In recent years, with the development of image sensor technology, the extensive use of high-definition products are based on network transmission technology, direct transmission in the HD era almost disappeared, but the debate has never been heard on the monitor image transmission is suitable for network transmission. Find a line with the practical application of technology to meet the costs as low as possible mode of transmission is particularly critical for monitoring applications.

Analog signal transmission

The vast majority of SD video signal is still using the analog signal transmission, bandwidth is 6M, application of 75-5 coaxial cable of 100 meters, at the expense of some high frequency details of the case, can transmit 200-300 m, and beyond . This performance, from the beginning of the monitoring application, this transmission mode has been there is no changed. But the HD era, 27M bandwidth due to the expansion of the transmission of high-definition signal bandwidth 1080P 75-5 traditional coaxial cable, a few meters on the attenuation down, can not be used in the monitoring project, and high-definition network camera transmission and SD compared to nothing changes, so when the HD era comes, it becomes a one-man show of the network camera directly transfer mode there is little application.

Digital transmission mode

Since the HD analog transmission can not be used in the monitoring of the practical application of, the only way to digital transmission. Analysis of the digital features of the HD signal, before the discussion of digital high definition transmission technology, the bandwidth of the signal of the 30 frames / sec 1080P image: 1920 × 1080 × 16 × 30 = 995.328M; 30 frames 720P image signal bandwidth: 1280 × 720 × 16 × 30 = 442.368M, that is to say with some blanking data, 30 / s 1080P image is no more than 1G, the 720P does not exceed 500M, in the field of data communication technology, to achieve such a large data traffic transmission. common coaxial cable, twisted pair and fiber optic media.

Coaxial cable transmission technology

Coaxial cable transmission technology in the SD era is widely used in standard-definition analog video signal transmission, and standard-definition analog video signal corresponding to the digital video signal standard SD-SDI interface (270 Mb / s), this interface due to cost and performance reasons rarely used in monitoring. The standard of high-definition digital signal is HD-SDI (of 1.485Gb / s), under the premise of application and descrambling technology, the originator of the pre-emphasis to the receiving end of equalization techniques, the transmission distance can be extended to 100 meters.

Plus descrambling technology is scrambling zero inverted (NRZI) instead of the early block coding. Scrambler before transmission, the original data stream, and transform the NRZI code, to ensure reliable recovery of the raw data at the receiving end. This concept of digital serial interface can be understood as a base band signal modulation. NRZI code is polarity-sensitive code. "1" and "0" level of high and low, if there is a long period of consecutive "1" or a "0", it will affect the receiving end to extract the clock from the digital signal. Because the serial digital signal interface does not separate the transmit clock signal, the receiver needs to extract the clock signal from the digital signal stream, so "1" and "0" to indicate whether the level-shifting NRZI code. NRZI stream is receiving, as long as the detection level transformation, we can recover data even if the signal frequency caused by the "1" signal, only the original clock frequency, and then through scrambling and continuous "1" reduce the chances of would cause the high frequency components to be further reduced. The receiving end of the data stream from the NRZI bit stream by the SDI decoder to restore the original data stream

Pre-emphasis is the originator of the signal amplification and equalization technology can be understood as the frequency compensation, usually filter through the filter to compensate for the distortion of the pulse, the judgment obtained by the demodulator output samples, as amended by the equalizer, had cleared the sample after the inter-symbol interference. Adaptive equalizer directly from the actual digital signal transmission according to some algorithm, adjusting the gain, which can adapt to random changes in the channel equalizer always maintain the best condition, and thus the distortion compensation performance. Adaptive equalizer typically includes two operating modes, training mode and tracking mode

The training mode first to launch a known fixed-length training sequence from the transmitter, so that the equalizer at the receiver can make the correct settings. The typical training sequence is a binary random signal or a bunch of pre-specified data bit immediately after the training sequence transmitted user data. Equalizer at the receiver through a recursive algorithm to evaluate the channel characteristics, and the correction filter coefficients to the channel to compensate. In the design of training sequences required to do even in the worst channel conditions, the equalizer through the training sequence to obtain the correct filter coefficients. This after receiving the training sequence, making the equalizer filter coefficients is close to the optimal value. Receive data, the equalizer of the adaptive algorithm can track the changing channel, the adaptive equalizer will continue to change its filtering features to protect the stability of data transmission.

But pre-emphasis and equalization techniques to extend the distance of the SDI signal transmission still exist many problems in practical applications, the characteristics of the cable transmission distance, coupled with the coaxial cable is a single-ended transmission system can not suppress total mode interference. The analog control in the project site if the line, it might lead to interference, which will seriously affect the SDI signal transmission distance is not difficult to understand why no agreement for the HD-SDI transmission distance. The theoretical transmission distance and the actual difference is too large, and most tangled HD-SDI signals over the transmission distance, the receiving end will not be able to solve any signal, which is far worse performance than the traditional standard-definition analog transmission system. HD-SDI transmission technology to a large area used to monitor the system still has many problems and difficulties