumeration of subpages in some sections occurs automatically, in others subpages must be called by dialing. The waiting time for the next page does not exceed 45 s. Teletext on the TV center program is organized in LIST mode. The packet of pages 100-497 is constructed so that the first pages of the magazines and the pages with the most important information are transmitted several times in each cycle. This significantly reduces the waiting time for such a page, although for the rest it is the same as in the ORT package. The NTV program broadcasts the "BLITZTEXT Magazine of Business People", consisting of pages 100-777, also in LIST mode. In the same mode, the "TV-6 text" packet is transmitted on the TV-6 channel. It consists of three magazines. Its peculiarity is that iteration of pages when searching for them is provided only within the pagination of the called journal. This means that in each half-frame the DHTV transmits one line from each log at the same time. Page waiting time does not exceed 5...8 s, which is much better than this indicator in any other program. To receive TXT signals, the TV must have a special device - a TXT decoder, and to control its operation - a remote control system with microcontroller command processing and appropriate software. Let's start with the TXT decoder. There are a large number of types of decoders that differ in how they are controlled, page memory size, and circuit design. According to the control method, decoders are divided into simple and advanced ones. A simple decoder is controlled by the microcontroller (CCU-TV) of the TV control system. It only works in LIST mode. The advanced decoder provides both LIST and fast modes (FAST, FLOF, TOP) operation. To do this, it must have its own microcontroller (CCU-TXT). Recall that the microcontroller is an eight-bit microprocessor, in the body of which a set of interface devices is introduced that converts the machine codes of the microprocessor into analog or other forms of signals to control external devices, including a digital bus. According to the amount of memory, decoders are divided into one-page (UNITEXT), four-page, seven-eight-page (EUROTEXT), ten-page or more (meaning the number of pages simultaneously stored when dialing a page number). The circuit design of teletext decoders (TXT), despite the large number of models, has only a few basic options. The first of them will consider the voltage synthesizer and decoder module MST-601. It consists of a PII remote control signal receiver, a CCU-TV microcontroller with a PROM-TV memory unit and a decoder. Block diagram of the module is shown in Fig.2. The CCU-TV (DD1) microcontroller receives and processes commands to control the reception of television programs and teletext. The procedure for processing commands related to the processes of controlling the TV (turning on / off, switching programs, adjusting analog parameters, displaying information on the screen - OSD) is described in [3]. With regard to teletext control, this chip has a built-in software (software) of the CTV322S variant, which allows you to control a simple decoder directly, and a decoder with advanced features through its microcontroller. The four-page decoder with extended capabilities of the MCT-601 module operates in LIST and FAST modes. It contains five chips: a CCU-TXT microcontroller with a PROM-TXT memory unit, a VIP2 video processor, an ECCT driver with a RAM-TXT memory chip. The CCU-TXT microcontroller processes the teletext control commands received via the two-wire I2C bus from the CCU-TV (a description of the structure and operation of the bus is given in [3]), writes information from the page headers into the EPROM-TXT (DD4) and determines the page number when pressed color button on the remote control. The module uses three memory chips. PROM-TV stores the settings generated by the CCU-TV for television programs, and when working with a simple decoder, TXT page numbers. ROM-TXT is only needed in decoders with advanced capabilities to store TXT page numbers. RAM-TXT contains the texts of the pages selected by the TV user from their stream. In the UNITEXT decoder, the list of page numbers for each program consists of one number, in the four-page decoder - of four, and in EUROTECHT - of eight numbers. The same number of page texts of one selected program is stored in RAM-TXT. The capabilities of TV control systems of different brands in terms of the number of memorized settings for television programs and teletext decoders for programs transmitting TXT are determined by the memory configuration of PROMTV and PROM-TXT. For four-page decoders controlled via the I2C bus, the configuration options listed in Table 1 are possible. Depending on the option selected, the 8571-byte PCF81/128 or 8570-byte PCF82/256A chips must be used. In order for the CCU to recognize the selected configuration and correctly address commands, the memory chips are listed in Table. 1 address. The address of the microcircuit is set by the voltages at its pins 1-3. To assign an AO address, you need to connect all these pins to a common wire. At address A2 or A4, +2 V is applied to pin 3 or 5, respectively, and the remaining pins are connected to a common wire. Table 1
