TV Terminology
Explanation of terms
1. Differential phase: Differential phase refers to the phase change of the color carrier component caused by the change of the luminance signal amplitude related to chroma. In the NTSC system, the change of the color signal vector angle represents the change of the hue, so the effect of the differential phase on the signal is very serious. The PAL system uses the line-by-line inversion technique, so the self-compensation effect causes the change in color saturation to replace the change in hue. In general, differential phase is a parameter used to describe the effect of changes in the amplitude of the luminance signal on the color tone. Second, differential gain: Differential gain refers to the functional relationship between the change in the amplitude of the chrominance signal and the change in the amplitude of the brightness signal. Simply put: Differential gain is the effect of changes in the brightness signal amplitude on color saturation. 3. Color-bright crosstalk: Color-bright crosstalk is the opposite of the differential gain. It represents the relationship between the amplitude of the luminance signal and the amplitude of the secondary chroma wave. Fourth, r (flail horse) correction: The so-called flail horse correction is to detect the dark part and the light part of the image signal, and increase the ratio of the two, thereby improving the image contrast effect. V. Surface Acoustic Wave Filter (SAWF)The surface acoustic wave filter is made of quartz, lithium niobate, and barium titanate crystals with piezoelectric effect. The so-called piezoelectric effect is the phenomenon that when a crystal is subjected to mechanical action, an electric field proportional to the pressure will be generated. When a crystal with piezoelectric effect is subjected to an electrical signal, it also produces elastic deformation and emits a mechanical wave (acoustic wave), which can convert the electrical signal into an acoustic signal. Since this acoustic wave only propagates on the crystal surface, it is called surface acoustic wave. The abbreviation for SAW filter is SAWF. SAW filter has small size, light weight, reliable performance, and does not require complicated adjustment. The key device to realize adjacent frequency transmission in the cable television system. The characteristics of SAW filters are: (1) The frequency response is flat, the unevenness is only ± 0.3- ± 0.5dB, and the group delay is ± 30- ± 50ns. (2) The SAWF rectangular coefficient is good, and the out-of-band suppression can reach more than 40dB. (3) Although the insertion loss is as high as 25-30dB, the amplifier can be used to compensate the level loss. SAW filters include surface acoustic wave TV image intermediate frequency filters, TV sound filters, and TV channel residual sideband filters. The typical technical specifications of SAW filters are shown in the table below.
6. Comb filter Comb filter is composed of many passbands and stopbands arranged at the same frequency interval, allowing only signals of certain frequency range to pass. The characteristic curve of the comb filter is like a comb, so it is called a comb filter. Comb filters have many applications in television technology. The comb filter is used to separate the two orthogonal components of the chrominance signal, the U color difference signal and the V color difference signal. The comb filter is generally composed of a delay, an adder, a subtractor, and a band-pass filter. For still images, comb filtering is performed between frames, that is, three-dimensional comb filtering. For moving images, comb filtering is performed within the frame, that is, two-dimensional comb filtering. Except for occasions with special requirements, most digital TV equipment or high-quality digital TV receivers use a line delay comb filter and a band-pass filter cascade to form a Y and C separation scheme to obtain a satisfactory image quality. The use of comb filters significantly improves the image quality. Solve the interference light spot and interference pattern caused by the bright color string and bright string color; eliminate the color edge creep caused by U and V mixing; eliminate the bright and color fringing.
7. Attenuator Within a specified frequency range, a circuit used to introduce a predetermined attenuation. It is generally indicated by the decibel number of the attenuation introduced and the ohm number of its characteristic impedance. Attenuators are widely used in cable television systems to meet the level requirements of multiple ports. Such as the control of the input and output levels of the amplifier, the control of the amount of branch attenuation. There are two types of attenuators: passive attenuator and active attenuator. The active attenuator is combined with other thermal components to form a variable attenuator, which is used in the automatic gain or slope control circuit in the amplifier. Passive attenuators have fixed attenuators and adjustable attenuators. The fixed attenuator is composed of resistors and does not affect the frequency characteristics. It is usually composed of T-type or π-type networks; (for common 75Ω impedance T-type and π-type resistance data with different attenuations, please refer to the book of common antenna TV system); The device is composed of a potentiometer and is used in debugging and level adjustment. It is required that the input and output impedance of the attenuator should match the interface, and the cable system should be 75 ohms. The frequency characteristics of the attenuator must meet the frequency range requirements of the system, and the attenuation of the attenuator in the frequency range should be independent of the frequency. Therefore, commonly used resistance components. Different frequency ranges have different forms of attenuators. Useful coaxial cables are used as attenuators; in waveguide systems, diaphragms that absorb electric field energy are commonly used as attenuators; there are also solid-state diodes (such as PIN diodes) that make waveguides or coaxial systems in the microwave frequency band that can be electrically tuned Attenuator. Attenuators are commonly used in a variety of telecommunications equipment and electronic instruments.
