Decoder Explanation
Relay
Relay is the action generator in the decoder, usually the decoder will be matched with nine relays, of which there are five larger relays, which are to control the up and down, left and right movements of the head, and the remaining four smaller relays are to control the three-variable lens aperture, magnification, and focus. Relays such as Panasonic and Ohm are first-class products, but they are more expensive and are usually used in areas such as access control. Monitor the decoder used in the relay in two brands more, Songle, Huigang, the quality difference is not big, in actual use of the relay failure rate is not high, I personally think that basically negligible. However, when the decoder works, the relay has to open and close frequently, and each time it opens and closes, there is an instantaneous current through. If the relay capacity is too small, the instantaneous current may exceed the capacity of the relay, the service life of the relay will be greatly shortened, must be used more than 7A rated current relay, and some manufacturers are too much to pursue the cost of selecting 1A small relay. There is another issue to note, some decoders designed to eliminate the lens control of the three relays, with transistors and resistors instead of this design in use, the failure rate is not high, basically within the normal range, but I personally have reservations, transistors have a limited ability to drive, when used in relatively complex environments, may be unstable.
Communication chip
The communication chip of the decoder is mainly responsible for the reception of data signals on the RS485 bus, and the most commonly used one is the 75176 chip. The 75176 chip is a relatively old one, but with lower price and better stability, but it is poorer in the driving ability. As the decoder belongs to the RS485 unidirectional communication bus, it doesn't have the problem of driving ability. For the competitive market now, the low-cost 75176 chip has become the choice of more than 95% of the production decoder manufacturers. However, it is recommended not to use 75176 as the communication chip code converter, due to the limited driving ability, when controlling more than 32 sets of decoder operation, the failure rate is very high.
Lightning protection system
Better brand decoders usually come with a simple lightning protection system, which mainly prevents induced lightning from damaging the decoder through the RS485 bus. The standard lightning protection system price will be several times higher than the decoder itself, so usually designed relatively simple lightning protection system. Ltd. of Shenyang first Kai Li Ke decoder, for example, lightning protection part to be completed by two lightning protection diode, can prevent 10 ms 600W induction lightning damage, basically can resist the common induction lightning attack. When the induced lightning energy exceeds the limit of the lightning protection diode, the lightning protection diode will burn out, while absorbing the induced lightning energy, to protect the decoder internal devices and power transformers from being destroyed. After the lightning protection tube is burned, the decoder can work normally after the lightning protection tube is removed, but it no longer has the function of lightning protection. Due to the complexity of the environment in which the decoder is used, lightning strikes occur frequently, and it is recommended that when choosing a decoder, choose a product with a lightning protection system.
Insurance
The fuse in the decoder basically has 2 major roles:
1. Prevent the 220V power supply from short-circuiting during the construction process or causing harm to the personnel, when the 220V power supply is short-circuited or the personnel is electrocuted, the fuse will automatically cut off the 220V power supply, to ensure the safety of the personnel and equipment. Usually 220V fuse will be designed with a transparent plastic housing to prevent electric shock caused by the metal connector of the fuse.
2. To prevent the camera short circuit caused by the power transformer burned, this usually do not use the fuse holder shell.
In some of the more well-designed decoders, there are three fuses, adding a 24-volt output protection for the external output AC, to prevent short-circuiting caused by the head of the decoder internal devices and power transformer burned.
Driver chip NLU2003:
The action of the decoder is realized by the CPU controlling the suction and closure of the relay, but the driving ability of the CPU is generally not strong, and it is necessary to use the driver chip to realize the control of the relay, for example, the first Keriker decoder of Shenyang first Keriker company uses 2 pieces of NLU2003 driver chip, when the CPU sends out the action instruction, the NLU2003 chip will be the driver chip to realize the control of the relay. NLU2003 chip to drive the relay suction and disconnection, 2 NLU2003 respectively control the PTZ control relay and lens control relay.
First of all, observe the glossiness of the decoder board, whether it is full, whether the font printing is clear, whether the components are neatly arranged. Whether there is a Chinese wiring instructions, decoder board thickness is thick enough, the standard should be more than 1.5 mm, thick enough decoder board tension is strong enough to adapt to more complex environments, circuit board deformation rate is smaller, the edge of the circuit board with a hand touching the feeling of scratching, if there is a description of the production of the board cutter machine is not advanced enough.
The decoder usually has two parameters that need to be set manually: the baud rate and the address code. Baud rate refers to the communication rate when the decoder communicates with the DVR or matrix via 485 bus. Different communication protocols have different communication rates, but they can be roughly categorized into 1200, 2400, 4800 and 9600. They are usually set with a binary dipswitch, the specific setup method for each brand is different. Take the first Ke decoder as an example, *** counting 8 bits of dip switches, of which 1, 2 bits for the communication baud rate setting switch, 3-8 bits for the address setting switch.