The full color television video signal of the PCTV containing TXT information is fed from the video path to pin 27 of the VIP2 (DD5) video processor - a specialized SAA5231 chip (analogue - KR1087XA7). The PDTV processing in VIP2 consists of VCS extraction and synchronous detection of the 3,46875 MHz subcarrier. The resulting TTD teletext signals are amplified and clipped. They are sequences of pulses and pauses of various durations transmitted during the THD - a current display of the digital codes of the characters of the lines of the TXT pages. To decode (digitize) signals of this kind, it is necessary to have a parallel synchronous and in-phase stream of synchronization pulses (TTC) with a frequency equal to or a multiple of the maximum repetition rate of TTD signals. The TTC pulse repetition rate is assumed to be 6,9375 MHz. It is formed in the video processor VIP2 by dividing by two the frequency of the oscillator signal with a ZQ3 quartz resonator. This ensures that the TTD and TTC streams are synchronized. Their in-phase is established by adjusting the phase of the TTS signal by the PLL system available in VIP2 when comparing the three-level SSC strobe pulses coming into VIP2 from the ECCT shaper with the VCS pulses extracted from the PCTV. If the PTSD has insufficient swing, the PLL becomes unstable and instead of VCS pulses, an artificial clock signal TCS generated in the ECCT is used to control it. Switching of VCS pulses to TCS at the input of the PLL system is provided in the VIP2 video processor by the PDTV level analysis node. Signal processing in VIP2 occurs at a frequency of 6,0002 MHz, generated by an oscillator based on the ZQ4 resonator. Pulses of this frequency (F6) are also transmitted to the ECCT to receive the TCS signal. At the same time, TTD, TTC, VCS signals pass to the ECCT from VIP2. The R, G, B signal conditioner of the ECCT teletext (SAA5243 P / R chip or analogue KR1568VG2) ensures the selection from the TTD, TTS streams of those parts that relate to pages to be written to memory and displayed on the screen. The selected pulses are divided into eight-bit groups corresponding to character bytes, which are then converted from serial to parallel codes. In this form, they are written to the page text RAM (RAM-TXT) via the eight-bit DO-D7 bus. In total, four pages with numbers N-601, N, N+1, N+1 are recorded in the RAM-TXT of the four-page decoder MCT-2 with each dialing of the page number N on the remote control. For each of them, 24 (according to the number of lines per page) set of 42 bytes of TTD signals and 42 bytes of TTC signals are entered. In total - 2016 bytes (the memory capacity of the DD7 chip is 8 kb or 8192 bytes). The first three sync bytes of the string are not written to RAM-TXT. The placement of these signal sets in RAM-TXT is controlled by the ECCT shaper via the 13-bit address bus AO-A12. Then it displays from RAM-TXT information related to page number N or (by command from the remote control) N+1, N+2. Page N-1 information in the MCT-601 module can only be displayed in FAST mode. The conversion of output signals from machine codes to symbolic form takes place in the character generator available in ECCT. This character generator (similar to the CCU-TV character generator), in addition to code conversion, generates the brightness value of the pixels of the image matrix of the desired character, and then deploys these brightness signals along the television lines in the place where the character should be located. Details of his work can be found in the section "84C44**, 84C64**, 84C84**" handbook [4]. The R, G, B and FB teletext signals formed in the ECCT, together with the same OSD signals from the CCU-TV, pass to the R, G, B DD8 signal switch (made on SN74LS241 or an analogue of KR1533AP4). This microcircuit is a two-channel buffer amplifier (four amplifiers in each channel) and, when connecting the outputs in pairs, forms a switch. It sends the TXT and OSD signals to the TV video processor for output after they are added to the screen. To ensure a stable position of the TXT page on the screen, it is necessary that the trigger pulses of the frame scan KIzap and the frame components of the VCS, TCS signals are in phase. Otherwise, there is an unpleasant jitter of the text of the page vertically. To exclude such a phenomenon, the position of the pulse fronts (KIzap) is corrected by POE