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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING TV sound. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Телевидение Owners of a number of imported TVs are not able to use such a function of foreign-made devices as stereo sound accompaniment of on-air and cable television programs. Often, only those who receive satellite programs can appreciate its benefits. How television sound is transmitted in existing standards and how to improve its reproduction is described in the published article. The technical base of domestic television has improved significantly in recent years. New equipment has appeared at television centers, modern means and technologies for preparing and conducting broadcasts are used. The image quality has improved, the number of broadcast channels is growing. The only characteristic that has not undergone significant changes in terrestrial and cable television is the soundtrack. It has remained monophonic for many decades. Monophonic sound seems to come from one point - the loudspeaker. In television, as in cinema, this way of reproduction conflicts with the image. It is partly acceptable only when showing close-ups, when the sound should come from the center of the screen. With medium and general plans, it is logically required to expand the sound picture in front of the viewer. A cardinal improvement in the perception of the sound panorama can only be provided by multichannel systems for the formation and reproduction of sound. These are numerous variants of two-channel stereo, four-channel quad, five-channel and more surround sound systems. All of them (except for quads, which have not yet found wide application) have been brought to a high circuit and quality level, mastered by industry and used all over the world. Recently, they have appeared in our country. Consider their main parameters. VHS video recorders of a simple design reproduce sound in one channel, and more complex ones (of the Hi-Fi class) also in two. The mode in which the audio is recorded is usually indicated on the video cassette. It can be STEREO, DOLBY STEREO, DOLBY SURROUND (with multi-channel audio). The absence of such inscriptions means a monophonic recording. On the media used in S-VHS VCRs and mini-DVD players, recordings are almost always made with multi-channel audio. All these devices process audio signals, as a rule, at a low frequency in analog form, and DVD players also in digital form. Telecentres of foreign countries transmit sound accompaniment in various ways. In the United States, the BTSC-MTS system (Broadcast Television Systems Committee - Multichannel Television Sound - multichannel television sound - the standard of the Committee on Transmitting Television Systems) is used. It represents the development of the monophonic television standard NTSC-M, which made it possible to additionally introduce multi-channel sound into it. The system provides for modulation of the 4,5 MHz subcarrier frequency not with mono sound, but with a complex stereo signal (CSS). The structure of this signal is shown in fig. 1a. The frequency of the suppressed subcarrier of the LR signal is 31,468 kHz, which corresponds to the second harmonic of the horizontal frequency, equal in the NTSC system to 15,734 kHz. In addition to the usual L + R, LR, subjected to amplitude (AM) and balanced (BM) modulation, and pilot signals, two additional frequency-modulated coded audio channels on subcarriers 78,67 and 102,27 kHz have been introduced into the BTSC-MTS CCC (for official use). Receivers with a mono audio path perceive only the L+R signal. Devices in which a stereo path is provided process all signals.
In Japan, sound signals are also transmitted in the form of a KSS (Fig. 1b), but constructed differently than in BTSC-MTS. The subcarrier of the LR signal is not suppressed. The pilot signal is also transmitted, but is used only for recognition of the mode of operation. When transmitting stereo programs, it is modulated with a tone with a frequency of 982,5 Hz, with a two-channel (bilingual) transmission - with a tone with a frequency of 922,5 Hz, and in the case of a mono channel, the pilot signal is not modulated. In the PAL-B / G standard for terrestrial broadcasting, stereo signals are in the PDTV on subcarriers of 5,5 and 5,742 MHz with FM modulation (Fig. 1, c). One of them transmits the signal L + R, the other - 2R. Using a 2R signal instead of an LR one can even out the noise in the channels, which are usually twice as loud in the L channel as in the R channel. This system is called Zweiton. In addition, the stereo signal is repeated in the PDTV in digital form encoded by the NICAM system (Near Instantaneous Companded Audio Multiplex - direct transmission of two-channel audio) using RPM (relative phase shift keying). The PAL-I PDTV (Fig. 1d) contains two simultaneously transmitted audio signals: a frequency-modulated analog mono signal on the 5,9996 MHz subcarrier and a digital stereo signal on the 6,552 MHz subcarrier, encoded using the NICAM system. The stereo signal of the NICAM system is formed at the television center by sampling the analog signals L and R in time with a sampling frequency of 32 kHz and quantizing at 256 levels (8 bits) in each sample. Information from both channels is transmitted in a common digital data stream DQPSK (Digital Quadrature Phase Shift Keying - a digital data stream with a quadrature phase shift) at a rate of 728 kbps. This stream modulates the audio subcarrier (5,85 MHz in PAL-B/G and 6,552 MHz in PAL-I) in DPSK. On the TV, the DQPSK stream is decoded into two-channel analog signals L and R. The structure of the decoder is shown in fig. 2.
