Conference audio system debugging and attention knowledge
Audio system connectors and cables Impedance matching: Audio equipment is usually designed with input impedance greater than the output impedance. The connection of the general audio equipment can make it work normally as long as the impedance of the load input is greater than the impedance of the signal output. At present, most of the output and input ports of professional audio equipment adopt IEC268-15 standard. The standard stipulates that the line output impedance of all audio equipment should be below 50Ω, and the line input impedance of the load should be above 10kΩ. In addition, the signal feed line of the microphone is generally long and requires strong anti-interference, so its input interface is generally around 1kΩ. Signal transmission level; signals transmitted through the cable connection outside the device in the audio system can be divided into the following categories: (1) Micro-level signal: microphone output signal (mV level), LP player output signal (mV level), audio source Output (-10dB). (2) Line level signal: mixer output (+4dB, 1.22V), peripheral input/output (+4dB, 1.228V), line transmission (0dB, 0.775V). (3) Power transmission signal: power amplifier output (high level, high current). In the system connection, attention should be paid to the matching of the output and input levels. Signal connection: The input and output terminals of professional audio equipment are balanced and unbalanced. The balanced mode signal transmission uses a three-wire system. Connected with a two-core shielded cable, the shielded mesh layer acts as a grounding wire, and the remaining two core wires are connected to the signal hot end (reference positive end) and cold end (reference negative end). Since the signal current flowing through the two signal cores is the same size and opposite direction, the external electromagnetic interference induced on the transmission line will be offset by the subtraction at the input end. In addition to the power transmission between the amplifier and the speaker, in order to improve the anti-interference ability of the system and ensure the signal-to-noise ratio, the signal connection in the professional audio system should be transmitted in a balanced manner as much as possible. When using some home audio equipment, when using unbalanced transmission, especially when the transmission level is low, the length of the connection cable should be as short as possible. Balanced and balanced, unbalanced and unbalanced ports can be directly fed with signals; in higher-demand situations, between balanced and unbalanced ports, special converters must be connected to each other. In some occasions where the requirements are not high, the unbalanced terminal of the signal and the balanced terminal can be directly fed. The wiring method is as follows: the thermal end of the balanced end is connected to the signal end of the unbalanced end, and the cold end of the balanced end is connected to the unbalanced end. The ground end, while the ground end of the balanced end terminates the shield of the signal feed line. Connector: There are many types of connectors (plugs) used in professional audio systems, mainly card connector, also known as standard connector, 6.25mm three-pin plug and 6.25mm two-pin plug, and RCA (lotus) Connector. (1) Card connector (cannon/XLR): The card connector is the most widely used type of connector in professional audio systems. Generally, balanced input and output terminals are connected using a card connector. According to the standard, pin 1 of the card connector is grounded, pin 2 is the signal hot end (refer to the positive terminal), and pin 3 is the signal cold end (reference negative terminal). (2) 6.25mm three-pin plug (TRS Cable requirements: (1) Microphone cable, microphone cable must be shielded cable. (2) Line level signal transmission line, used for connection between various devices of the audio system, and shielded lines should also be used to prevent interference. (3) Speaker cable First, the speaker cable should have the lowest possible resistance, which should be as thick and short as possible. Secondly, the material of the speaker cable also has an influence on the sound quality. It cannot be replaced by a single-core audio coaxial cable. The special speaker cable of oxygen-free copper (OFC) should be used. The higher the purity, the better the sound quality. Sound system settings Peripheral equipment connection: (1) Between the sound source and the mixer: It can be specially processed for a specific sound source. This method can achieve good results, but the cost is high. (2) Use the plug-in interface to string into the mixer: Power on and off: (1) When starting up, first confirm that the power amplifier level adjustment knob has been adjusted to the minimum, then pull all the attenuators on the mixer to the lowest position, turn on the mixer power, and then follow the "first open" The order of the signal, the big signal is turned on, the power of the sound source and peripheral devices is turned on in order, and finally the power of the power amplifier is turned on. (2) When shutting down, first draw all the attenuators on the mixer to the minimum, adjust the power amplifier