1.1 Conference TV system composition
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity. 1.1 Conference TV system composition
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity. 1.1 Conference TV system composition
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity.
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity. 1.1 Conference TV system composition
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity. 1.1 Conference TV system composition
Conference TV is a communication means that uses TV technology and equipment to meet in two places or multiple places through a transmission channel. It realizes the integrated processing of audio, video, text data, graphic data and other multimedia, and is in the same information network. Running in the road, unified and real transmission.
If the cable that transmits these signals passes through a strong electromagnetic field, interference will be superimposed on the signal. The longer the cable, the more obvious the interference.
1.2 Types of interference sources and interference methods
In the conference television system , the noise interference affecting the video and audio systems, in addition to the thermal noise of the equipment and the transmission line itself and the continuous "white noise" interference superimposed on it, can be mainly divided into two categories according to the types of interference sources. Pulse interference and hum noise. Impulse interference is caused by the strong electromagnetic field generated by the pulse device coupling into the channel: the motor and the engine engine spark plug are ignited, and the switching power supply will generate 60Hz-2MHz interference. The harmonic components of these interferences will fall into the audio and video bands; lightning Cosmic noise also produces 2KHz-100MHz impulse noise. The audible interference is mainly due to the unreasonable design of the grounding system. There is a potential difference between different grounding points, which causes the ground current to form a loop. The high-voltage transmission line and the AC electrified railway can also cause AC noise, such as the AC electrified railway. In addition to the fundamental frequency of 50 Hz, there are also odd-order harmonics such as (2N+1)×50Hz.
In addition, according to the external interference source electromagnetic energy propagation path and the coupling mode to the audio and video equipment can be divided into radiation mode interference and conduction mode interference. Conduction mode interference is transmitted to the affected equipment through circuit (including stray capacitance and mutual inductance, etc., which can be represented by lumped parameters), such as pulse interference and ac noise, mainly acting on the victim device through conduction; radiation Mode interference is transmitted from the space to the affected equipment through the action of the antenna, such as the interference of the high-voltage transmission line to the victim device.
The audio signal of the conference television system has a frequency range of 300 Hz - 3.4 KHz and a video signal bandwidth of 6 MHz. Audio signals are usually transmitted in a balanced manner. Since the induced noise on the two lines cancel each other out, the interference is much lighter and even impossible to detect. Video signals are usually transmitted in an unbalanced manner, and the interference is much more serious. Therefore, audio interference analysis is different from video interference analysis: for video interference, it is mainly discussed from the interference mode; because the audio signal has a larger wavelength, the shielding effect of the communication building is more obvious. In contrast, the radiation interference can be neglected. Excluding. The pulse interference generated by the rectifying device such as the power supply has the same mechanism for the sound and video signals, and the solution is also consistent. Therefore, the focus of the audio interference discussion is on the humming interference.
2 Video interference analysis and solutions
In actual engineering, video cables are usually laid in metal pipes in the communications building. Although the metal tube skin is connected to the building's construction site, it may still be disturbed. On the monitor, irregular thin lines will be seen rolling from top to bottom. The interference is generated by the ignition of the thyristor. Caused by interference. The pulse interference waveform generated by the UPS power supply is more complicated. In addition to the interference generated by the rectifier ignition, there are pulse interference generated by the inverter and its harmonics. The radiation interference generated by the power rectifier and the UPS power source is everywhere, but it is mainly transmitted through the AC power supply line distributed in the conference room, which is conducted by the conduction mode.
2.1 Conduction mode interference suppression
2.1.1 Suppression of pulse interference
For pulse interference, the solution is to install a filter network. As shown in FIG. 3, a high-voltage, large-capacity capacitor is connected to the input terminals of the three live wires and the output terminal of the rectified power supply, respectively, to form a low-pass filter circuit.
Since the interference of the high-power UPS power supply is more serious, in addition to the larger amplitude of the interference pulse, the waveform of the interference pulse is complicated, and the frequency component is also rich. To suppress interference, install capacitors at the incoming and outgoing ends of the UPS power supply. Increase the capacitance, although the pulse interference level can be further reduced, but after increasing to a certain amount, the effect is not obvious. At this time, a "Γ" or "π" low-pass filter circuit can be used, that is, the cable is wound around the ferrite magnetic ring, and is equivalent to an inductor in series in the effective frequency range.
