Sharing the six phenomena of video image interference
2020-02-25 11:05:58
[ Huaqiang Security Network News ] Speaking of video interference, we should talk about the traditional way of video surveillance signal transmission video baseband transmission. The so-called video baseband transmission refers to the transmission mode in which the video signal is directly transmitted to the monitoring end by the image pickup end through the coaxial cable without any processing such as frequency conversion, and the image is directly transmitted by using the coaxial cable at 0-6 MHz during transmission, which is very easy. Interference caused the image to appear textured, horizontal stripes and noise to monitor the effect. For baseband transmission video interference, it is divided into AC acoustic interference and spatial electromagnetic interference from the perspective of interference source, and is divided into conductive interference and radiated interference from interference interception. Let's analyze the common video interference phenomenon and its causes. 
1, power frequency interference <br> interference phenomenon: the image appears snow noise, netting or a wide dark horizontal band continues to roll.
Cause of interference: This phenomenon is caused when the camera and the monitoring device are grounded at the same time. Due to the ground resistance and the cable sheath resistance, the load imbalance or the grounding mode of the power system between the two places causes a 50 Hz potential difference. Caused by power frequency interference. The ground potential causes a voltage drop between the two grounding ends. The voltage drop is applied to both ends of the shielding layer and forms a ground current with the ground (ground resistance). The ground current forms an interference voltage through the cable shielding layer, and some harmonic components of the ground current fall. Into the video core wire, causing an interference potential between the core wire and the shielding layer, so that the interference signal is added to the video signal to interfere with the monitoring image.
2, space electromagnetic wave interference <br> interference phenomenon: the image appears dense diagonal network, in severe cases will flood the image.
Cause of interference: When the monitoring cable is erected in the air, the space electric field generated by the electromagnetic wave interference signal in the air will act on the monitoring transmission line, causing a considerable electromagnetic interference voltage at both ends of the line, and the frequency is about 200 Hz to 2.3 MHz. Due to the potential difference in the cable, the cable shielding layer generates interference current. Under normal circumstances, the camera end and the monitoring device end are grounded, which causes the interference current to form a loop through the grounding point at both ends of the cable and the ground, resulting in the terminal. The load generates an interference voltage, and the interference signal is coupled into the video signal to generate an image interference situation.
3, low frequency interference (20Hz-nKHz low frequency noise interference)
Interference: The image shows a still horizontal stripe.
Symptoms: Since sound, data and other signals are low-frequency signals, the narrow frequency band is only used when transmitting 20Hz~nKHZ. It can be transmitted by almost any kind of cable, and is generally only interfered by hum. The coaxial cable used for transmitting video signals has a shielding layer anti-interference curve characteristic, indicating that the higher the interference signal frequency, the better the shielding performance, and the pale frequency interference such as carrier phone and cable station is pale and weak. Low-frequency interference signals also generate interference voltages on the transmission cable, which affects image quality.
4, high frequency interference <br> interference phenomenon: the image appears snowflake points or highlights.
Symptoms: Although the coaxial cable used for video transmission is more resistant to high-frequency interference than the low-frequency interference, strong high-frequency interference signals can also interfere with image transmission. Large charge load start-stop, frequency converter and high-frequency machine, in addition to output high-intensity fundamental wave, also produce high-intensity second harmonic. Although the harmonic intensity is much lower than the fundamental wave, the higher harmonic band is wide and the composition is complex, so the harmonics of the fundamental wave will cause different degrees of interference to the video signal transmitted by the video baseband (ie, within the 6 MHz bandwidth). . After many precision experiments, the fundamental and harmonic frequencies of the high-frequency interference signal are all within 45MHz.
5, reflection interference <br> interference phenomenon: the image appears ghost.
Causes of interference: The chrominance, brightness and saturation of the video signal will be correspondingly attenuated during transmission. When the coaxial network impedance of the transmitted video does not match (also called mismatch), the video signal will be partially chromatic when transmitted to the terminal. The brightness and saturation produce micro-reflection, and the reflected signal will return to the emission to form a re-reflection, which is superimposed with the video signal and reaches the terminal through delay and loss. Multiple reflected signals will generate inter-symbol interference (ISI) at the receiving end. ISI will cause the monitor to receive the wrong input signal amplitude and phase and display it, which will make the transmitted image look as if the image is clear. There is a faint image phenomenon, that is, ghosting.
6. Static interference <br> Interference phenomenon: There are noise when there are nets in the image, and sometimes there is no.