level adjustment knob to the minimum, and turn off the power in the order of “first turn off the big signal, then turn off the small signalâ€. Amplifier power, mixer power, and other device power supplies. The initial setting of the system level: (1) turn on the selected source and adjust its volume to the maximum undistorted output state. (2) Turn off the main fader (pull to the end). (3) According to the selected signal source, select the typical signal level, press the PFL button to display the signal level through the meter. (4) For the maximum value of a stable signal, adjust the gain control (GAIN) so that the meter display level is 0 dB. If the source is a high signal level with a strong transient, the meter should reach +6dB/+9dB. (5) Set the main fader on the mixer to 0 dB, lift the PFL fader, and adjust the fader so that the meter display level is about 0 dB. (6) Adjust the other input channels in the same way, and send the adjusted signals to the output of the subsequent devices. (7) Adjust the volume adjustment knob on the power amplifier to meet the maximum amplification volume. (8) At this time, if the system has entered the clipping state, it should consider replacing the power amplifier system with a larger power. In order to make the system have a certain ability to withstand unexpected large peak signal impact, the system should leave a dynamic margin of about 6dB, that is, when the maximum sound volume is satisfied, the volume is turned up by 6dB without distortion. Suppression of acoustic feedback The formation of acoustic feedback: In the sound reinforcement system, in addition to the positive electric transmission channel of the microphone-amplification system-speaker, there is also a feedback channel for the speaker to directly feed back to the microphone. The feedback sound is then passed through the microphone to the amplification system and sent to the speaker. By repeating this cycle, the entire system will be self-excited and the system will not work properly. In the outdoor sound reinforcement system, the acoustic feedback is mainly caused by the direct sound of the speaker. In the indoor sound reinforcement system, in addition to the direct sound of the speaker, there are reflected sounds from each interface. Common causes of acoustic feedback in practical applications: (1) Sound field environment reverberation time is too long can cause sound feedback. (2) There is a problem with the position of the speaker and the microphone to cause acoustic feedback. (3) The sensitivity of the microphone is too high and will cause acoustic feedback. (4) Incorrect acoustic input can also cause acoustic feedback. (5) The microphone input gain is too large to cause acoustic feedback. (6) The self-excitation of the line will cause acoustic feedback. (7) The incorrect adjustment position of the sound field adjustment equalizer will cause acoustic feedback. (8) Incorrect adjustment of the mixer microphone channel EQ will cause acoustic feedback. (9) The effect of the effect added by the singing microphone is incorrect, which will cause acoustic feedback. (10) The incorrect method of holding the microphone by the speaker or the singer may also cause acoustic feedback and the like. The method of suppressing acoustic feedback: (1) Controlling the acoustic conditions of the hall and appropriately reducing the reverberation time can effectively suppress the acoustic feedback. (2) Adjust the relationship between the microphone and the speaker, try to make the microphone's pickup range and the speaker's radiation range avoid overlapping, select the microphone with strong directivity and low sensitivity, and select the speaker with strong directivity, which can effectively suppress Acoustic feedback. (3) Special equipment can be used to eliminate acoustic feedback, including equalizers, frequency shifters and feedback suppressors, but it will have a certain impact on the sound quality. Sound system noise problem The noise problem caused by the sound system is multi-faceted, among which the strong electric interference is the most serious; the other is the cable shielding end, and the balanced and unbalanced connection lines; the third is the grounding of the equipment and the system. Prevent strong electrical interference: Most high-power electrical appliances can interfere with the sound system through the power grid. Such interference is more serious with thyristor dimmers, high-voltage arc lamps, etc., the power supply of the audio system should be avoided as much as possible. The system's power supply is together, try to use a separate power supply system. In addition, the power phase of each device in the audio system must be consistent and not in the same phase as the power of other system devices. Grounding network: All equipment of the sound system must enter the same common grounding network; its role is to establish a shielding system. The entire grounding network consists of two parts, one part is the shielding system and the other part is the common grounding system. The iron casing of the audio equipment and the shielding layer of the signal feeder shield the entire system of the sound system to form a shielding system. Grounding not only plays a role in preventing electric shock in sound