On the market has been applied to monitor the optical fiber digital transmission equipment of the industry, from the development trend of the future look, at this stage of optical fiber digital transmission is still a conceptual transition products. Optical fiber analog transmission equipment because of its advantages of real-time lossless transmission mature technology and low price, is still a good means of transport. On optical analog transmission scheme is discussed and engineering applications, analog transmission and optical fiber digital transmission and optical fiber to make a comparison.
Analog transmission scheme
1. Choose AM or FM?
Analog optical fiber transmission technology is divided into two major categories of AM and FM modulation and its modulation method is shown in table 1. Because AM modulation technology are more mature than FM technology, and the structure is simple, so early in the remote monitoring of transmission engineering, mainly AM modulation technology. But FM modulation strong anti-interference ability, high fidelity, in linear good medium transmission requirements for nonlinear distortion is not high, with the mature of FM modulation technology, the improvement of precision optical components, electronic parts, FM modulation gradually replace AM modulation has become the mainstream in the market.
2. The transmission distance
Video analog signals in the optical fiber transmission are two key indicators: S/N and DG, represent the transmission signal noise size and linear. General TV transmission line standards can be divided into two categories: one is the high standards of information transmission lines like room, ask S/N (peak - peak/RMS) = 56 dB, differential gain DG = 0.3 dB; The other is ordinary quality user network, S/N (peak - peak/RMS) = 40 db, differential gain DG = 2 db. Fiber analog TV transmission system is in accordance with the requirements for these two kinds of design respectively, simulation of closed-circuit television (CCTV) optical fiber transmission technology standard is designed according to the information transmission lines like room standard.
In long distance transmission design, should give full consideration to optical transmission equipment manufacturers to provide technical manuals. If the transmission distance is too long or certain indicators beyond normal standard, should be clarified. Force transmission distances application: analog TV signals over long distances, due to the accumulation of noise linear distortion, cause the terminal image flicker, vague concepts, outline is not clear, the image quality degradation, if looking at the screen for a long time will cause harm to the human eye. One-sided to strengthen the effect of fiber optical power, although in a short period of time image can reach an acceptable level to the naked eye, but along with the time growth, accompanied by the ageing of the photoelectric device, image quality is in a relatively short period of time drops rapidly to the point of hard to accept. So required at the end of the project, should use a dedicated video test instrument, such as vector oscilloscope, etc.) test whether or not the actual effect observation indexes meet the design index.
3. The safety of the transmission system
Telecommunications on optical fiber transmission equipment, to protect the safety of the system, regular light path rearrangement of the system, to ensure the reliable transmission; Closed-circuit television (CCTV) in optical fiber transmission system, the basic is a point-to-point transmission, there is no rearrangement of equipment, so the security of the system as a whole should be considered in design.
With the optical fiber prices fall further, should try to avoid using the large capacity (e.g., 32 road 16 road video audio equipment) of the optical fiber transmission equipment, and USES the small capacity (such as 8 road, even four-way audio equipment) transmission equipment to ensure the safety of the system as a whole. So, although temporary cost will increase a little, but from a long-term point of view, to cost in safety practice is worth it.
Design also want to consider the maintainability of the system, such as system lightning protection, electric plug, take self recovery fuse; Simple structure, low power consumption; Can be adaptive to adjust output optical power without distance and so on.
Three kinds of special engineering application
1. RS - 485 bus control
Due to the RS - 485 bus transmission distance long, industrial control, the RS - 485 bus mode control many sets of industrial equipment, some applications require the use of secondary transmission signal or half-duplex RS - 485 four wire transmission signal full duplex RS - 485.
2. The COAX control
Along with the development of the monitor market, fast ball camera has gradually become the hotspot of application, which used line type has two kinds: one kind is the control signal is separated from video signal lines; The other is a video signals and control signals share a coaxial cable, commonly known as coaxial visual control. General optical fiber transmission equipment of the control signal and video signal is separated, but in order to meet the fast ball camera, also must be developed coaxial visual control of the optical fiber transmission equipment.
3. Data held hands connection
The matrix code output port is less, so to control more equipment distributor must be increased when the code.
Analog optical fiber transmission and digital optical fiber transmission
As mentioned earlier, monitoring industry, digital optical fiber transmission is the trend of future development, and the existing digital optical fiber transmission system on the market just conceptual transition products, generally can be divided into two kinds: one kind is compressed digital optical fiber transmission; Another kind is compressed digital transmission. Their applications are point-to-point transmission, the same as the analog optical fiber transmission mode.