Address code setting, because the 485 communication bus are generally connected to multiple decoders in order to distinguish them, respectively, each decoder to set an address. The address code is also expressed in binary.
Communication protocols:
The protocol is the information storage format for data communication. There are more than 100 kinds of communication protocols for decoders, and there are several versions of the more common ones, and the most commonly used one is the Pairco D protocol, which has about 7 versions. The most commonly used protocol, Pair of high D, there are about seven versions. Take Pair of high D as an example, analyze the data structure of the protocol.
The communication data is 7 bytes, respectively: 1-2-3-4-5-6-7
1, flag bit: to determine the type of protocol;
2, address bit: to determine the type of protocol;
2, address bit: to determine the type of protocol.
2, address bit: the decoder address corresponding to the action issued;
3, 4, action bit: the specific work instructions;
5, 6, speed and preset bit: defines the speed of the action and the preset to the coordinates;
7, check bit: check whether the data is correct.
Usually the decoder will recognize the type of protocol according to the format of the protocol, which is commonly referred to as the self-recognition protocol decoder, also known as the 10,000-code decoder. After the DVR sends out the control command, the serial interface RS-232 of the DVR will send out the control protocol code with the baud rate and data format of the protocol set by the software, and the RS-232 to RS-485 code converter will convert the RS-232 signal into RS-485 signal, and then transmit the RS-232 signal to the RS-485 signal. -485 signal, and then transmitted (theoretically can be transmitted 1200 meters), the system through the 485 bus each decoder will receive the control protocol code, and then compared to their own address, the control protocol code and the address is the same decoder will perform the action. The use of 232 to 485 code converter is mainly RS232 signal transmission distance is relatively close to about 15 meters, while the 485 signal can be transmitted 12,000 meters, so that the control distance is greatly lengthened. Embedded DVR itself has a 485 communication interface, do not have to add RS-232 to RS-485 pass-through protocol converter.
The communication between the DVR and the decoder is a one-way communication, the data DVR issued to the decoder to receive until the decoder, the decoder does not send any data to the DVR. Therefore, in order to reduce the cost of monitoring systems using a significant proportion of the code converter is a single-power code conversion, that is, from 232 signals into RS485 signals. As a result, using a surveillance system code converter in a two-way communication system such as access control may result in poor communication.
Decoder power supplies
Decoder power supplies generally use linear transformers with multiple voltage outputs, which can be roughly divided into three types:
1. EI-type transformers;
2. R-type transformers;
3. toroidal transformers.
EI-type transformer
EI-type iron core transformer manufacturing process is simple, relatively low cost. Because the iron chip and its matching coil skeleton have formed a series and mass production, so it is widely used.
EI-type core transformer core material used is very complex, there are hot rolled silicon steel plate (commonly known as low selenium), cold rolled non-oriented silicon steel strip, cold rolled oriented silicon steel strip (commonly known as high selenium). Now the use of cold-rolled non-oriented silicon steel strip is mainly 0.5 mm thick (commonly known as medium selenium), 0.35 mm (commonly known as domestic high selenium) has been eliminated. These materials in the cold-rolled oriented silicon steel strip material is best, its thickness is below 0.35 mm, cold-rolled non-oriented silicon steel strip followed by hot-rolled silicon steel plate material quality is the worst. Thus, the same size transformer, different core materials, transformer output power difference is large.
R-type transformers
R-type transformers than EI transformers 30% smaller, 40% thinner, 40% lighter.
R-type transformers have the smallest magnetic leakage, 10 times smaller than EI-type transformers.
R-type core transformers generate a minimum of 50% less heat than EI-type transformers.
R-type transformers do not generate noise, a feature that is far superior to EI-type transformers or cut core transformers with a gap in the core.
R-type transformers are more robust and reliable than toroidal transformers, with strong insulation and easy installation.
The construction of R-type transformers is simpler than EI and C-type transformers but the reliability and quality are higher than them.
Toroidal transformer
The core of the toroidal transformer is made of high-quality cold-rolled silicon steel sheet (sheet thickness is generally 0.35mm or less), seamlessly rolled, which makes its core performance superior to that of the traditional stacked-sheet core. The coil of toroidal transformer is evenly wound on the core, and the direction of the magnetic line of force generated by the coil is almost completely coincident with the magnetic circuit of the core, so the excitation energy and core loss will be reduced by 25% compared with the stacked piece type. Toroidal transformer has the following characteristics:
(1) high electric efficiency heart without air gap, stacking coefficient can be as high as 95% or more, core permeability can be taken 1.5 ~ 1.8T (stacked piece of core can only be taken 1.2 ~ 1.4T), the electric efficiency of up to 95% or more, no-load current is only stacked piece of 10%.