engineering, but also plays an important role in preventing interference and improving the signal-to-noise ratio of the whole system. In order to prevent some interference from being introduced into the sound system through the ground wire, the sound system should be set up with a dedicated grounding wire, and try not to share a ground wire with other devices, especially the controllable light device. One point of grounding: The grounding network must not have a closed loop structure. The reason for generating a closed loop is generally because the shield layers of the plurality of signal lines are grounded at both ends or between the shield layer and the power supply ground. All audio cable shields between devices should be grounded at one end (except for the microphone cable). The best way to ground is to ground the end of the signal transmission line. For grounding between balanced and unbalanced ports, select Balanced end. Sound system operation and maintenance The maintenance and maintenance of electro-acoustic equipment in the sound system is mainly to prevent moisture, shock and overload. Need to pay attention to the following points: (1) The attenuator of the mixer and the power amplifier, that is, the fader on the mixer and the attenuator on the power amplifier should be placed at the position with the largest attenuation when the system is turned on or off. The system power is turned on, and then slowly adjust to the appropriate position as required. (2) Prevent liquids, debris, dust, etc. from falling into the gaps of the mixer's faders. (3) Prevent the equipment at all levels of the system from being over-excited and damaging the input stage. (4) It is strictly forbidden to plug the connector in the hot plugging system to avoid the impact of instantaneous overload current on the rear equipment. (5) Prevent the signal from being too strong and damage the amplifier and speaker system. Helicopter and Single Sheave Stringing Blocks Ningbo MARSHINE Power Technology Co., Ltd. is a professional engaged in the development, design and manufacture of power engineering construction equipment and tools.
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Jack's internal wiring is: the top of the plug T (Top) is the signal hot end, the plug ring R (Ring) is the signal cold end, and the plug sleeve S (Sleeve) is the ground end. This three-pin plug can be used to transmit signals in one direction, in which case balanced transmission is used. It can also be used for bidirectional transmission of signals. In this case, unbalanced transmission is used. At this time, it is generally specified: top-output, loop-return, sleeve-ground. In addition, the three-pin plug can also be used as a connector for input and output of stereo equipment. Generally, it is defined as: top-left channel, ring-right channel, sleeve-ground. (3) The shape and size of the 6.25mm two-pin plug and the three-pin plug are basically the same, except that there is one less electrical contact R, which has only two electrical contacts, T and S. Generally, the unbalanced input and output ports are connected using a large two-core connector. The sophomore core plug defines the top T (Top) as the signal end and the sleeve S (Sleeve) as the ground end. For the case of signal input, inserting the two-pin plug into the three-pin socket (that is, sending the unbalanced signal to the balanced input port) can automatically realize the unbalanced balance connection. At this time, the two-pin plug will connect the cold end of the signal in the three-pin socket. Connected to the ground. (4) RCA plug, also known as lotus plug, is mainly used for home audio and audio-visual equipment, and is an unbalanced transmission method when used as an audio plug.
The INS is inserted as a high-level line interface, and the low-level microphone signal can be pre-amplified to a high level signal by the mixer's preamplifier. The output level of the INS insertion interface can be controlled by a gain control unit. Since the peak indicating device on the input channel is generally configured before (or after) the INS functional unit, it can monitor the signal peak at the transmitting (return) end of the INS interface without exceeding the dynamic range of the device. The INS interface is generally more suitable for compression expanders, effects, and the like. (3) Use the auxiliary bus and connect to the mixer: If you want all input channels to feed an effect processing device at any depth, you can use the effect output bus system. The position of the signal must be after the channel volume fader, so that the effect signal can be changed synchronously with the loudness adjustment of the original signal; the processed signal can be fed back to the total output bus or bus through a dedicated effect return channel; Return to the mixer through other input channels. At this time, the corresponding auxiliary output of the channel must be turned off to avoid feedback. (4) String between the mixer and the amplifier: If the purpose of using peripheral devices is to improve the hall environment defects, or to simulate the sound quality of a specific hall, you can string the peripheral devices between the mixer and the amplifier.
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