8. Equalizer In telecommunication equipment, it is used to correct the attenuation (ie transmission loss) and phase difference networks caused by different frequencies. Those who can correct the relationship between attenuation and frequency are called "attenuation equalizer"; those who can correct the relationship between phase difference and frequency are called "phase equalizer". Equalizers are often used in cable television systems. The equalizer is usually connected in series in the amplifier circuit, which is set to balance the attenuation of the high-frequency and low-frequency signals caused by the balanced cable transmission. Because the attenuation characteristics of the cable increase with increasing frequency. Commonly used attenuation equalizer is also called amplitude equalizer. It is generally composed of coils, capacitors, resistors and other components. The characteristic impedance of the attenuation equalizer is equal to a fixed value, and its equalization value is the decibel difference of the attenuation between the high and low frequency reference points of the cable. The frequency characteristic of the equalizer is opposite to the frequency characteristic of the cable, but the frequency is low and the attenuation is high, the frequency is high The attenuation is small, using this opposite characteristic to play a balancing role. The equalizer is also often made into a small printed board plug-in structure, divided by the amount of equalization.
9. Mixer The device that mixes more than two sets of radio frequency programs (signals) with different frequencies to form a broadband RF (signal) multi-channel program output is a mixer. In the front-end of the cable TV system, the mixer is the distribution point of the system signal, that is, all the multi-channel RF signals processed by the technology are collected at the input end of the mixer, and then the signal output is distributed to the system network and sent to the user at the output end of the mixer. . The main technical requirements of the mixer. Operating frequency: If the mixer is broadband, the frequency should meet the requirements of the entire frequency band in the system. If the mixer is a channel type, the frequency should meet the requirements of each channel to be mixed; access loss: When the signal passes through a passive network, it is always hoped that the access loss (insertion loss) is as small as possible. The ratio of the input power to the output power of the mixer is called the access loss of the mixer. Access loss is usually expressed in decibels. When expressed in decibels, it is the difference between the decibel level at the input and the decibel level at the output. Different mixers have different access losses; input and output impedance: In order to match each interface in the entire system, the input and output impedance of the mixer should be 75 ohms; the input terminals are isolated from each other; in ideal In this case, when a signal is added to any input terminal of the mixer, the signal cannot appear at the other input terminals, and when any input terminal has an open circuit or a short circuit phenomenon, the other input terminals should not be affected. But in fact there is always a certain influence. In the case of matching between the terminals, a signal is added to an input terminal, and the difference between the signal level and the signal level appearing at other input terminals is the mutual isolation between the mixer input terminals, generally expressed in decibels . There are different requirements for different mixers, generally requiring greater than 20 decibels.
10. Intermodulation ratio (IM)
When the amplifier in the cable TV system amplifies the TV signals of multiple channels, due to the nonlinear effect of the amplifier (mainly the quadratic term), the transmitted signals are mixed with each other, and the sum frequency or difference frequency generated falls within the frequency range of the desired channel When the internal and useful signals enter the TV receiver, interference will occur, which is called intermodulation, or intermodulation for short. Intermodulation is closely related to frequency. Intermodulation interference, which produces moire or twill interference. The intermodulation ratio is defined as IM = 20lg carrier level effective value / intermodulation product effective value. The national standard stipulates that IM≥57dB, and it should be 58dB during design.
11. Crosstalk modulation ratio (CM)
When the amplifier in the cable TV system amplifies the TV signals of multiple channels, due to the influence of the nonlinear devices in the amplifier (mainly the third term), the image carrier of the desired channel is interfered by the amplitude change of the modulated wave of other (interfering) channels , This is called crosstalk modulation or cross modulation for short. A common phenomenon is that a negative image that interferes with the channel image appears on the background of the image to be received. Sometimes the horizontal synchronization signal of the interfering channel is flipped on the image screen to be received, becoming a vertical white bar, and moving left and right (fixed when the horizontal frequency is consistent), like the wiper of the front window of the car, so it is also called "wiper interference" .