The audio subcarrier, modulated by the DQPSK stream and a pilot signal with a frequency of 1 kHz, enters the DD54,6875 chip from the PDTV demodulator. In the DD1 chip, the subcarrier is demodulated and the received digital stream is cleared of interference in the digital filter. The DQPSK stream and the pilot signal are transmitted to the decoder DD2. Decoding consists in dividing the DQPSK stream into digital signals L and R, as well as dividing them into groups of bits (words) corresponding to samples. Digital-to-analog converters in the DD2 chip turn digital samples into pulses, which, after smoothing, form analog signals L and R • At the same time, the sound transmission method is also recognized. If the pilot signal is modulated at a frequency of 117,5 Hz, then a stereo program is transmitted, if at a frequency of 274,1 Hz, two mono signals, and if not modulated, one mono channel. The decoder is controlled by the microcontroller of the TV control system via the I2C digital bus. All of the systems discussed are compatible with a fleet of mono TVs. Television broadcasting in satellite channels is organized with the transmission of signals in analog, digital-analog and digital forms. In analog form, satellite broadcasting continues in NTSC, PAL, SECAM systems. In the SECAM-D/K system, the soundtrack, as before, remains monophonic. On satellite channels, unlike terrestrial broadcasting, it is transmitted on 6,8 subcarriers; 7 or 7,5 MHz. In the PAL system, analog audio is organized into one, two, or four channels. In the first case, one of the subcarriers 6,5 is selected; 6,6; 6,65; 6,8; 7; 7,5 MHz. Two- and four-channel audio transmission is provided by the Wegener-Panda 1 system. As shown in fig. 1, e, it provides for the inclusion in the PCTV of four additional frequency-modulated audio subcarriers 7,02; 7,2; 7,38; 7,56 MHz. Two of them are used to transmit the stereo sound accompaniment of a television program, the rest - for simultaneously transmitted broadcast programs. More details about such a system can be found in [1]. In digital form, the sound accompaniment of the analog PAL television signal is transmitted over satellite channels after encoding using the NICAM system. In digital-analog form, television signals are used in MAC and MUSE systems. The MAC (Multiple Analog Components) system is a transitional version from analog to digital methods of transmitting a television signal over communication channels. It uses analog and time-separated transmission of luminance and color signals and digital transmission of audio and other information signals (synchronization signals, teletext, service signals). Their processing on the transmitting and receiving sides is provided by digital methods. There are several options for building a system: A-MAC, B-MAC, C-MAC, D-MAC, D2-MAC, HD-MAC, HD-B-MAC. Their main differences are in the methods of signal coding, carrier modulation, and the number of audio channels. Audio signals from analog form are converted to digital after they are sampled at 32 kHz and quantized using 14 bits per sample. After that, they are recorded in real time in the buffer memory, where they are combined with digital information signals in packets of 751 bits. During the frame, 162 packets are formed in the C-MAC, D-MAC systems (82 packets in the D2-MAC system). During the blanking intervals, packets are read from the buffer memory at a rate of 20,25 MHz in chunks of 195 bits per line (10,125 MHz and 99 bits in the D2-MAC system) and are digitized into the transmitted television signal. In the A-MAS and C-MAS systems, digital signals are placed on their 7,25 MHz subcarrier, while in the A-MAS system they are transmitted continuously. Digital packet signals are a stream of bits that control the carrier phase of the television signal, which can take two or four fixed values. The A-MAC system is single-channel. in BD versions, up to eight channels of sound can be organized. In the receiver, digital sound signals are separated from digital information, entered into buffer memory, from which they are read for digital-to-analog conversion at normal speed. The MAC system has not stood the test of time. In the summer of 1999, out of more than 5000 satellite channels, only 56 operated in the D2-MAC standard and 20 in the B-MAC standard. The HD-MAC and HD-B-MAC variants refer to high definition television systems (HDTV or HDTV) with 1250 scan lines. They retain the principles used in previous versions: digital audio and time-separated analog luminance and chrominance signals. More details about the MAC system are written in [2 and 3]. The MUSE (Multiple Sub-Nyquist Sampling Encoding) system is developed and used in only one TV channel in Japan. In it, as in the MAC system, analog signals of brightness and color are transmitted with digital signals of sound and digital information. Like HD-MAC, it is a high definition system (1125 lines) The audio signal in the MUSE system, together with digital information, is transmitted in the blanking intervals of the image fields using quadruple