pulses generated in the ECCT and transmitted to the frame scanner of the TV. The MST-601 module is used in the HORIZONT-STV601/602 TV sets together with the PDU-6 or PDU-6-1 remote controls. The circuit diagram of the module is published in [5]. According to the scheme shown in Fig. 2, simple decoders are also built that operate only in the LIST mode. Their example is the MDT-655 teletext module used in the HORIZONT-CTV655 and TVT2054TX/2154TX TVs. It does not have a CCU-TXT and a PPZUTHT (DD3 and DD4 in Fig. 2, circled with a dash-dotted line). The connection of the remaining microcircuits and their functioning remained the same. The MDT-655 module uses a CCU-TV of the same type as in the MST-601, but with a different modification: PCA84C640R-030. Both microcircuits have the same characteristics and pinout, but different software. In modification 030, the CTV320S software is installed, which differs from CTV322S in the lack of control of the "Picture in Picture" module. The ability to control the TXT decoder for software and modifications of microcircuits is the same. By the way, CTV320S software also has modification 019 of this microcircuit, which can serve as a replacement for PCA84C640P-030. Schematic diagram of MDT-655 is considered in [5].
For use in the TV HORIZONT-CTV655, a teletext module with advanced capabilities MDT-656 was developed. Its scheme is also presented in [5]. Compared with the device according to the scheme in Fig. 2, instead of VIP2 and ECCT, it uses the teletext processor SAA5281ZP / R, which replaced these two microcircuits. The limited functions performed by CCU-TXT led to the fact that they were later transferred to more complex LP processors, which combined CCU-TXT, VIP2, ECCT in one package. This simplified the decoders while maintaining multi-mode operation. A decoder built using such a SAA5246P / R chip is used in TVT2594TX / 2894TX TVs. Its block diagram is shown in fig. 3. TXT signal processing technology as described above. The number of quartz resonators has been halved. The schematic diagram of the decoder is given in [6]. This construction of decoders was used by SONY in the KV-M14 model and by GRUNDIG in devices with the SAA5273 processor. The Russified version of the decoder for SONY-KV-M14 TVs is produced by the domestic company "Komplekt" under the designation TTK-10.
The next step towards improving the decoders was the integration of CCU-TV and the TXT processor. With the advent of such chips, the TV control system and the decoder turned into a single-chip (except for memory chips) module. Recall that the first models of TV control systems contained two dozen microcircuits, and another ten microcircuits were in the TXT decoder. There are several types of such chips, in particular SDA5250 and SAA5296. The first one is used in the control system and TXT decoder of SONY-KV-21X receivers. The block diagram of the system is shown in fig. 4, and the circuit diagram is given in [7]. The difference between the decoder and those considered earlier is as follows: the R, G, B, FB signals of both OSD TV programs and TXT are removed from pins 45-48 (the switch for these signals to common pins is located in the microcircuit). The processor uses two external memory chips RAM-TXT (DD3) and PROM-TV, TXT (DD2), as well as five internal memory blocks: control command RAM, TXT page RAM, OSD and TXT combined character generator ROM, microprocessor ROM and RAM.
The microcontroller of the control system and ten-page decoder TXT SAA5296 is currently one of the best microcircuits of this type. It has numerous interfaces and software that allow you to control an analog or digital TV of any complexity and process the TXT signal in any mode. Information about it and the switching scheme are presented in [8]. All the decoders discussed above are controlled via the I2C bus. Let's talk about decoders that use other buses. The Ml digital bus was proposed by PHILIPS for controlling decoders that use the SAA5020, SAA5030, SAA5040, SAA5050 and other simpler chipsets. There are ten microcircuits in the decoder in total. Its typical scheme is shown in [4]. The decoder is controlled by the SAA1251 or SAA1293 chip. All these microcircuits are now obsolete, and with them the Ml bus has fallen into disuse. The IM bus is still used today to control TXT decoders along with I2C. One of the reasons for this can be indicated by the presence of a chipset that allows using the IM bus to implement a simpler construction of a digital color block than is currently possible on chips controlled via the I2C bus. Using the IM bus, TXT decoders with TTD digital signal processing have been created. An example of such a device is the MTT-57 module of ELECTRON-TK551/557 TVs. Its block diagram is shown in Fig. 5, and the circuit diagram is considered in [9].
