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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Two-standard block of pure sound. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Телевидение Due to the significant expansion of the fleet of TV models in use, the availability of various VCRs and the increase in the number of TV channels in the MB and UHF bands, it is of interest that many old and some new devices can be upgraded to the level of two-standard sound with a noticeable improvement in its quality. This will be discussed in this article. In an attempt to eliminate the shortcomings of the “pure” sound block (PSU) and the two-standard IF (adjustment of the standard TV circuit and the need for manual switching), which are described in detail in [1], a fairly simple and stable second IF audio converter was developed. Its circuit diagram is shown in Fig. 1 (the designations in parentheses will be discussed later), and the appearance is shown in Fig. 2.
The main purpose of the converter is to convert the second IF sound of 6,5 MHz into the second IF of 5,5 MHz. However, it can convert vice versa: 5,5 to 6,5 MHz. At the same time, the converter with equivalent quality of sound of TV programs works both on the UPCHZ with an IF of 5,5 MHz, and on the UPCHZ with an IF of 6,5 MHz. You only need to replace the Z3 piezoceramic filter with the appropriate frequency or eliminate it if there is a filter at the input of the UPCHZ itself. The possibility of the converter operating in both versions is due to the choice of the frequency of the quartz resonator connected to pins 11 and 13 of the DA1 microcircuit. To provide the required functions, the converter uses a double balanced mixer on the K174PS1 (DA1) chip. The signal of the second IF sound through parallel-connected piezoceramic filters on the surfactants Z1, Z2 and capacitor C4 is supplied to pin 7 of the microcircuit. Since the reference voltage has a frequency of 12 MHz (pins 11 and 13 of the microcircuit), the output of the converter (pin 2) will produce a difference frequency of 5,5 MHz with an input signal frequency of 6,5 MHz. If the second sound IF is 5,5 MHz at the converter input, then a difference frequency of the output signal of 6,5 MHz will appear, and it will be delayed by the Z3 piezoceramic filter. However, the converter output is free for the 5,5 MHz input signal itself. Consequently, the converter provides automatic reception of signals of both the D/K television audio standard and B/G. Piezoceramic filters Z1 and Z2 completely suppress image signals at the converter input and prevent signals from the second IF sound from entering the image path. Capacitor C3 in the converter is a correction capacitor. Capacitors C2 and C5 set the operating mode of the local oscillator. They are subject to increased requirements for capacitance stability at the operating frequency. The converter is mounted on a one-sided foil printed circuit board, the drawing of which and the arrangement of parts on it are shown in Fig. 3. Any ceramic capacitors can be used, the dimensions of which ensure the possibility of their installation on the board. Resistors - MLT.
If installed correctly, the converter does not require configuration. The terminals of the microcircuit are set to constant voltages indicated in the diagram. The current consumed by the converter does not exceed 2,5 mA. The converter was supposed to function only in conjunction with the BCZ, but it is also possible to use the converter independently, provided that a second IF sound signal of good stability is supplied to its input and it is protected from the influence of video signal components. As a result of the constructive combination of the converter with the BCZ, a two-standard block of “pure” sound (DBChZ) was obtained, which does not have the disadvantages noted at the beginning of the article. A schematic diagram of the BCZ for such use is shown in Fig. 4. A converter assembled according to the diagram in Fig. is connected to it. 1, and the numbering of parts in this case is indicated in brackets on the diagram.
The appearance of the DBCHZ is shown in the photo in fig. 5.
The main purpose of the block is to automatically provide sound in a video camera according to two standards D/K and B/G. This turned out to be possible thanks to the block performing the functions of extracting the first IF sound of 31,5 (32,5) MHz from the PCTV, converting it into the second IF sound of 6,5 (5,5) MHz and converting the second IF sound of 6,5 MHz into 5,5 MHz. In addition, DBChZ allows you to improve the quality (“purity”) of the audio of television programs due to the selection of the 31,5 (32,5) MHz sound carrier in the first full-level IF mode after the channel selector. This significantly increases the sensitivity and noise immunity of the radio channel of the television receiver. When installing the unit, no modifications or adjustments to the device are required. The unit has minimal dimensions and is powered by a DC voltage source of +12 V. The current it consumes does not exceed 35 mA. From the symmetrical output IF1, IF2 of the channel selector, the signals of the first IF image and audio are supplied to the symmetrical input (pins 1 and 16) of the DA1 microcircuit (see Fig. 4) and processed there. It should be noted that with an asymmetrical selector output, the IF2 signal input (pin 16) of the DA1 microcircuit is connected to the common wire through capacitor C1. When the L1C7 circuit is tuned to the desired frequency, connected to pins 8 and 9 of the DA1 chip, the selected signal of the second IF sound of 6,5 or 5,5 MHz through the piezoceramic bandpass filter Z1 or Z2 (Fig. 1) on the SAW comes to the input (pin 7 ) DA2 converter chip of the second IF sound and is further processed in it. The requirements for parts, fastening and installation of the DBChZ are similar to the requirements for the BCZ [1]. All elements of the block are mounted on a printed circuit board made of one-sided foil-coated fiberglass, the drawing of which and the placement of parts on it are shown in Fig. 6.
