[ Huaqiang Security Network News ] The application of high-definition surveillance has changed the high-definition era of the direct transmission method. Due to the application of a large number of high-definition surveillance products, security vendors are looking for a way to match the actual application, the technology can meet, and the cost is as low as possible. Transfer mode. At present, according to the China Security First Network, the hot security network technical personnel introduced, the new mode of high-definition surveillance transmission, now mainly: analog signal transmission mode, digital transmission mode, coaxial cable transmission technology three transmission modes. 
Analog signal transmission
At present, most of the standard definition video signals still use analog signal transmission mode, the bandwidth is 6M, and the coaxial cable of 75-5 is used to transmit 100 meters. In the case of sacrificing some high frequency details, it can transmit 200-300 meters or even further. . Due to this performance, this transmission mode has not changed since the beginning of the monitoring application. However, in the high-definition era, due to the expansion of the transmission of high-definition signal bandwidth, 1080P requires 27M bandwidth, transmitted by the traditional 75-5 coaxial cable, a few meters will be attenuated, can not be used in the monitoring project, and HD network camera The transmission method has no change compared with the standard definition, so when the high-definition era comes, it becomes a one-man show of the network camera, and the direct transmission mode has few applications.
Digital transmission mode
Since the HD analog transmission method cannot be used in monitoring actual applications, the only method is digital transmission. Before discussing the digital high-definition transmission technology, the digital characteristics of the high-definition signal are analyzed. The bandwidth of the 30-frame/second 1080P image signal is: 1920×1080×16×30=995.328M; the bandwidth of the 30-frame 720P image signal is: 1280×720× 16×30=442.368M, that is to say, with some blanking data, the image of 30 frames/second 1080P does not exceed 1G, and the 720P does not exceed 500M. In the field of data communication technology, there are many ways to realize such large data traffic. Commonly used media are coaxial cable, twisted pair and fiber.
Coaxial cable transmission technology
Coaxial cable transmission technology is widely used in standard-definition digital video transmission. The digital video signal standard corresponding to standard-definition analog video signal is SD-SDI interface (270Mb/s). This interface is cost and performance. It is rarely used in monitoring. The standard for high-definition digital signals is HD-SDI (1.485Gb/s). The transmission distance can be extended to 100 meters under the premise of applying de-scrambling technology, starting pre-emphasis technology and receiving equalization technology.
The descrambling technique replaces the early block coding with a non-return-to-zero inversion (NRZI) of the scrambling code. Prior to transmission, the original data stream is scrambled and converted to an NRZI code to ensure reliable recovery of the original data at the receiving end. This conceptually understands the digital serial interface as a baseband signal modulation. The NRZI code is a polarity sensitive code. Use "1" and "0" to indicate the high and low levels. If there is a continuous "1" or continuous "0" for a long time, it will affect the receiver to extract the clock from the digital signal. Since the serial digital signal interface does not separately transmit the clock signal, the receiving end needs to extract the clock signal from the digital signal stream, so the NRZI code indicating whether there is level conversion by "1" and "0" is used. When receiving the NRZI code stream, as long as the level change is detected, the data can be recovered. Even if it is all "1" signal, the signal frequency is only half of the original clock frequency. After scrambling, the chance of continuous "1" is reduced. Therefore, the high frequency component is further reduced. At the receiving end of the data stream, the original data stream is recovered from the NRZI code stream by the SDI decoder.
The pre-emphasis technique is to amplify the signal at the origin. The equalization technique can be understood as frequency compensation, which is usually realized by using a filter. The filter is used to compensate the distortion pulse, and the demodulated output sample obtained by the decider is corrected by the equalizer. Passed or cleared samples after intersymbol interference. The adaptive equalizer continuously adjusts the gain from the actual digital signal transmitted according to an algorithm, and thus can adapt to the random variation of the channel, so that the equalizer always maintains an optimal state, thereby having better distortion compensation performance. The adaptive equalizer generally includes two working modes, a training mode and a tracking mode.
The training mode first transmits a known fixed length training sequence by the transmitter so that the equalizer at the receiver can make the correct settings. A typical training sequence is a binary random signal or a sequence of pre-specified data bits, and the user data is transmitted immediately after the training sequence. The equalizer at the receiver will evaluate the channel characteristics by a recursive algorithm and modify the filter coefficients to compensate for the channel. When designing the training sequence, it is required that the equalizer can obtain the correct filter coefficients through this training sequence even under the worst channel conditions. This allows the equalizer's filter coefficients to be close to the optimal value after the training sequence is received. When receiving data, the equalizer's adaptive algorithm can track the changing channel, and the adaptive equalizer will constantly change its filtering characteristics to ensure the stability of data transmission.
However, the pre-emphasis and equalization technology to extend the signal transmission distance of SDI still has many problems in practical applications. The characteristics of the cable have a great influence on the transmission distance. In addition, the coaxial cable transmission is a single-ended transmission system that cannot suppress the total. Mode interference. In the analog monitoring, if the line is not good, it will cause interference. These will seriously affect the transmission distance of the SDI signal. It is not difficult to understand why the current HD-SDI transmission distance is inconsistent. The theoretical transmission distance and the actual difference are too large, and the most entangled is that if the HD-SDI signal exceeds its transmission distance, the receiving end cannot solve any signal, which is far worse than the performance of the traditional standard definition analog transmission system. Therefore, there are still many problems and difficulties in the application of HD-SDI transmission technology to a large-scale monitoring system.
