Detailed Explanation of the Compressor of the Signal Processing Equipment of the Mixer

Compressor is a combined device of compressor and limiter. Because of its many characteristics, it has many applications in sound reinforcement system, recording system and broadcasting system. For example: add a solid feeling to the drum sound; add extra-string sound to the guitar and string music; make the singing sound more rounded; enhance the mixed low-level signal; reduce the fluctuation of the pickup of the handheld microphone; reduce the fluctuation of the music; protect the sound reinforcement system from overload; Digital / analog conversion; compression processing, etc. At the same time, most of these devices are equipped with expansion doors, which are used to cut off the noise of music quietness or intermittent, making its application range wider and more flexible. In addition, the side chain input and output jacks are attached for external control, making its applications more diverse. To give full play to the role of the pressure limiter, we must fully understand its principle characteristics and corresponding function adjustment.

(1) The working principle of the pressure limiter is explained by taking LC-6 produced by Furman Sound Company of the United States as an example.
GC2020II type, American dbx266XL, American DOD866II are similar. The principle block diagram of LC-6 is shown in Figure 4-4. It consists of six parts: input, voltage controlled amplification (VCA), output, detection / control, expansion door and power supply.
The input sound signal enters the input gain amplifier through the balanced input amplifier (the gain adjustment range is from -20dB to + 20dB), makes proper gain adjustment, and directly sends out or inverts it for balanced output. The other signal goes to the output level detector (some machines use the input signal of the voltage control amplifier, called the input level detector). The output level detector uses the compression threshold (compression threshold) to remove the level of this signal. For identification, the compression threshold is adjustable (-40dB to + 20dB). For signals below this threshold, the output level detector does not control the voltage controlled amplifier. The voltage controlled amplifier does normal amplification output. For signals above this threshold, The output level detector controls the gain variation of the voltage-controlled amplifier with a certain dynamic compression ratio, start-up time, and recovery time (by impedance transformation). At the same time, a side chain jack is arranged between the output of the voltage controlled amplifier and the input of the output level detector. It is similar to the insertion jack (INS) on the mixer, and the side chain jack output (Side Chain Output) can directly output the output signal of voltage-controlled amplification and crying, and cut off the path into the output level detector. The side chain input jack (Side Chain Input) can introduce external control signals that exceed the compression threshold, and control the output of the voltage control and amplifier through the output level detector (some side chain jacks are installed at the input end of the voltage control amplifier. The effect is exactly the same). In addition, before the input gain amplifier (some after it), the signal is output to the expansion gate detector, and the gate control threshold is adjusted. If the induced signal is lower than the gated threshold, it will be attenuated by 35dB. If the signal is higher than the gated threshold, the output signal of the gate detector is extended to control the operation of the voltage controlled amplifier. Its function is opposite to the output level detector, and Work independently.

(B) The working characteristic curve of the compressor The working state of the compressor depends entirely on the adjustment of the level detector. For simplicity, the input gain and output gain of the compressor are placed at the 0dB level. The input signal and the output signal are expressed in relative dB, the compression threshold is set at -40dB, the compression ratio is 2: 1, the abscissa indicates the input signal (dB) sent to the output level detector, and the ordinate indicates the output signal of the whole machine (dB), based on the compression threshold, the input signal changes by 20dB and the output signal changes dynamically by 10dB. The compression ratio is equal to the ratio of the input signal dynamic change to the output signal dynamic change, which is 2: 1, as shown in Figure 4-5. Similarly, if the input signal changes dynamically by 40dB, the compression is 2: 1, and the output signal changes dynamically by 20dB. It can be seen from Fig. 4-5 (b) that the compression ratio is not an absolute ratio, that is, it is not the ratio of the input signal dB value to the output signal dB value. The figure also shows the meaning of startup time and recovery time. Compression ratio âˆž: 1, the compression threshold becomes the limit threshold. Figure 4-5 (b) shows the case where the compression threshold moves from -40dB to + 20dB. It is worth noting that the abscissa in the figure represents the input signal to the output level detector. The adjustment of the output gain changes the compression curve. Vertical base point position.