Crosstalk modulation is the modulation of the interference signal transferred to the carrier of the useful signal. Define the cross-talk modulation: XM = 20lg peak-to-peak value of transfer modulation on the carrier under test / peak-to-peak modulation on the carrier under test is opposite to the cross-talk modulation definition, and define the cross-talk modulation ratio (CM): CM = 20lg under test The peak-to-peak value to be modulated on the carrier / peak-to-peak value of the transfer modulation on the carrier under test stipulates that CM ≥ 46dB, and the design should take 48dB.
12. Distributor The distributor is the most commonly used component in the distribution network of a cable TV transmission system, and is used to distribute signals. Its function is to divide an input signal equally into several outputs, usually there are two distribution, three distribution, four distribution, six distribution and so on. The frequency of the cable TV network continues to increase, and the functions continue to increase, so the requirements for the distributor continue to increase. The main technical requirements of the distributor. Frequency range: The distributor is used in the entire cable TV network, so it should have broadband frequency characteristics; Input and output impedance: The impedance of various RF interfaces in the cable TV network should be 75 ohms to achieve impedance matching, so the distributor input The impedance of both the end and the output should be 75 ohms; Distribution loss: In the system, it is always hoped that the loss of the access distributor is as small as possible. The distribution loss Ls is related to the distribution channel number n. In an ideal case, Ls = 10lgn. When n = 2, the distribution loss of the second distributor is 3dB. In fact, in addition to the loss of the halved signal, part of it is due to the tensile attenuation of the distribution device itself, so it is always larger than the calculated value. For example, the distribution loss of the second distributor is usually 3.5dB, and the loss of the 4 distributor is usually 8dB; mutual isolation: mutual isolation is also called distribution isolation. If a signal is added to one output terminal of the distributor, the difference between the signal level and the signal level of other output terminals is isolated from each other. Generally, the isolation of the output terminal of the distributor is required to be greater than 20dB. The greater the isolation of the output end of the distributor, the better, it means that the mutual effects and interference between the output ends of the distributor are small; VSWR: VSWR is an important indicator to measure the transmission quality of the distribution network, and it represents the degree of impedance matching. In an ideal situation, the input impedance and output impedance of the distributor and the impedance of the coaxial cable to which it is connected are completely equal. At this time, the standing wave ratio is 1, and in fact the standing wave ratio is often greater than 1. If the standing wave ratio is too large, the transmitted signal will be reflected at the input or output of the distributor, which will adversely affect the image quality, such as ghosting. Distributors are also divided into over-current distributors, outdoor distributors, indoor distributors, etc. in the project.
Thirteen, monaural A sound channel, the process of picking up sound with a microphone and playing sound with a speaker is called mono. In TV broadcasting, the quality of mono soundtrack is not good, especially when it comes to the advantages of literary and artistic programs, especially when it is live broadcast of high-level music performances. In addition, the mono sound can only be broadcast in one language, which is not applicable to a multi-ethnic and multi-dialect country in China. It should be developed as a dual sound (dual channel) to play two languages ​​at the same time, and also create conditions for stereo broadcasting. .
14. Dual Sound / Stereo Use dual channels to achieve dual-channel TV sound, and play sounds in two different languages ​​at the same time, such as standard Mandarin in one way; the local ethnic language or dialect in the other way, and the two can be free select. You can also use standard Mandarin or ethnic languages ​​all the way; use a foreign language on the other, and you can switch between the two at will. For example, when playing the original foreign TV movie, although Chinese subtitles can be played on the TV screen, it always looks unsmooth, and the double sound can overcome the defects of playing subtitles. Two channels create conditions for stereo broadcasting. Because people's ears can distinguish the distance and direction of each sound source, the listening sound has a sense of space (or three-dimensional). In a sound reproduction system, two or more sound channels are used to make the relative spatial position of the sound source felt by the listener close to the relative spatial position of the actual sound source. This kind of playback sound is called stereo sound. Stereo has two-channel stereo, four-channel stereo, Dolby stereo, Dolby surround sound, Dolby AC-3 digital surround sound and so on. Radio broadcasting using stereo technology is called stereo broadcasting. Take two-channel stereo broadcasting as an example. Two-channel stereo broadcasting is to broadcast two sound signals corresponding to two directions of the listener's front left and front right through one or two broadcast channels with different frequencies. The listener uses a stereo radio with a two-channel playback system to receive, and can identify the relative position of the sound source to produce a sense of stereo; the same program content can also be received with an ordinary radio, but there is no stereo sense. In order to meet monophonic compatibility, FM stereo broadcasting with a pilot system is mostly used. It uses only one FM broadcast channel, and uses the modulated basic sound frequency to send the "left plus right" signal, and the subcarrier AM band and pilot to send the "left minus right" signal. In theory, in order to obtain the best stereo effect, the ideal method is to use an infinite number of microphones to pick up the sound signal, and then use an infinite number of sound channels to transmit the sound to an infinite number of speakers and replay. As long as there is a one-to-one correspondence between the position of the speaker and the position of the microphone, the reproduced sound can accurately reproduce the sound of the scene, so that the listener has a three-dimensional sense of the immersive scene.