The decoder consists of an analog-to-digital converter ADC DD2, a scan processor DD3, an LP processor DD5 with a memory chip DD6. Its operation is significantly different from the principles of operation of the previously discussed devices. The decoder is controlled by commands from CCU-TV (DD1) SAA1293A-03 or TVP02066-A26. The PCTV from the video path goes to the DD2 chip, which consists of a synchronous detector, a limiting amplifier and an ADC. The TTD signals obtained as a result of detection come to the ADC, where they are converted into groups of a seven-bit parallel code corresponding to the bytes of the original TXT signals. These codes pass through the seven-bit VO-V6 bus to the scan processor and without the most significant bit through the six-bit VO-V5 bus to the LP processor. In the DD3 scan processor (DPU2540/43), synchronization and blanking pulses are extracted from the TTD digital stream, which are then transmitted to the TXT processor. The digital representation of the TTD stream greatly simplifies signal separation. The fact is that lowercase sync pulses are detected by the appearance of units in the upper bits of the code at the output of the ADC. In this case, the sync selector is a simple device that selects the V6 bit of the code and turns it into a synchronization pulse of the required shape and amplitude. The processing of the TTD digital stream in the LP processor ends with the formation of page texts in machine codes transmitted via the DO-D3 bus to RAM-TXT (DD6) in four-bit words with their addressing via the eight-bit AO-A7 bus. The amount of memory is 128 KB, which allows you to store up to 128 pages in it. At the command of the remote control, information from RAM is called into the LP processor, which has an internal character generator, and after being converted into a symbolic form, it is transferred to the TV's video processor. The module was produced in two versions: MMT-57 - for operation in LIST, PAST, FLOF and TOP modes and MMT-57-1 for operation only in LIST mode. The second component of any teletext system is the SDU TV: remote control, CCU-TV. digital bus and software. Ways to organize communication between the TXT decoder and CCU-TV. bus and software were considered when describing decoders. Let's move on to the issues of organizing control of a decoder with a remote control. There are many different models of remote control. differing in the number of given commands, buttons and their purpose. In SDU RC-5 used in equipment with digital bus l2C. A set of 40 commands is provided to control the TXT decoder. Their complete list is given in [4]. As a rule, only a part of them is used in the remote control, chosen at the discretion of the hardware developers. As a result, the procedure for controlling the decoder in TVs of different brands is organized differently. For analysis, let's take the PDU-6 remote control used to control the previously discussed MST-601 module. It has 36 buttons, of which 26 are used to control the decoder. They are shown in fig. 6 in the form of filled (black and colored) rectangles, their purpose is also indicated there. The remote control allows you to work with information in LIST, FAST and FLOF modes.