The formula of the magnetizing inductance LP is as follows:
Where: N—the number of turns of the primary coil of the transformer;
Μ—the relative magnetic permeability of the toroidal core;
It is not difficult to see from the above formula that to increase the inductance, the following methods can be used:
(1) reducing the inner diameter of the magnetic ring and the cable gap;
(2) selecting a magnetic ring with high magnetic permeability;
(3) using high frequency magnetic materials;
(4) Increase the number of magnetic rings.
2.1.2 suppression of hum noise
As mentioned above, the hum noise is mainly caused by the ground current forming loop, which acts on the video receiving device by conduction. For this purpose, the video source and receiver can be isolated by a transmission line transformer.
2.1.3 Design of transmission line transformer
The parameters that need to be clarified before design are:
(1) the highest operating frequency Fmax and the lowest operating frequency Fmin;
(2) determining the output load resistance R1 and the signal source internal resistance Rs;
(3) It is necessary to clarify that the transmission line transformer only serves as an isolation function here.
At the time of design, it is not difficult to find the minimum length Lmin of the transmission line, the maximum length Lmax, and the characteristic impedance Zc from the equations (2-1), (2-2), and (2-3).
Practice has proved that the high-strength enameled wire with Φ=(0.27-0.77) mm is wound, and when it is wound, Zc is about (60-80) Ω; when the operating frequency is lower than 50MHz, manganese-zinc oxide (MXO) ferrite is used. The magnetic permeability of the ferrite is determined by the formula (2-4).
Transmission line transformers require good amplitude-frequency characteristics, low insertion loss, and high return loss. Since the transmission line transformer is a passive device with no gain distortion and no differential phase distortion, no new noise is introduced. After accessing the transmission line transformer, it was found that the pulse interference was greatly suppressed, and no interference was observed at all on the monitor screen. Therefore, in the long-distance transmission of video signals, the installation of the transmission line transformer at the end can significantly suppress the interference of stray electromagnetic fields and the audible interference.
2.2 Suppression of radiation interference
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation to interfere with communication equipment in this band. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
2.2.1 Dispersed grounding method
Decentralized grounding is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipments, and the grounding of other equipments to separate grounding systems. Due to the increasing number of grounding systems, the potential coupling between the grounding lines is often It is difficult to avoid, and dispersing the grounding is likely to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought about by the grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
2.2.2 Joint grounding method
The joint grounding method is also called single point grounding mode, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type pressure equalizing body. For direct lightning strikes, the same layer in the building is relatively uniform; for the inductive mine, the cage-type pressure body and the frame structure of the building interfere with the external incoming magnetic field. Provides 10-40dB shielding effect;
(2) Generally, the grounding resistance of the joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) It can save metal materials and occupy less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of the external high-voltage transmission line.
3 audio interference analysis and solutions
There are two types of balanced and unbalanced types of signal line input and output interfaces between audio equipment: balanced type - two-line differential type, with strong common mode interference suppression capability, small audible interference, commonly used Signal transmission under long distance or strong interference conditions. Unbalanced - single-wire single-ended, mostly used for signal transmission between close-range or internal devices. In order to suppress hum noise, the following points should be noted:
(1) Avoid connecting the signal ground wires between two devices with different ground potentials directly or forming a ground loop.
(2) Try to avoid or weaken the direct connection between the two devices.
(3) The part of the electrical connection is shielded in a system, and the signal ground or shield is grounded on one side of the system.
(4) The long-distance transmission signal adopts the balanced transformer transmission mode. There must be a balance transformer at both ends, and the shield layer is grounded at one end, or it can be left unconnected. Grounding can be used as a shielding function, and it can also prevent electric shock accidents when the electric power is connected. When not grounded, the balance transformers at both ends can provide insulation isolation, balance the center of the transformer, and discharge static electricity.
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