carrier phase modulation at a transmission rate of 2,048 Mbps. More detailed information about the system is contained in [3]. There are also widely used digital television information compression systems MPEG (Moving Picture Experts Group - a development carried out by a group of moving picture experts): MPEG-1, MPEG-2, MPEG-4. Their description is given in [2 and 4]. In television broadcasting, information is compressed according to the MPEG-2 standard system, which is used when scanning up to 625 lines. It consists of standards of 20 levels of complexity, allowing you to create algorithms for compressing information in systems for various purposes. The audio part of the standard is the MUSICAM (MPEG-Audio) information compression system for audio channels, which allows processing up to six high-quality wideband audio channels. MPEG are lower level digital television standards. In addition to them, there is also a set of harmonized standards that ensure the transmission of several television programs in one frequency satellite (DVB-S), cable (DVB-C) or terrestrial (DVB-T) channels. To resolve the contradiction between picture and mono sound, stationary TVs sometimes use a "mono surround" system, consisting of two loudspeakers located on the sides of the screen. In high-end TVs, external speaker systems (ACs) are added to them. In foreign-made equipment, as a rule, the same type of small-sized broadband sound emitters are used for this purpose. In TV sets produced in the former USSR, a broadband head with a power of 3 ... 4 W was usually installed on the right side of the case, and a high-frequency, lower power one on the left. Both loudspeakers were connected in parallel to the output of a common 3H amplifier. At the same time, the sound expanded spatially. At the same time, a pseudo-stereophonic effect of separation of reproducible frequencies in the space in front of the viewer was partially achieved, which improved the perception of the sound picture. But the placement of several sound emitters in a common open TV case could not create a tangible expansion of the sound volume. It is possible to improve the quality of reproduction of monophonic programs using monoambiphony methods, when an audio signal is fed to one emitter without additional processing, and to the other after a certain delay. This allows you to improve the acoustic properties of the room, giving it the desired boom. This method has not found wide application in monophonic television and has been in demand only recently in systems with multi-channel surround sound. You can use another method - pseudo stereophony with a spatial separation of the frequency spectrum of the sound, feeding low frequencies to the right speaker, and high frequencies to the left. As for two-channel stereophonic sound reproduction systems, there are two main options for their construction: simple and extended stereo. In the first case, the audio signals received through the channels L and R. after amplification are transmitted to the speakers without additional processing. The disadvantage of such systems is well known - a narrow spatial sound panorama unfolds not around the listener, but in front of him in the form of a flat sound wall. An attempt to expand it by spreading the speaker leads to the appearance of a clearly perceived failure in the center of the sound "picture". Extended stereo increases the size of the stereo image by passing part of the L signal to the R channel, and vice versa. If the transmitted signals are subjected to phase and time processing (delay), the sound panorama can be significantly expanded even when the sound emitters are located in a common housing at a small distance from each other. There are two main options for such a system: ISS (Incredible Surround Sound - incredibly surround sound) and the Qsound system. In both cases, audio signals are processed by microcircuits - sound processors (SP), which provide volume, balance, treble and bass control. They also handle mono, pseudostereo, simple stereo, and extended stereo audio. A number of microcircuits have appeared that implement these functions. This is TDA8421/24/25/26, TDA9860/61, CXA1735AS, LMC1982CIN/CIV with I2C digital bus control. These include the TDA3810 processor, which performs only regime processing of signals without their adjustment. ZP is quite widely used in TVs of different companies. So, the TDA8425 chip is installed in the TVT-C24F4R TV and forms a pseudo stereophony mode in it when receiving Terrestrial signals of the SECAM-D / K system [5]. It is also used in the PHILIPS-FL receiver [6]. The CXA1735AS processor works in the PANASONIC-TX-28WG25C (ODD) digital TV [7]. The SONY-KV-28WS4R TV contains the MSP3410 chip, which combines the functions of the RFP and the NICAM system decoder [7]. An interesting solution for the low-frequency part of the sound path is used in the PHILIPS - FL TV. It has a converter of a two-channel audio signal into a five-channel one with a pseudo-quadraphonic conversion algorithm. Its block diagram is shown in Fig. 3.
