Power over Coaxial (PoC) is a standard for video and power transmission over coaxial cable
Power over Coaxial (PoC) is a technology that supports simultaneous transmission of power, high-resolution analogue video signal and OSD control signal over coaxial cable at the distance of up to 400 m.
Fig. 1. Transmission of camera’s video signal and power: (a) standard, (b) PoC
1 - Camera 2 - DVR 3 - Power adapter
The first provisions to standardize the video signal and power transmission over coaxial cable were introduced at the “Power systems for coaxial cable” conference at the Institute of Electrical and Electronics Engineers (IEEE) in 1994 in Vancouver, Canada. The PoC technology has not yet been standardized.
In the PoE technology, the devices of different brands are compatible. Connecting a camera of one brand to a switch or recorder of another brand will not affect system operation. In case of PoC based devices, only the devices of the same brand are usually compatible. Connecting the devices of different brands may cause issues with correct operation and lead to failure.
The first devices (still available on the market) for transferring video, audio and alarm signals to the recorder, power to the camera and control signals (RS-485) both ways were developed at the turn of 2004/2005. The devices allow transmission over RG-59 at the distance of up to 300 m. However, only SD video signal can be transmitted.
At present (Q4, 2017), a comprehensive PoC-based solution (cameras and recorders) for Full HD or higher definition video signals was implemented by Hikvision only. In September 2016, Hikvision patented and marketed in 2017 a solution which allows transfer of power, OSD control and video signal up to 4K/UHD. Transmission range is up to 300 m - depending on the type of cable used, signal resolution and PoC version. Two types of devices using this technology are available. PoE 802.3 standard nomenclature is used. In device version PoC.af, the output power is 6 W, whereas PoC.at guarantees 12 W. Other manufacturers, e.g. Dahua, are currently implementing their own PoC solutions. The devices will be available at the beginning of 2018.
Also, PoC solutions dedicated for standard recorders and cameras are available (Fig. 2).
Fig. 2. Example system using video signal and power transmission over coaxial cable.
1 - Camera 2 - DVR 3 - Power adapter 4 - The transmitter “supplying” the power to the coaxial cable. 5 - The transmitter “separating” the power from the video signal.
The principle of PoC operation is based on the difference between the video signal (alternating current) and power (direct current). Most cameras are supplied with a direct current; direct connection of the power supply to the cable transmitting the video signal would damage the electronics.
To simplify, the issue was solved by using two basic passive electronic components - a coil and a capacitor. For the direct current, the coil is shorted, and the capacitor is the gap in the circuit. For the alternating current, it is reversed: the coil is the gap in the circuit, while the capacitor is shorted. The solution is theoretical and is the basis for the design of other complex systems separating the power from the video signal. Fig. 3 shows the solution diagram.
Fig. 3. Power (DC) separated from the video signal (AC) using the capacitor and the coil.
The design of the PoC devices allows both the camera and the recorder (and the transmitter set) to include systems transmitting the analogue video signal and preventing the direct current flow through the video line. In the power supply system, the alternating current is blocked, and the direct current is passed through. Fig. 4 shows an example diagram of a camera - recorder system using PoC technology.
Fig. 4. Diagram of the PoC device design
1 - Camera 2 - DVR 3 - Coaxial cable 4 - Power controller 5 - Power source
The recorders by leading manufacturers featuring the “power over coaxial” also include special systems protecting other non-PoC devices against damage - the connected devices are recognized automatically. If the compatibility of the camera with PoC technology cannot be determined, the recorder will cut the power supply off. The power is also cut-off if short-circuit is detected in the system to prevent damage both to the end devices and the recorder.
The main advantages of using the PoC technology for the CCTV systems include:
- shorter design and development time; - reduced workmanship and material costs: the system does not require installation of power supplies, power cables, connectors and sockets; – number of cables reduced by half (saves space); – PoC technology can also be used to retrofit legacy analogue systems; – due to lower attenuation of the coaxial cable compared to UTP cables, PoC technology allows to transfer signals and power at three times the distance available to PoE devices; – using a single central power supply reduces the device susceptibility to interferences and failure due to power system instability.
At the current level of PoC technology development, the main drawback is the lack of compatibility between the solutions offered by different manufacturers.