CCTV Video Signal Transmission Options
From The D.U.C.K. Project
Standard RG-59U coaxial cable can carry the video signal to distances up to 1000 ft. For longer runs, you should use RG-11U, which has a thicker center conductor and can carry the video signal up to 2400 ft. For extremely long cable runs it is necessary to add inline amplification.
Cable Type Distance w/o Distance with Amplification Amplification ------ ------ ------ RG-59U 1000 ft 3400 ft RG-11U 2400 ft 8600 ft
Capacitance is affected by the cable length. Long runs can lead to high frequency signal loss and poor picture quality.
The most common means of conducting video signals for cctv is coaxial cable. Not only is coax the most commonly used cable, but also the least expensive, most reliable, most convenient, and easily maintained way of transferring electronic images in a CCTV system. The most common type used is RG59/U, but this designation actually represents a family of cables with widely varying electrical characteristics. Other varieties like RG59/U are RG6/U and RG11/U; these are used predominately in CCTV and video work.
The RG reference is the cable specification for use as a "radio guide ",while the numerical value helps differentiate the specifications of each individual cable. Common "coax" cable RG59U,RG6U, and RG11U is circular. Each has a center conductor surrounded by dielectric insulating material, which in turn is covered by a braid to shield against electromagnetic interference (EMI). The outer covering is the "jacket".
- The RG-59 has the highest attenuation of the two other types and you can expect to get a distance of about 750 - 1,000 ft.
- The RG-6 has lower attenuation characteristics than RG-59 and you can expect distances of about 1,000 - 1,500 ft
- The RG-11 type has the lowest attenuation characteristics and you can expect distances of 2200 ft - 3000 ft. If you need to go beyond 3,000 ft., then you need to use amplifiers or use fiber optic cable as a method of transmission.
Attenuation is the opposite of amplification, it is a reduction in amplitude and intensity of a signal.
Up to 15 miles range between line of sight points can be achieved using Microwave transmission for the video signal. One way signal from the camera to the receiver is common, however, two way is necessary if control of the camera or pan and tilt is used. To use Microwave transmission proper FCC licensing is required.
Microwave signals are impacted by weather conditions such as heavy rain or snow.
Slow Scan Telephone
Due to bandwidth limitations, only still images may be transmittioned. The still frame images are typically transmitted every 3 to 70 seconds. The combination of transmitter and receiver uses traditional phone lines to transmit the video signal. The image is converted line by line to low frequency audio tones. The receiver listens to the sound tones and converts back to video signal.
Radio Frequency (RF) signal may also be used to transmit cctv video. Using this method a line of sight view is not necessary depending on the RF used. With FM frequency the signal can go though structures such as buildings and trees. No license is required for video only signal, however, adding audio requires FCC licensing.
Using fiber optic cable and LEDs video signal may be transmitted over great distances. This method is expensive. Fiber optical cable offers exteremly high quality video signal and is highly resistant to interference. Standard video signal must be converted to the optical signal and then converted back at the receiving end. One big advantage to fiber optics is that of multiple device integration within one system. The same fiber can carry video for cctv, computer data, network, and other types of information for other systems.
Twisted Pair or Cat5
This system is not recommended when proper cable may be ran. However, when existing cable is present, such as a building wired for Ethernet, it is sometimes necessary to use the twisted pair wire for cctv. Converters are necessary. Connecting a transmitter to an incoming video device via a 75 ohm cable converts the video image to a composit signal, which is sent to the amplified receiver over the spare wires and ultimately to the monitor or other device.
This science fiction sounding method is actually useful for short distance light of sight transmission. There are restrictions on the power that may be applied to the laser transmission, limiting total distance to up to one mile. At the tramsmitter the input signal from the camera is converted to a modulated laser light beam and directed to a photoconductor at the receiving end.