Transmission is signal propagation through various means (via media such as wire, coaxial cable, microwave, optical fiber, or radio frequency, etc.). There are two points in this. One is the means of transmission, that is, the transmission medium used, and the other is the signal to be transmitted.
The signals that need to be transmitted in the security system mainly include power, video, audio, alarm I/O, control signals, etc.
The main transmission methods and media are: coaxial cable, twisted pair, optical fiber, wireless (Wi-Fi and 4G, etc.), microwave, satellite, etc.
The details are described below.
Security cameras generally use DC12V power supply, some dome cameras use AC24V power supply, and some home Wi-Fi smart cameras use DC5V power supply.
Hard disk video recorders DVR/NVR, ordinary switches generally use DC12V power supply or directly use AC220V power supply.
The POE switch uses AC48-52V power supply.
Back-end equipment, such as various servers, matrixes, and TV walls, generally use AC220V power supply.
As mentioned in the article on the classification of security cameras, according to the different output signals, cameras can be divided into three types: analog, digital, and network.
An analog signal refers to a signal whose mathematical form is a continuous function in the time domain, which corresponds to a digital signal.
The analog video signal transmits a set of brightness and chrominance data of a video image that changes over time. Expressed by a digital function, time is the independent variable and the signal itself is the dependent variable. This function takes values continuously on the time axis.
If only sampling at a fixed time node, such a signal is called a discrete-time signal, and a finite word length is used to represent the value of all sampling points. Such a discrete-time signal is called a digital signal.
Conceptually, a digital signal is a quantized discrete-time signal, and a discrete-time signal is an analog signal that has been sampled.
Because the value is continuously taken, it is obvious that the analog signal is more susceptible to noise interference. At the same time, almost all transmission media, such as coaxial cables, radio waves, optical fibers, etc., can be applied to digital or analog signals, but digital signals can use these media more effectively than analog signals.
Network signals, to be precise, should be called encoded digital signals, which are transmitted through a computer network, and they are still digital signals.
Sound is a sound wave generated by vibration, which is transmitted through a medium (gas, solid, liquid) and can be sensed by human or animal auditory organs.
The frequency of sound is generally expressed in Hertz, recorded as Hz, which refers to the number of periodic vibrations per second. The decibel is a unit used to express the intensity of a sound, denoted as dB.
The audio signal is the same as the video signal, and the audio is simulated by the analog signal, so the audio signal can be transmitted through the same medium as the video signal. In particular, audio can also be encoded like video signals and transmitted over the network.
For more details, please refer to the article on Audio Function in Network Camera.
The control signals in security mainly include RS-485 pan/tilt control signal and RS-232 serial port signal.
RS-485 uses the voltage difference between the two ends of the cable to represent the transmission signal, and the different voltage differences are respectively identified as logic 1 and logic 0. It is valid when the voltage difference between the two ends is at least 0.2V and above. Any difference not greater than 12V or not less than -7V is considered correct for the receiving end.
RS-485 only specifies the electrical characteristics of the receiving end and the transmitting end. It does not specify or recommend any data protocol. (In the early use of the PTZ control function, you need to set the address code, protocol, and baud rate of the dome camera. Now the requirements are not so strict, and any protocol or baud rate may be able to control the PTZ equipment normally)
RS-485 is recommended to be used in point-to-point networks, linear and bus types, not star or ring networks. If you must use a star network, you need to use a special RS-485 splitter/repeater.
RS-232 is an interface standard for serial data communication formulated by the Electronic Industries Alliance (EIA), and is widely used in computer serial interface peripheral connections.
In the RS-232 standard, characters are transmitted serially in a serial bit string one after another. The advantage is that there are fewer transmission lines, simple wiring, and a longer transmission distance.
The cables and connectors used by RS-232 may be 3 to 25 pins, and the typical application is 4 to 6 pins.
RS-232 serial communication requires multiple settings in the software settings. The most common settings include baud rate, parity, and stop bit.
Alarm I/O signal
The alarm input and output signals are generally high and low levels or switch values. Its essence is a digital circuit.