If the piezoceramic filter Z2 at 5,5 MHz is not installed in the unit, the standard wire is not disconnected from the UPChZ device, and the DBChZ output is connected to the UPChZ input without disturbing the installation. This is the versatility of using the block and the variability in its manufacture. The K174UR8 chip is replaceable with an analogue TDA2545 [2] from PHILIPS. But you can also use the KR1021UR1 microcircuit with the following inclusion features. Pins 4, 5, 7, 10 are left free, pins 3, 6, 13 are connected to the common wire, and an RC circuit is connected to pin 14 of the microcircuit according to the diagram shown in Fig. 7. All other connections are the same as in the diagrams in Fig. 1 and 4. The KR1021UR1 microcircuit is also replaceable with an analogue TDA3541 [2] from PHILIPS. The use of the K174UR8 microcircuit is due to its availability and low cost.
Adjusting the unit in the device consists only of adjusting the L1C7 circuit of the DA1 microcircuit detector. Its trimmer achieves the best “clean” sound and maximum volume. The setting is clarified on all working TV channels until a silent sound is obtained. For the purpose of conscious use of DBChZ by radio amateurs in various devices (foreign and CIS countries), we will consider some options for its practical connection, taking into account the peculiarities of the structure of radio channels [3]. In the diagrams of Fig. 8-10 show options for enabling DBZZ in TV models 1512, 4462, 4465 from PHILIPS, respectively. The numbers next to the blocks show the corresponding terminals of devices or elements in them. The forming chains of nodes are omitted. The places where the conductors break are indicated by a cross.
If the DBCHZ is assembled in full accordance with the diagrams in Fig. 1 and 4, when installing it, the standard SAW filter is disconnected from the desired output of the device microcircuit (Fig. 8 and 9), and the DBCHZ output is connected to the output. In the case of manufacturing a DBCHZ without a Z2 filter, its output is soldered to the shown pins of the microcircuits without disturbing the standard installation of the device. In this case, a so-called quasi-parallel channel is obtained in a “pure” form. According to the diagram in Fig. 9 it can be seen that the radio channel of the model 4462 contains different ones than in the diagram in Fig. 8, microcircuits. Moreover, the UPCHZ is located in the primary processor after the PCTV processing channel selector. In addition, a switching device (CD) is included in the radio channel, which supplies a 3H signal either from a VCR or from a radio channel to the input of the amplifier. The Criminal Code is not directly related to the issue under consideration; you can familiarize yourself with it in [3]. TV radio channel 4465, as seen in Fig. 10, includes a multi-system audio device (MSD), which we will discuss in more detail. Knowing the essence of its functioning will make it possible to understand the meaning of using the DBCHZ in this model. A simplified schematic diagram of the MSU is shown in fig. eleven.
The purpose of the device is to ensure all-standard sound. The audio IF signal enters the amplification and detection path on the TBA120U (7410) chip via wire A203 through capacitors 2303, 2304 and an input four-frequency filter. The filters that make it up are switched by diodes 6403-6406 when exposed to a television system selection signal coming from the TV control unit to pin 2 of the first operational amplifier of the 7405 chip. In order to ensure all-standard detection, the frequency detector of the TBA120U microcircuit uses a circuit with varicaps 6435-6437, which change its tuning frequency. The tuning occurs when the second operational amplifier (pins 5-7) of the 7405 microcircuit is operated simultaneously with the switching of the MSU input filters. The tuning mode is set with a trimming resistor 3426. Through pin 8 of the 7410 microcircuit, the audio signal passes to the audio signal (see Fig. 10) of the device. Why do you need DBCHZ in such a universal TV? This is necessary, first of all, to improve the technical characteristics of the radio channel. For this purpose (case 1), all surfactant filters are removed from the DBCHZ and connected according to the diagram in Fig. 10, disconnecting the standard wire from the MSU input. In this case, the converter is also not used. In addition, if the first operational amplifier of the 2 microcircuit fails (case 7405) (if the second op-amp is working), it is impossible for the MSU to switch its input filters to the desired audio frequency. In such a situation, the output of the DBCHZ, made in full accordance with the diagrams in Fig. 1 and 4, connect to pin 14 of the TBA120U chip with the standard wire disconnected from it. This provides high-quality dual-standard audio with 31,5 and 32,5 MHz subcarriers. And finally (case 3), if both op-amps in the 7405 chip have failed and there is no way to replace it, but the TVA120U chip remains operational, the DBCHZ is connected in the same way as in case 2. However, from pins 7 and 9 of the detector of the chip TBA120U disconnect the standard circuit and connect to them a circuit similar to L1C7 in DBChZ. It is adjusted with a trimmer until the most “clean” sound is obtained at the output of the radio channel. In Fig. Figure 12 shows a block diagram of the radio channel of televisions in which the TDA8362 chip (or its modifications) serves as the video processor. It is known that it has a number of disadvantages in terms of the sound channel. They are associated with the use of a wideband difference IF audio amplifier, a frequency demodulator with a PLL system, and the unfortunately close design location of the UPCHZ input (pin 5) and the PCTV output (pin 7). Even with minor interference at the input of the UPCH, this leads to breakdowns of the PLL system and, as a consequence, to the appearance of interference in the loudspeakers.
The use of DBCHZ in such televisions ensures an increase in the noise immunity of the UPChZ by obtaining the maximum permissible signal level at its input (pin 5) of the microcircuit. In this case, the input signal of the block is not affected by video components, since it is isolated in the first full-level IF mode and detected in the DBZZ. In addition, the technical characteristics of the entire radio channel are improved. The considered options will allow radio amateurs to freely resolve issues of using DBChZ. Literature
Author: E.Gaidel
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