When you turn on the TV in the operating mode (button 1), the decoder software is automatically set to the start address and the FAST mode is entered. In order to start familiarizing yourself with TXT information in this mode, you should tune in to the program that broadcasts TXT in this mode and press button 10 on the remote control. The title bar with the number N will appear on the screen. If N#0. you need to press the button 7 to call page 100. When page 100 appears on the screen, the desired magazine is selected and the number of its first page is known, you should dial this number using the buttons "0" - "9" of the dialing field 13. If the magazine starts from page 101, then press button 12 to go to it. On the first page of the magazine, in addition to the table of contents, there is a status line, and in it there are four colored fields (red, green, yellow and blue) with page numbers. On the remote control, press one of the buttons in zone 8 of the color that matches the color of the field with a number equal to or close to the number of the desired page. The desired or some other page will appear on the screen with other numbers in colored fields. By repeating this procedure, you need to go to the desired section. Page enumeration in it will be provided either automatically every 30...45 s. or by calling them. To go to TXT. transmitted by another program, you need to turn off the decoder with button 10. turn on the desired channel, press button 10 again and. if it also works in FAST mode, proceed as follows. as described above, or switch to LIST mode. The LIST mode is switched on with button 2 ("P+"). to return to FAST mode, press it again. When selecting information in this mode, you should call up page 100 (using button 7) in the same way as in FAST mode. Then, using the buttons "0" - "9" of the dialing field 13, dial the number of the first page of the journal, and after it appears, dial the number of the first page of the desired section. Each time pages with numbers (N-1)-(N+2) are entered into memory. of which page N is displayed. To pages N+1. N + 2 go by pressing the button 12. To call the next group of pages with the continuation of the section, you need to dial the page number N + 3 using the buttons "0" - "9" of the dialing field 13 or press the button 12 again. After some waiting, the next four pages will be stored in memory, and page N+3 appears on the screen. In the LIST and FAST modes, other operations listed in Table 2 are also possible. 5 indicating the necessary actions. More complex operations are also possible: calling hidden information (buttons 4 and XNUMX). watching a TV program while the decoder is working to search for the desired pages in the TXT signal stream. These operations require pressing several buttons on the remote control in a certain sequence and are not performed on all domestic TV channels.
The composition of the CCU-TV software is described in [3]. As for the order of its operation to control the teletext decoder, each time the remote control button is pressed, a decoder control command is generated, CCU-TV sends it to the 1gC bus at the CCU-TXT address and waits for confirmation from it. If there is no CCU-TXT in the decoder (no acknowledgment received). CCU-TV repeats the command to ECCT. If confirmation of receipt of the command is not received this time. The CCU-TV recognizes the situation as the absence of a decoder, perceives the command as impossible and reports this by forming a yellow rectangle in the upper left corner of the screen. If acknowledgment is from CCU-TXT or ECCT. then the CCU-TV processor turns on the programs for processing the remote control commands for controlling the teletext decoder and writes the numbers of the called pages into the PROM-TV. In FAST mode, it generates and sends the "List assignment" command to the CCU-TXT when switching to the next TV channel. The CCU-TXT software is represented by standard sets of programs named CTV900/940/970-976/988/990/991S. Their main characteristics are listed in [10]. In particular, the PCF84C81 processor (see Figure 2) uses the CTV972S software. Any set of TXT software includes the following standardized software packages: 8/30 - ensuring the operation of the character generator; 24 - serving modes FAST and FLOR 26 - for processing texts with an extended character set (for example, Spanish, the alphabet of which contains 32 characters); 27 - linking pages. There are five modifications of the 0/30 package, depending on the set of languages being processed. They are designated by the symbols A, E, H, K, R in the type of microcircuit containing the character generator. For example, in the SAA5243P / R chip, the package processes Cyrillic characters (Russian and Latin characters), in the SAA5243P / E and SAA5243P / H chips - Latin characters in broadcast systems with a scan of 625 lines, SAA5243P / A - Latin characters with a scan of 525 lines , and SAA5243P/K are Latin and Arabic characters. Let us dwell on the question of possible signal distortions in TXT systems. They manifest themselves in the instability of reception and the disappearance of characters or their replacement by others (for example, Cyrillic to Latin). Unstable reception occurs when the signal strength at the receiving location is insufficient or the sensitivity of the TV is reduced. The disappearance or replacement of characters is a consequence of the increased sensitivity of the TXT digital signal to impulse noise and echo signals caused by reflections from local objects and inhomogeneities of the antenna cable. From this point of view, the TXT signaling system adopted