Fifteen, surround sound Surround sound refers to the direct sound and reflected sound to surround the listener's playback mode. Therefore, the more speakers, the stronger the sense of being surrounded by the listener. Two-channel stereo can only discern the relative position of sound sources. The four-channel stereo system uses four microphones and four speakers. Two of the four microphones are close to the stage, picking up the direct signal of the stage, and the other two are farther from the stage, picking up reverberation signals that reflect the effects of ambient sound. The signals picked up by the four microphones are transmitted to the four speakers by four independent sound channels. Corresponding to the position of the microphone, the speakers are front left, front right, rear left, and rear right; where the front left and front right are used to replay the direct sound of the stage, and the rear left and rear right are used to replay the mixture reflecting the environmental effects. sound. The listener has speakers in front and rear, which not only has a sense of presence in the horizontal direction, but also feels surrounded by sound, so it is also called surround sound.
16. Dolby AC-3 Digital Surround Sound AC-3 technology originated from providing high-quality sound for high definition (HDTV). AC-3 technology is developed from Dolby AC-1 and Dolby AC-2. AC-1 reduces the number of channels by half in a 4-2-4 multi-channel matrix (so that the transmission capacity can be reduced), and then uses incremental modulation (delta modulation) technology for digital encoding. Therefore, the compression factor of AC-1 is twice. With the advancement of sound coding technology and digital signal processor (DSP), the AC-1 system has developed into an AC-2 system based on transform coding technology. While improving quality, the compression factor has further changed to 4 times, but multi-channel Matrix processing technology remains. AC-3 is developed on the basis of AC-1 and AC-2. It inherits many advantages of AC-2, such as window processing, transform coding, adaptive bit allocation, etc., but also overcomes their deficiencies And limitations. AC-3 has 5.1 channels, that is, left, center, right, left surround, right surround and 0.1 low-effect sound channels. The left and right surround channels here are independent channels made separately, which are more lively and realistic. . Based on Dolby AC-3 technology, there are currently 6.1-channel and 7.1-channel digital sound return systems. AC-3 is the sound system of HDTV in the United States. This system has been recognized worldwide. Note: 4-2-4 represents four-channel recording when recording. When the recording is placed on film, the 4 channels are compressed to two channels by encoding technology. When the sound is restored, 4 sounds are still reproduced. Original sound.
17. Panda Sound (PANDA-1)
In the early 1980s, the American Wegener Communication Company designed a set of practical TV audio high-fidelity multi-channel multi-language audio demodulation circuit technology, which was named "PANDA-1" (Chinese translation "Panda-1" high-fidelity Language stereo sound), and has obtained technical patents in the United States. This sound technology is an analog noise suppression system. Its function is to transfer a normal sound, generally with a high frequency bandwidth of 280KHz, and it uses half the high frequency bandwidth of 130KHz to transmit the baseband audio signal with a bandwidth of 15KHz. The dynamic range of the audio signal is compressed at a specific ratio of tens of times to a very narrow bandwidth. In this way, each set of image programs can simultaneously provide up to six audio channels for stereo or bilingual broadcasting, and can simultaneously transmit independent radio programs, that is, three sets of stereo or six mono channels can be transmitted on different frequencies Road. PANDA-1 sound is used in analog channels and is also analog compressed, so the sound quality is quite different from the digital sound signals currently used, but because of the bandwidth savings, there are several more channels. To receive PANDA-1 sound, the receiver needs to have an expansion circuit audio carrier demodulator, otherwise the received sound is high noise and high distortion sound. Any receiver with PANDA-1 demodulation function can choose to listen to six accompanying sounds or two-channel stereo sound at will, through remote control operation, screen display, and audio programming reception.
