Since surveillance cameras generally use the most common transmission method is video baseband transmission (baseband transmission refers to the method of directly transmitting television signals without any processing such as frequency conversion). The advantage of this transmission method is that the structure of the transmission system is simple. Within a certain range, the distortion is small and the noise is low (the system has a high signal-to-noise ratio). The disadvantage is that the transmission distance cannot be too long and must be transmitted within the range of cable characteristics requirements. A video coaxial cable can only transmit one TV signal at the same time.
Due to the high stability of this transmission method, the advantages of simple equipment and convenient wiring in the system, it has been widely used in real life. However, the use of high-bandwidth and actual low-frequency cables causes the amplitude and phase of the signal to be transmitted in the cable to be very different between the low frequency band and the high frequency band, especially when the phase distortion is too large. The circuit is compensated. And the low-frequency part of the baseband transmission is easily disturbed by interference sources such as strong electricity, transmission towers, base stations, motors, and frequency converters. For example, the common interference source in engineering:
1, broadcast interference:
When the cable is erected in the air, the cable itself is equivalent to a long antenna. As a result of the antenna effect, a considerable broadcast interference voltage is generated in the cable, and an interference current is generated on the cable sheath. This current forms a loop through the grounding point at both ends of the cable and the ground, thus causing broadcast interference on the terminal load. The voltage of the signal causes the interfering signal to mix in the video signal. Such interference signals appear as dense stripes, vertical stripes, diagonal stripes, etc. on the image, and even overwhelm the entire video image in severe cases.
2, high-frequency interference:
The shielding effectiveness of the cable shield is lower for signals with lower frequencies. The high-frequency interference signals introduced for this reason include carrier telephones and radio signals. They cause horizontal streaks in the image.
3, power interference:
When the system needs both the beginning and the end of the system to be grounded at the same time, due to the difference in the grounding potential at both ends and the presence of the cable sheath resistance, a 50-Hz ground potential difference is caused between the two grounds, thereby generating an interference signal voltage. When the interference signal is superimposed on the video signal, a wide horizontal band or the like appears on the normal image.
4, harmonic interference:
Harmonic interference is mainly manifested around the power line with large current or high voltage. It is a radiation signal from the power cable to the surrounding area. Its frequency is 2500 Hz and 125000 Hz, and it mainly interferes with the low frequency band of the video signal.
5, transmission line interference:
The quality of the video cable is poor, and the shielding performance is poor (a sparse shield or a non-copper dielectric shield, etc.). The cable resistance is too large and the video signal is severely attenuated.
6, unclean power interference:
For example, in this power grid, there are high-power thyristor frequency modulation speed control devices, thyristor rectifier devices, thyristor AC-DC converter devices, and so on, all of which will cause pollution to the power supply. Unclean power makes the camera and other active devices unstable and interfere.
The above several parts make the video image of the scene suffer from strong interference, we can use the video anti-jamming device to solve the above kinds of interference. The specific solution is as follows:
1. Frequency shifting: Adopt frequency shift technology to shift the video signal (0-6MHz) to a 49-300MHz range; the video signal is transmitted over long distances by the anti-jammer transmitter, and the signal is restored to normal video at the receiver. Signal output. As the frequency is increased, multiple interference signals are effectively avoided while transmitting at a long distance;
2. Encoding: After the video signal enters the anti-jammer, the digital encoding processing is performed, and the decoding processing is performed before reaching the monitoring center equipment. So as to avoid the interference of the source to the video signal and the crosstalk and attenuation of the video signal itself;
3. Enhancement: The signal attenuation is greater as all signal transmission distances are longer. Therefore, we must strengthen the capability of the signal carrier at the transmitting end so that it can transmit farther;
4. Self-adaptation: It contains the automatic adjustment and adaptation function of the signal strength inside the device, so the device has zero transmission distance to its longest distance automatic transmission adaptability;
5, debugging-free: a good device should not have a lot of debugging part, so our equipment already has a 90% market adaptability at the factory, basically can be used on the connection, no need for additional debugging;
6, security: wide voltage (12V-24V) design concept, to avoid the sudden increase in power supply voltage or suddenly burn down equipment, to protect the safety of equipment and operators.