(2) small dimensions, light weight toroidal transformer than stacked transformer weight can be reduced by half, as long as the core cross-sectional area is equal to keep the toroidal transformer is easy to change the length of the core, width, height ratio, can be designed to meet the requirements of the dimensions.
(3) less magnetic interference toroidal transformer core has no air gap, the windings are uniformly wound in the ring on the core, this structure leads to a small leakage, electromagnetic radiation is also small, without the need to add another shielding can be used in high-sensitivity electronic equipment, such as the application of low-level amplifiers and medical equipment.
(4) less vibration noise, the core does not have an air gap to reduce noise.
Connection of RS485 communication line for decoder
Chain connection diagram is the standard wiring method, all decoders are connected to the RS485 bus, communication distance is far, data transmission is stable, the last decoder needs to be jumpered through the 120Ω resistor to improve the quality of communication, and it is recommended to utilize this wiring method during construction.
Star connection diagram in the decoder are alone through a RS485 bus with the sending device is connected, when the 485 communication data through the node to more than two directions, its transmission distance will be greatly reduced. It is recommended to avoid utilizing this wiring method during construction. If you use this wiring method, it is recommended to use RS485HUB to solve the problem.
External structure of decoder
The external structure of decoder can be divided into two kinds, the first one is painted iron box, which is more common and accounts for more than 85% of the decoder market. The second one is cast aluminum shell, which occupies 10% of the market and has the features of good waterproof and generous appearance. Due to the high cost of cast aluminum shell, the use of less, usually brand decoders will be selected.
In the indoor environment, there is no difference between the two shells of the decoder function. In the outdoor environment, there is no difference between the iron box decoder and the cast aluminum decoder function, but the cast aluminum shell decoder sealing is better can be placed directly on the ground, will not occur in water and other situations, while the iron box decoder due to sealing is not as good as the cast aluminum structure, is usually mounted on the wall or monitor pole, configure waterproof junction head, you can prevent the rain from entering the, due to the cast aluminum structure decoder is usually more than the price of the iron shell structure is higher than 1 times! Because the price of cast aluminum decoder is usually 1 times higher than iron shell structure, so in the middle and low end of the market, cast aluminum structure shell is not common anymore.
There are many kinds of outer structure of the iron shell, in which there is a kind of pull-out structure design is more successful, and now the application is also the most, the pull-out structure consists of two parts, one part is a lower end face of the empty iron box cube, the other part is like a drawer type L-type iron plate, the decoder board, power supply, etc., is installed in the iron plate, in the iron plate will be inserted into the iron box, fixed with galvanized (rust) screws, in the outlet port Configure 2 waterproof heads. The L-type iron plate can be inserted under the direction of construction to prevent rainwater from pouring in and protect the internal electrical components. When debugging, you can remove the two screws on the L-type iron plate, the iron plate can be pulled down, when all the decoder board is exposed, the hook on the L-type iron plate automatically hangs the decoder shell, more convenient to install.
[edit]Wireless decoder
Frequency range
Frequency range refers to the wireless decoder in the specified distortion and rated output power conditions of the operating bandwidth, that is, the wireless decoder's lowest operating frequency to the highest operating frequency between the range. Unit Hz (Hertz). The actual operating frequency range of the wireless decoder may be larger than the defined operating frequency range.
Frequency Stability
Frequency Stability identifies how stable the wireless decoder's operating frequency is. The unit is ppm (parts per million). Typically, the frequency stability of a wireless decoder should be around ±1.5ppm.
Channel Spacing
Channel refers to the value of the frequency occupied when transmitting and receiving. The frequency difference between neighboring channels is called channel spacing. The specified channel spacing 25KHz (broadband), 20KHz, 12.5KHz (narrowband) and so on.
Modulation mode
The modulation mode of wireless decoder mainly has the following categories:
(1) GMSK
Gaussian filter minimum frequency shift keying. GMSK modulation is a modulation mode in which Gaussian low-pass pre-modulation filters are inserted before the MSK (Minimum Frequency Shift Keying) modulator.
(1) GMSK
GMSK enhances the spectrum utilization and communication quality of digital mobile communication. GMSK improves the spectrum utilization and communication quality of digital mobile communications.
(ii) CPFSK
Continuous Phase Frequency Shift Keying. The use of CPFSK modulation makes the receiver easy to realize, compared with the modulation of QPSK phase stability requirements are not high, not easily affected by external temperature noise, and in the signal demodulation processing to achieve low power consumption.
(C) QAM
Quadrature amplitude modulation. QAM is a digital signal to modulate the amplitude and phase of the carrier, so that the amplitude and phase of the carrier is controlled by a digital signal, commonly used 16QAM, 32QAM, 64QAM and so on. This modulation due to the amplitude and phase of the carrier with information, so it is higher than the QPSK method of transmission of the digital rate.