18. NICAM NICAM is a quasi-instantaneous companding sound multiplexing. It is a digital sound processing technology. Its main features are high signal-to-noise ratio, wide dynamic range, and sound quality comparable to CD. Therefore, it is named Liyin. Called Liyin, Liyin is a common name for the digitalization of radio and television sound. In China's terrestrial broadcasting and satellite broadcasting, the frequency of TV sound is FM. Liyin adds a digital sound sub-carrier frequency on the basis of the original sound sub-carrier frequency. The sounds form a dual-carrier mode and do not interfere with the original mono signal. Using AM-FM, FM-FM playback mode. The Liyin mode has a stereo mode, that is, left and right channels; a dual voice mode, that is, two voices are transmitted from the left and right channels at the same time, or two mono broadcasts, or one mono broadcast and one data. When transmitting, it is processed with a special modulator and then transmitted together with the TV image signal and analog audio signal. When receiving, you can listen to the digital stereo sound program comparable to CD after processing with a dedicated Liyin demodulator. There are more than 20 kinds of Liyin systems in the world, and China adopts NICAM and D Liyin systems. On the basis of retaining the original analog FM audio secondary carrier frequency, a second secondary carrier frequency of 7.28MHz is added, and quasi-instantaneous companding coding technology is adopted. At present, in the first and second sets of satellite transmission systems of China Central Television, three sets of Central People's Broadcasting Station programs of Central One, Central Two, and Central Three are inserted using NICAM-728. When receiving satellite digital sound broadcasting signals, the satellite receiver needs to have a baseband output, connect it to the NICAM-728 receiver, and receive stereo signals after processing it, so that you can easily obtain high-quality CCTV broadcasting program signals source. The transmission of Liyin signals in the broadcasting of terrestrial TV stations marks the beginning of the era of digital stereo sound in TV sound. At present, some TV sets have the NICAM receiving function, but they need to be in the same format to receive Liyin. Otherwise, they cannot listen, and it is necessary to distinguish whether they meet the Chinese Liyin standard. Nineteen, DVB gateway function: convert IP stream, data file and other sources into DVB standard transport stream (TS) output. For example, when performing data compression, an 8-megabit TV channel can transmit 8 MPEG-2 TS streams, but the QPSK data broadcast signal cannot be compressed and transmitted together with the MPEG-2 signal because their digital encoding methods are inconsistent , So it can only occupy an 8 trillion TV channel alone, causing waste of resources. After having a gateway, you can multiplex QPSK data broadcast signals and IP stock information MPEG-4 or MPEG-1 and MPEG-2 data streams into a TS stream for playback, so that within an 8-megabit bandwidth You can watch MPEG-2 DVD digital TV programs, you can watch MPEG-1 VCD quasi video on demand programs, you can also use a computer (TV) to watch data broadcasts and real-time stock information. This greatly saves frequency resources. Transmission costs will also be reduced. The control system of the VCR and DVD player is different from that of the TV. The TV uses a CPU to complete the display, various control, tuning, remote control receiving and processing functions, and the VCR and DVD player are generally composed of two CPU to complete these functions. One is called the panel CPU, and the other is called the main control CPU. The two CPUs communicate through a data line and a clock line. The main functions of the panel CPU are: 1. Processing signals sent by the remote control or keyboard signals to process corresponding data signals to the main control CPU for analysis, and driving the display tube for corresponding display; 2. Timing function; 3. Receive various working status signals sent by the main control CPU and decode them and display them through the display screen. The main functions of the main control CPU are: receiving signals from the panel CPU and various peripheral sensors to complete the control and detection of various working states, such as completing the drum motor, spindle motor, loading motor, optical head focusing and tracking, etc. control. Understanding their division of labor is of great help to fault diagnosis and maintenance.
Color burst gate circuitWe know that the color burst signal and the chrominance signal are inconsistent in time, as shown in Figure CS, so we can use the time separation method to select the excellent sync signal. This is the color burst gate circuit. The color burst signal is an amplitude carrier pulse train located behind the horizontal sync signal. To separate it, a gate circuit is used. The time difference between the color burst signal and the chrominance signal is used. Usually the gate circuit is in the closed state, when the color burst The gate only opens when the signal arrives. This gate circuit needs to input a gate pulse to close and open the gate circuit. According to the above analysis, this gate pulse must arrive at the same time as the color burst signal in each row, that is, the color burst signal is Consistent. In this way, the color burst signal is just passed. The gate pulse one here uses the line mud pulse. This pulse and the color burst signal are not consistent in time, so the line mud must be delayed. Delay it to make the color burst signal consistent in time.
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