The main problem studied in digital circuits is the logical relationship between the state of the output signal (“0” or “1”) and the input signal (“0” or “1”), that is, the logical function of the circuit. (Under positive logic, “0” is low level, “1” is high level, and there is no clear boundary between high and low levels)
There is a detailed description of the alarm input and output interface and signal in the article on the external interface of the camera, which will not be repeated here.
Transmission equipment and media
For the requirements of various cables used in security, please read the article Cables used in security. Here mainly talk about some special transmission functions and others.
Power line transmission
In addition to transmitting power signals, power lines can also transmit video signals, network signals, etc., with the help of power carrier technology.
Power line carrier communication is PLC, which is the abbreviation of English Power line Carrier. Power line carrier is a unique communication method of power system. Power carrier communication refers to the technology that uses existing power lines to transmit analog or digital signals at high speed through carrier waves. The biggest feature is that there is no need to re-establish the network, as long as there are wires, data can be transmitted.
When using power lines to transmit video signals, when network signals, you need to use the corresponding power line bridge, similar to a wireless bridge, one transmitter and one receiver, or many-to-one use.
Power line carrier transmission problems:
It can only be transmitted within the area of a distribution transformer;
It can only be transmitted on single-phase power lines;
The power line causes a high reduction in the carrier signal;
The power line has inherent impulse interference.
Because of the above drawbacks, power line transmission technology is only a useful supplement, and it is difficult to apply and develop on a large scale.
Coaxial cable transmission
Similar to power carrier technology, coaxial cable can also be used to transmit network signals. This technology is called EOC (Ethernet Over Cable).
Similarly, EOC transmission also requires the help of corresponding bridges, one for transmitting and one for receiving, or many-to-one use.
The main problems of EOC: poor anti-interference ability, short transmission distance, and numerous standards. More importantly, network bandwidth has long been popularized, and the market stock using coaxial cable TV is becoming less and less. Therefore, it is less and less necessary to use EOC to transform the network.
Twisted pair transmission
Twisted pair is a kind of cable that is twisted into a spiral by two insulated double wires covered with plastic insulation material and copper cables inside.
In the two wires, the current induced by each wire is almost equal. The twisting of the two wires ensures that the average distance between the two wires and the source of interference is the same, and the effects are the same. The noise therefore generates a common-mode signal, which can be eliminated at the receiver only by detecting the differential signal.
Therefore, the twisted pair can reduce the attenuation in signal transmission, reduce crosstalk and noise, and improve the ability to suppress external electromagnetic interference.
Twisted-pair cables used to be mainly used to transmit analog signals, but they are now also suitable for digital signal transmission and belong to the transmission medium of information communication networks.
The use of twisted pair cables to transmit analog video requires the use of corresponding twisted pair transmitters (one receiver and one transmitter are used in pairs). When used to transmit digital network signals, use RJ-45 connectors, commonly known as crystal heads.
Multiple network cameras, network devices are transmitted through network cables, and they are gathered together using switches. For related introductions to switches, please refer to the articles on Switch Selection in Security Video Surveillance and PoE Power Supply.
Twisted pair specifications
EIA/TIA defines model specifications based on different qualities for twisted-pair cables.
CAT-1 is mainly used for voice transmission
CAT-2 transmission frequency is 1MHz, the highest transmission rate is 4Mbps
CAT-3 transmission frequency is 16MHz, the highest transmission rate is 10Mbps
CAT-4 transmission frequency is 20MHz, the highest transmission rate is 16Mbps
CAT-5 transmission frequency is 100MHz, and the highest transmission rate is 100Mbps
CAT-5e has low attenuation and low crosstalk
CAT-6 transmission frequency is 250MHz, transmission speed is 1Gbps, standard outer diameter is 6mm
CAT-6A transmission frequency is 500MHz, transmission speed is 10Gbps, standard outer diameter is 9mm
CAT-6e transmission frequency is 500MHz, transmission speed is 10Gbps, standard outer diameter is 6mm
CAT-7 transmission frequency is 600MHz, transmission speed is 10Gbps
CAT-8 transmission frequency is 2000MHz, transmission speed is 40Gbps
Fiber optic transmission
Whether it is a coaxial cable or a twisted pair cable, the transmission distance is limited. When a wired transmission is required for several kilometers or tens of kilometers, optical fiber is required.