(iv) QPSK
Quadratic phase-shift keying modulation.QPSK is a quadratic phase keying modulation method that can be viewed as two orthogonal two-phase modulation synthesis. The four combinations of the successive code elements (00, 01, 10, 11) correspond to the four phases of the carrier (0, ±π/2, π).
Data Interface
The common interface for wireless decoders is the RS-232 port.The RS-232-C interface (also known as EIA RS-232-C) is one of the most commonly used serial communication interfaces today. It is in 1970 by the U.S. Electronic Industries Association (EIA), the joint Bell System, modem manufacturers and computer terminal manufacturers **** with the development of standards for serial communications. Its full name is "data terminal equipment (DTE) and data communications equipment (DCE) between the serial binary data exchange interface technology standard" the standard provides for the use of a 25-pin DB25 connector, the connector of each pin of the signal content to be stipulated, but also on the level of the various signals to be stipulated.
Interface rates
The common interface rates for wireless decoders are: 1200, 2400, 4800, 9600, 19200, 38400bps (bits per second) and higher N*64Kbps.
Interface checksum
The common checksums for wireless decoder interfaces are Odd checksum and Even checksum.
Odd parity states that the number of 1's in a byte of the correct code must be odd, and if it is not odd, add 1 to the highest bit b7. e.g.
1 0110, 0101
0 0110, 0001
Even parity states that the number of 1's in a byte of the correct code must be even, and if it is not, add 1 to the highest bit b7. e.g.
0 0110, 0001
0 0110, 0001
The even parity states that the number of 1's in a byte of the correct code must be even. 1. e.g.
1 0100, 0101
0 0100, 0001
The parity check is able to detect part of the error code in the information transmission process (1-bit error code can be detected, and 2-bit and more than 2-bit error code can not be detected), and, at the same time, it can not correct the error. After the discovery of the error, it can only ask for retransmission. However, due to its simple realization, it is still widely used.
Antenna impedance
Antenna impedance refers to the antenna circuit containing resistance, inductance and capacitance, the obstruction of the alternating current, the unit Ω (ohms). Common antenna impedance is 50Ω or 75Ω.
[edit]Codec Difference
In multimedia, the encoder is mainly analog audio-visual signals compressed data coded file, and the decoder to data coded file to analog audio-visual signals process. Classification of decoders:
Decoders are classified into AC decoders and DC decoders according to the power supply voltage of the head. The AC decoder provides AC 230V or 24V voltage for AC head to drive the head to rotate; the DC head provides DC 12V or 24V power supply for DC head; if the head is variable speed control, the DC decoder is also required to provide 0-33 or 36V DC voltage signal for the head to control the variable speed rotation of the DC head.
According to the communication mode is divided into one-way communication decoder and two-way communication decoder. Unidirectional communication decoder only receives communication signals from the controller and translates them into voltage/current signals to drive front-end devices; Bidirectional communication decoder has the performance of a unidirectional communication decoder and also sends communication signals to the controller, so that the working status of the decoder can be transmitted to the controller for analysis in real time, and in addition to the alarm detector and other front-end device signals can be directly inputted into the decoder by bi-directional communication. In addition, the signal of front-end equipment such as alarm detectors can be input directly into the decoder by two-way communication to recite the alarm detection signal on the scene, reducing the use of cables.
According to the communication signal transmission can be divided into coaxial transmission and twisted pair transmission. General decoders support twisted-pair transmission of communication signals, while some decoders also support or support coaxial cable transmission, that is, the communication signal is modulated with the video signal at a different frequency *** with the same transmission on the same video cable.
The circuitry of the decoder is based on a microcontroller and consists of a power supply circuit, a communication interface circuit, a self-test and address input circuit, an output driver circuit, an alarm input interface, and other circuits.
The decoder generally cannot be used alone and needs to be used in conjunction with the system host.
[Edit] Precautions
● Decoder to the PTZ, the lens of the connecting cable should not be too long, because the control of the lens of the voltage of DC 12 volts or so, the transmission is too far away from the voltage drop is too large, it will lead to the lens can not be controlled. In addition, because multi-core control cable is more expensive than shielded twisted pair, so the cost will increase.
● Outdoor decoder to do a good job of waterproof treatment, in the inlet with a waterproof glue seal is a good way, and easy to operate.
● From the host to the decoder usually use shielded twisted-pair cable, a line can be connected in parallel with more than one decoder, the total length does not exceed 1500 meters (depending on the site). If the number of decoders is too large, you need to add some auxiliary equipment, such as adding a control code distributor or connecting a matching resistor in parallel on the last decoder (subject to the manufacturer's instructions).