in the NTSC (2,5 MHz subcarrier phase modulation) standard is more resistant to impulse noise and provides reliable reception. The appearance of interference may also be the result of inaccurate tuning of the TV to the carrier of the television signal. Tuning offset results in either narrowing of the bandwidth (loss of high frequencies in the video signal) or penetration of the FM modulated audio components into the video signal. In both cases, the decoding of the TXT signal stream occurs with numerous errors that cannot be corrected by the protection means introduced into them. According to [1], clear, error-free operation of the TXT decoder is ensured when the receiver local oscillator detunes by no more than ±250 kHz. The residual value of the detuning during the operation of the APCG system is normalized at a level of no more than ±100 kHz, but in practice it is very often more. In case of distortion of teletext signals, first of all, you need to check the quality of television reception and, if necessary, adjust the APCG system of the TV. There are distortions of another kind. On fig. 1 it can be seen that the TXT signals are located in the middle part of the image brightness change interval. corresponding to gray signals. During normal operation of the backscattering system, they will not be visible on the screen. If the vertical blanking pulse duration is set incorrectly, bright dots appear from bit 1 of the TXT signals at the top of the screen. They will appear in its lower part if the levels of the frame and line components of the SSC signal or the ratio between them are incorrectly set on the TV, and therefore the black fixation level. These phenomena are explained by the presence of TXT signals in the PTTV and can occur on any TV, even without a TXT decoder. Their appearance should be considered as a signal of shortcomings in the operation of the systems for fixing the level, damping and limiting the beam current. The reason for this may be incorrect brightness adjustment. Many TV owners set the brightness to a level that produces high whites. In this case, the range of reproducible brightness gradations not only decreases, but their entire interval shifts towards increased brightness. The restoration of the black level is achieved by increasing the contrast. This distorts the image and contributes to the appearance of the above defect. It is no coincidence, therefore, that a number of publications on this topic have already expressed the opinion that that brightness control should be excluded from the number of operational ones. It is proposed to provide the necessary changes at different intensities of external illumination by adjusting the contrast. This topic requires a separate discussion and is only touched upon in connection with possible interference from TXT signals. In conclusion, I will briefly mention a new direction in the development of the teletext system - the RDS system [11]. Today, such signals are transmitted not only on television, but also on radio broadcasting channels. Such broadcasts in Moscow are carried out by the Silver Rain radio station, which informs listeners about the weather, finances, etc. In radio receivers with a signal decoder with CCU and software. these messages are displayed as a running line on an alphanumeric display. The operation of such devices is based on principles that coincide with those used in television, but on completely different technical solutions. Let's move on to some practical advice. If you have an imported TV with a TXT decoder. whose character generator does not contain the Russian alphabet, and the decoder is a removable module, it should be replaced with a Russified version. On sale there are such modules for almost all types of TVs. You just need to make sure that the new module is designed specifically for use in your TV. A good new module will work without any adjustment. If the desired model of the Russified module is not on sale or the decoder is mounted on the TV board, it is recommended to replace in it the microcircuit that contains the TXT character generator. to a new one with the P / R index in the designation (see above). Depending on the design of the decoder, these will be ECCTs. LP or CCU. Soldering from a microcircuit board with a large number of pins requires accuracy and special skills. As for the replacement of the CCU in a single-chip TV and decoder control system, it seems irrational and impractical. In this case, you will have to come to terms with the need to read Russian text written in Latin characters. When the TV does not have a decoder, but there is a connector for installing it, purchase a Russified version for this model of the device and install the module in it. TV adjustment is not required. If you want to install a TXT decoder in a third or fourth generation TV, you should, in my opinion, opt for the MCT-601 module. You will also need a remote control panel-6 and a network switching unit for the MST-601. The scope of work on installing the module in a TV set is identical to that described in [2, 12. 13) for installing the MSN-501, adjusted for a different layout of the connecting cables and the difference in the amplitudes and shapes of the voltages supplied through them. If, on the TV of the third or fourth generation, the voltage synthesizer MSN-501 is already installed. then for the introduction of TXT, you can use the MDT-656 module. This work is not so difficult and can be handled by radio amateurs who have experience installing the MCH-501 in their TVs. Literature
Author: V.Brylov, Moscow
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