(3) Related function keys (1) Input gain (Input
Gain): Adjust the signal level relative to the compression threshold. 0dB means that the signal input to the balanced amplifier is not boosted or attenuated. Positive dB means boost, which means more output signal is compressed.
(2) Gate threshold (Gate
Threshold): Adjust the gating limit, and the signals below this limit, including noise, attenuate the gain by 35dB. Clockwise adjustment means that the gate control limit is increased, and more signals are attenuated under the gate control limit. Fully counterclockwise, the door threshold is closed. This gate threshold is not affected by input gain control.
(3) Start time (Attack
TIme): indicates the time required to compress the signal above the compression threshold detected by the output level detector to the specified compression ratio. Adjust clockwise to slow down the process, and adjust it counterclockwise to speed up the process. It is the speed response of the detection part and the voltage control part to the increase of the signal level.
(4) Recovery time (Release
TIme): indicates that the compression part (the output level detector and the modern amplifier part) responds to the signal level drop quickly. The time required to return from the compressed state to the normal state below the compression threshold.
(5) Compression ratio (RaTIo): adjust the strength of compression. 3: 1 means that every 3dB increase of the signal above the compression threshold will only increase 1db at the output; 1: 1 means that the compressor will not compress; if it exceeds 5: 1, it will have obvious compression; if it exceeds 10: 1, it will obviously The limiting effect of âˆž: 1 means that the pressure limiter acts as a limiter, and the output of the voltage controlled amplifier no longer increases. The compression threshold becomes the limit threshold.
(6) Compression threshold (Compress
Threshole): Adjust the compressed signal level reference. Adjusting counterclockwise, there are many compressed signals. Adjust clockwise to full. At + 20dB, most signals are not compressed.
(7) Output Gain (Output Gain): used to restore the overall gain during signal compression processing, it only affects the output of the whole machine.
(8) Normal / Linked stereo switch (Normal / Linked
Switch): Press it when the dual channel is used for stereo, the gain reduction is controlled by channel 1, the original balance and sound image of the sound are retained, and the start time, recovery time, compression ratio and compression threshold of channel 1 are controlled Two channels. However, the input gain and output gain controls of channel 2 still work for balance. The general adjustment is the same as channel 1. But when this button pops up, the two channels work independently.
(9) Gain Reduction Table (Gain ReducTIon
Meter): indicates the magnitude of gain attenuation. Taking 5dB as the first level, the first light indicates that only 1dB gain attenuation is performed.
(10) Ground switch
Switch): It is a switch on the back cover of the machine. The common end of each circuit board of the machine is connected to the power ground wire and the metal railing of the rack to form a ground loop, which may cause humming. This switch is connected upward to separate the rack from all signal ground wires and cut off the ground loop. However, the power ground wire is still connected to the rack to ensure safety and shield against radio frequency interference.
(11) Side Chain input and output
output): There are special applications (see later chapters for details).

Four) Relevant adjustments (1) Adjustment of the expansion threshold: Generally, the sound reinforcement equipment is of higher grade, connected to the sound reinforcement system, all devices are opened, the function keys are placed in normal working state, and the entire sound reinforcement system is in static work State, no sound source signal is added, the system noise is not loud, the ear can tolerate, and there is no annoying feeling. At this time, the expansion door can be closed without being used. If the noise of the sound reinforcement system is loud, the door threshold can be raised (some machines are indicated with red lights) until only a little noise can be heard, to ensure that the lowest music signal can pass through and play naturally. When there is music, the door control is opened, and the music signal passes (some machines are indicated by yellow lights), when there is no music signal, the door is closed, and the noise below the door threshold is attenuated by 35dB.
(2) Adjustment of start-up time: When adjusted to less than 1ms, it will produce a "gasp" effect. If it is shorter, the potential transient danger is not generated, which is conducive to the protection of the subsequent highly sensitive machinery and equipment, but it is not conducive to the retention of the transient characteristics of the music signal (for example: percussion). For percussion, it should be adjusted to greater than 10ms, exaggerated to make Leling full. The start-up time is adjusted longer, and the compression has a "soft knee" characteristic. The recovery time is adjusted longer, the compression effect is smooth and natural, but it hinders the performance of the small signal followed by the large signal.
(3) Adjustment of compression ratio: 1: 1 means that the compressor does not compress, higher than 5: 1 compression, the compression is violent, the sound is flattened, the average volume is increased, and the listening is prone to fatigue. Compression higher than 10: 1 plays a significant limiting role. âˆž: 1 plays a complete limiting role. At this time, the compression threshold becomes the limiting threshold, and the signal before the threshold is in the amplified state. As far as ordinary music data is concerned, the startup time is adjusted between 1ms and 5ms, the recovery time is adjusted above 500ms, and the compression ratio is adjusted below 5: 1.