Causes of interference: The ground potential difference between high-voltage (1000V or more) output, severe mechanical friction and high electromagnetic environment grounding in power plants, coal mines and industrial enterprises is between 400VP-P~1500VP-P. The phenomenon of potential difference between ground and earth is a kind of electrostatic phenomenon. When there is static electricity, the ground (including cold ground and hot ground) and the earth are equivalent to a capacitor with positive and negative charges. According to the working principle of the capacitor, it is known that the charge capacity will be discharged when it reaches a certain level. Then, when the electrostatic discharge occurs, a potential difference is formed between the different ground terminals, so that the shielding layer on the transmission line forms a ground current, so that the interference signal is coupled into the video signal and sent to the monitoring device. The interference of static electricity on video transmission depends on the magnitude of the electrostatic voltage difference. In severe cases, the interface chip may be damaged or damaged.
Cause of interference: This phenomenon is caused when the camera and the monitoring device are grounded at the same time. Due to the ground resistance and the cable sheath resistance, the load imbalance or the grounding mode of the power system between the two places causes a 50 Hz potential difference. Caused by power frequency interference. The ground potential causes a voltage drop between the two grounding ends. The voltage drop is applied to both ends of the shielding layer and forms a ground current with the ground (ground resistance). The ground current forms an interference voltage through the cable shielding layer, and some harmonic components of the ground current fall. Into the video core wire, causing an interference potential between the core wire and the shielding layer, so that the interference signal is added to the video signal to interfere with the monitoring image.
2, space electromagnetic wave interference <br> interference phenomenon: the image appears dense diagonal network, in severe cases will flood the image.
Cause of interference: When the monitoring cable is erected in the air, the space electric field generated by the electromagnetic wave interference signal in the air will act on the monitoring transmission line, causing a considerable electromagnetic interference voltage at both ends of the line, and the frequency is about 200 Hz to 2.3 MHz. Due to the potential difference in the cable, the cable shielding layer generates interference current. Under normal circumstances, the camera end and the monitoring device end are grounded, which causes the interference current to form a loop through the grounding point at both ends of the cable and the ground, resulting in the terminal. The load generates an interference voltage, and the interference signal is coupled into the video signal to generate an image interference situation.
3, low frequency interference (20Hz-nKHz low frequency noise interference)
Interference: The image shows a still horizontal stripe.
Symptoms: Since sound, data and other signals are low-frequency signals, the narrow frequency band is only used when transmitting 20Hz~nKHZ. It can be transmitted by almost any kind of cable, and is generally only interfered by hum. The coaxial cable used for transmitting video signals has a shielding layer anti-interference curve characteristic, indicating that the higher the interference signal frequency, the better the shielding performance, and the pale frequency interference such as carrier phone and cable station is pale and weak. Low-frequency interference signals also generate interference voltages on the transmission cable, which affects image quality.
4, high frequency interference <br> interference phenomenon: the image appears snowflake points or highlights.
Symptoms: Although the coaxial cable used for video transmission is more resistant to high-frequency interference than the low-frequency interference, strong high-frequency interference signals can also interfere with image transmission. Large charge load start-stop, frequency converter and high-frequency machine, in addition to output high-intensity fundamental wave, also produce high-intensity second harmonic. Although the harmonic intensity is much lower than the fundamental wave, the higher harmonic band is wide and the composition is complex, so the harmonics of the fundamental wave will cause different degrees of interference to the video signal transmitted by the video baseband (ie, within the 6 MHz bandwidth). . After many precision experiments, the fundamental and harmonic frequencies of the high-frequency interference signal are all within 45MHz.
5, reflection interference <br> interference phenomenon: the image appears ghost.
Causes of interference: The chrominance, brightness and saturation of the video signal will be correspondingly attenuated during transmission. When the coaxial network impedance of the transmitted video does not match (also called mismatch), the video signal will be partially chromatic when transmitted to the terminal. The brightness and saturation produce micro-reflection, and the reflected signal will return to the emission to form a re-reflection, which is superimposed with the video signal and reaches the terminal through delay and loss. Multiple reflected signals will generate inter-symbol interference (ISI) at the receiving end. ISI will cause the monitor to receive the wrong input signal amplitude and phase and display it, which will make the transmitted image look as if the image is clear. There is a faint image phenomenon, that is, ghosting.
6. Static interference <br> Interference phenomenon: There are noise when there are nets in the image, and sometimes there is no.
Causes of interference: The ground potential difference between high-voltage (1000V or more) output, severe mechanical friction and high electromagnetic environment grounding in power plants, coal mines and industrial enterprises is between 400VP-P~1500VP-P. The phenomenon of potential difference between ground and earth is a kind of electrostatic phenomenon. When there is static electricity, the ground (including cold ground and hot ground) and the earth are equivalent to a capacitor with positive and negative charges. According to the working principle of the capacitor, it is known that the charge capacity will be discharged when it reaches a certain level. Then, when the electrostatic discharge occurs, a potential difference is formed between the different ground terminals, so that the shielding layer on the transmission line forms a ground current, so that the interference signal is coupled into the video signal and sent to the monitoring device. The interference of static electricity on video transmission depends on the magnitude of the electrostatic voltage difference. In severe cases, the interface chip may be damaged or damaged.
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