At present, most of the standard definition video signals still use analog signal transmission mode, the bandwidth is 6M, and the coaxial cable of 75-5 is used to transmit 100 meters. In the case of sacrificing some high frequency details, it can transmit 200-300 meters or even further. . Due to this performance, this transmission mode has not changed since the beginning of the monitoring application. However, in the high-definition era, due to the expansion of the transmission of high-definition signal bandwidth, 1080P requires 27M bandwidth, transmitted by the traditional 75-5 coaxial cable, a few meters will be attenuated, can not be used in the monitoring project, and HD network camera The transmission method has no change compared with the standard definition, so when the high-definition era comes, it becomes a one-man show of the network camera, and the direct transmission mode has few applications.
Digital transmission mode
Since the HD analog transmission method cannot be used in monitoring actual applications, the only method is digital transmission. Before discussing the digital high-definition transmission technology, the digital characteristics of the high-definition signal are analyzed. The bandwidth of the 30-frame/second 1080P image signal is: 1920×1080×16×30=995.328M; the bandwidth of the 30-frame 720P image signal is: 1280×720× 16×30=442.368M, that is to say, with some blanking data, the image of 30 frames/second 1080P does not exceed 1G, and the 720P does not exceed 500M. In the field of data communication technology, there are many ways to realize such large data traffic. Commonly used media are coaxial cable, twisted pair and fiber.
Coaxial cable transmission technology
Coaxial cable transmission technology is widely used in standard-definition digital video transmission. The digital video signal standard corresponding to standard-definition analog video signal is SD-SDI interface (270Mb/s). This interface is cost and performance. It is rarely used in monitoring. The standard for high-definition digital signals is HD-SDI (1.485Gb/s). The transmission distance can be extended to 100 meters under the premise of applying de-scrambling technology, starting pre-emphasis technology and receiving equalization technology.
The descrambling technique replaces the early block coding with a non-return-to-zero inversion (NRZI) of the scrambling code. Prior to transmission, the original data stream is scrambled and converted to an NRZI code to ensure reliable recovery of the original data at the receiving end. This conceptually understands the digital serial interface as a baseband signal modulation. The NRZI code is a polarity sensitive code. Use "1" and "0" to indicate the high and low levels. If there is a continuous "1" or continuous "0" for a long time, it will affect the receiver to extract the clock from the digital signal. Since the serial digital signal interface does not separately transmit the clock signal, the receiving end needs to extract the clock signal from the digital signal stream, so the NRZI code indicating whether there is level conversion by "1" and "0" is used. When receiving the NRZI code stream, as long as the level change is detected, the data can be recovered. Even if it is all "1" signal, the signal frequency is only half of the original clock frequency. After scrambling, the chance of continuous "1" is reduced. Therefore, the high frequency component is further reduced. At the receiving end of the data stream, the original data stream is recovered from the NRZI code stream by the SDI decoder.
The pre-emphasis technique is to amplify the signal at the origin. The equalization technique can be understood as frequency compensation, which is usually realized by using a filter. The filter is used to compensate the distortion pulse, and the demodulated output sample obtained by the decider is corrected by the equalizer. Passed or cleared samples after intersymbol interference. The adaptive equalizer continuously adjusts the gain from the actual digital signal transmitted according to an algorithm, and thus can adapt to the random variation of the channel, so that the equalizer always maintains an optimal state, thereby having better distortion compensation performance. The adaptive equalizer generally includes two working modes, a training mode and a tracking mode.
The training mode first transmits a known fixed length training sequence by the transmitter so that the equalizer at the receiver can make the correct settings. A typical training sequence is a binary random signal or a sequence of pre-specified data bits, and the user data is transmitted immediately after the training sequence. The equalizer at the receiver will evaluate the channel characteristics by a recursive algorithm and modify the filter coefficients to compensate for the channel. When designing the training sequence, it is required that the equalizer can obtain the correct filter coefficients through this training sequence even under the worst channel conditions. This allows the equalizer's filter coefficients to be close to the optimal value after the training sequence is received. When receiving data, the equalizer's adaptive algorithm can track the changing channel, and the adaptive equalizer will constantly change its filtering characteristics to ensure the stability of data transmission.
However, the pre-emphasis and equalization technology to extend the signal transmission distance of SDI still has many problems in practical applications. The characteristics of the cable have a great influence on the transmission distance. In addition, the coaxial cable transmission is a single-ended transmission system that cannot suppress the total. Mode interference. In the analog monitoring, if the line is not good, it will cause interference. These will seriously affect the transmission distance of the SDI signal. It is not difficult to understand why the current HD-SDI transmission distance is inconsistent. The theoretical transmission distance and the actual difference are too large, and the most entangled is that if the HD-SDI signal exceeds its transmission distance, the receiving end cannot solve any signal, which is far worse than the performance of the traditional standard definition analog transmission system. Therefore, there are still many problems and difficulties in the application of HD-SDI transmission technology to a large-scale monitoring system.
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