Optical fiber, the full name of optical fiber (Optical fiber), is a fiber made of glass or plastic, and a light transmission tool that uses the principle of total internal reflection to transmit light in these fibers.
The tiny optical fiber is encapsulated in a plastic sheath so that it can be bent without breaking. Generally, the transmitting device at one end of the optical fiber uses a light-emitting diode or a laser beam to send light pulses into the optical fiber, and the receiving device at the other end of the optical fiber uses a photosensitive component to detect the pulses. Cables containing optical fibers are called optical cables. Because the transmission loss of information in optical fiber is much lower than that of electricity in wire transmission, and because the main production material is silicon, it has a large reserves and is easier to mine, so the price is very cheap, which promotes the use of optical fiber as long-distance information Transmission medium.
In addition to fiber-optic cables, fiber-optic data transmission requires the use of fiber-optic transceivers, which can be used in pairs for receiving and transmitting data. The optical fiber transceiver function can also be built into the switch, that is, the optical fiber interface (SFP) of the switch to realize the transmission of optical signals.
The use of light-emitting diodes to transmit multiple beams of light is called multimode fiber. It is a single-mode fiber that uses a laser to emit a single beam of light. Single-mode and multi-mode fibers cannot be mixed, and the corresponding optical transceivers cannot be mixed.
Generally, we call the optical fiber equipment that transmits network digital signals an optical fiber transceiver, and the optical fiber equipment that specializes in transmitting video signals is called an optical transceiver.
Fiber optic connector
Common fiber optic connectors are divided into FC and SC.
FC is the Ferrule Connector round with thread, which is mostly used on patch panels. SC is the Snap-in Connector card-connected square type, which is mostly used in routers and switches.
All of the above are wired transmission, and the wireless transmission is introduced below. In any case, the stability and reliability of wireless transmission are worse than wired transmission, and wireless transmission only exists as a useful supplement to wired transmission.
In theory, security equipment that uses wired network transmission, such as network cameras, DVRs, NVRs, etc., can expand the wired network to wireless Wi-Fi by adding a wireless Wi-Fi network card.
The transmission distance of wireless Wi-Fi is generally limited. For special needs, we can use high-power wireless bridges to wirelessly transmit network digital signals or video signals over a long distance.
Wireless bridges are also used in pairs, one for receiving and one for sending. Of course, it can also be used in many pairs. The transmission distance can be several kilometers to dozens of kilometers. When performing wireless transmission, there should be no obstacles between the bridges. When the distance is long, the antenna angle between the bridges needs to be adjusted to achieve the best wireless transmission effect.
Microwave communication refers to a comprehensive technology that transmits signals using microwaves with frequencies between 0.3 GHz and 300 GHz (wavelengths between 0.1 mm and 1 meter) as carriers. The millimeter wave band used in 5G communications is microwave.
Microwave communication is a communication that directly uses microwave as a medium, and does not require a solid medium. Microwave transmission can be used when there is no obstacle within a straight line distance between two points. Generally, a parabolic antenna is used to transmit and receive microwave signals.
Microwave communication has good disaster resistance performance, and is generally not affected by natural disasters such as floods, wind disasters, and earthquakes. However, microwaves are transmitted through the air and are susceptible to interference. The same frequency cannot be used in the same direction on the same microwave circuit. Therefore, the microwave circuit must be constructed under the strict management of the radio management department.
Radio communication uses electromagnetic waves to transmit signals. These waves are transmitted in a straight line, so they will be blocked by the curved surface of the earth. Therefore, communication satellites can be used to transmit signals on the surface of the earth to achieve long-distance communication on the ground.
The satellite orbit is high above the ground, the antenna beam can cover a large area of the earth, and the radio wave propagation is not restricted by the terrain. The satellite is equipped with a transponder composed of receiving and transmitting equipment, which amplifies and shifts the frequency of the received signal and transmits it to the ground.