From the source of analog signals or from the NICAM decoder, the stereo signals L and R enter the DA1 RFP, from it directly to the 3H amplifiers A1 and A3, and then to the AC L and R connected to them. In parallel, they come to the adders S1 and S2, in which L+R and LR signals are formed. The first of them passes through a low-pass filter through amplifier A2 to the central speaker M. The signal LR after amplifier A4 enters the rear left and right AC SL and SR, connected in series with oppositely connected windings. This ensures the antiphase of the signals arriving at the AC. Extended stereo and pseudo-quadraphony systems improved the quality of sound reproduction, but failed to solve the problem of obtaining high-quality sound. Today it is formulated as follows: the sound field should be voluminous, envelop the listener from all sides and from above, ensuring that directions to apparent sound sources coincide with their actual position in space during transmission. The problem of reproducing such sound was first solved in cinema when multichannel surround sound systems appeared in concert halls - Dolby systems. Surround, THX and CS. At the same time, home video recording equipment on magnetic tape in the VHS format, which was widely used, led to the mass transfer of movies to video cassettes for home viewing. At the same time, naturally, the need arose to preserve surround sound when dubbing a movie onto a video cassette. This has led to the creation of Dolby Surround video variants - four-channel Dolby Pro Logic Surround with analog representation of audio signals and six-channel Dolby Digital with digital representation. Dolby Pro Logic Surround converts multi-channel audio information to two-channel when recorded on tape and converts it back to multi-channel at the viewer. Sound information is folded and unfolded according to an algorithm that is more complex than that used in pseudoquadraphony. Of the available sources, the most complete description of the principles of operation of this system can be found in [8]. The conversion on the receiving side takes place in the audio decoder (DZ). An example of using the Dolby Pro Logic Surround system is the SONY-KV-28WS4R TV [7]. in which the DZ is the TC9337F-015 chip. There are other similar chips. For example. NJW1102AF. The acoustic system of the KV-28WS4R model is built similarly to that considered according to the diagram in fig. 3. To emphasize the stereo effect and better localize the direction to the sound source, the remote sensing adjusts the gain of the amplifiers in all channels so that it remains unchanged in the channel with the maximum signal level and is reduced in the rest. There are other options for constructing the acoustic part of the apparatus with surround sound. An additional broadband speaker is sometimes installed in the center above the TV to reproduce sound from sources moving vertically. Rear speakers can be located not behind the viewer, but on the side, in line with him. Instead of mono, pseudo stereo signals can be fed to them. The logical conclusion of the process of improving sound reproduction systems in television was the creation of the concept of a home video theater. Its composition and capabilities are described in detail in [8 - 10]. Its video part is a large-screen TV or video projector, a high-end VCR, equipment for receiving satellite programs. The audio part is a multi-channel amplifier with multi-mode RF and DZ, a set of speakers. So what can radio amateurs do to improve television sound reproduction? First, I recommend implementing the existing ability to view videos with stereo sound. True, this will require a music center or any stereo installation, a VCR with a stereo path and video cassettes with STEREO, DOLBY STEREO indices. Useful practical advice can be found in [11]. If you go further along this path, you will also get surround sound recorded on video cassettes with the DOLBY SURROUND index in the DOLBY Pro Logic version. But this will entail a serious alteration of the audio system: remote sensing, a four-channel amplifier and five external speakers will be needed. Secondly, one can limit oneself to pseudo-stereo reproduction of the sound accompaniment of on-air and cable programs. But for this you will have to modify the audio path of the TV by introducing into it the RFP, the second 3H amplifier and speakers. More detailed information about the RFP is given in [12]. Literature
Author: V.Brylov, Moscow
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