(5) The connection of the compressor on the main channel of the sound reinforcement system. The key to the connection of the compressor on the main channel is whether the compressor is placed before or after the equalizer that performs room equalization compensation, as shown in Figure 4-6 As shown. Someone particularly emphasized putting the compressor after the room equalizer. The main reason is to protect the connected amplifier and speakers, and it can cut off the noise introduced by the room equalizer. In fact, this is because I do nâ€™t know much about the function of the compressor, ignoring the relationship between the adjustment of its function keys.
If the compression ratio is adjusted to âˆž: 1, the compressor limiter plays a limiting role; the compression threshold is adjusted to + 20dB, basically let the sound signal pass; the output gain is adjusted to + 20dB, the output of the compressor is connected to the exciter, The signal of electronic frequency division is as high as + 40dB, so even if the pressure limiter plays a limiting role, it will not help, and it is difficult to protect the power amplifier or the speaker. What's more, the electronic crossover has three high, medium and low frequency band adjustments. Improper adjustment can overload the amplifier, damage the amplifier, and burn the speaker. Therefore, comprehensive adjustment must be made, otherwise serious consequences will occur.
Some people think that the compressor with noise gate placed before the room equalizer will greatly increase the noise of the sound reinforcement system. In fact, this is a surface phenomenon. We may wish to make a basic estimate. The static noise of the compressor itself is -95dB, and the static noise of the room equalizer is -95dB. If the compressor is placed in front of the room equalizer, the scale frequency of the room equalizer will be increased. Put in the extreme state + 12dB, ignore the noise of the exciter and power amplifier connected to the room equalizer, and take into account the amplification of their noise + 38dB, then the noise from the output of the power amplifier is -95dB + 12dB + 38dB = â€”45dB. If you put the compressor in the room after equalization, use its noise gate to cut off the noise introduced by the front room equalizer, and take into account the apparent noise of the compressor itself, then the noise from the output of the power amplifier is -95dB + 38dB = â€”57dB. It can be seen that these two situations have little effect on the noise of the sound reinforcement system.
From another point of view, when the room equalizer is placed before the compressor, the compressor is used as a compressor. Although the stereo connection button on the compressor is pressed, the compression effect on the stereo is still obvious, and the stereo playback effect Poor, the signal is dynamically compressed, thereby making the music layer feel worse. If the compression ratio of the compressor is adjusted to a larger value, it is sometimes difficult to use the room equalizer to compensate the room equalization. For example, the frequency of certain scales is increased by 12dB, and the compression ratio of 4: 1 is compressed. After the compressor is compressed to a 3dB boost, which is unfavorable for some dance halls. If the compression threshold is placed at -20dB, the use of the room equalizer to suppress some howling frequencies is not obvious, such as attenuation of 12dB. The limiter only attenuated about 3dB. It can be seen that in the sound reinforcement system in karaoke halls, karaoke halls, concert halls, multi-function halls, it is best to connect the room equalizer after the compressor, which can compress the more violent music signals. , Use the room equalizer for equalization compensation, so that the stereo playback effect, music level and presence can be restored (not the expansion function of the expander). But after this connection, the room equalizer will increase 12dB to some frequency points, is it harmful to the power amplifier, and it is easy to cause overload? Generally, when the power amplifier is working, there are 3 to 4 reserves. The maximum undistorted power of the power amplifier is equal to twice the rated power of the power amplifier. Most power amplifiers can follow the peak power of the music. The peak power of the music is equal to 4 times the rated power of the power amplifier. Therefore, the room equalizer boosts the music signal of 12dB (4 times) at certain frequency points without overloading the amplifier. At present, many karaoke rooms are no longer equipped with compressors. As long as the operator is skilled and the power amplifier has a reserve, it will generally not cause power amplifier overload and speaker burnout. In the professional DISCO ballroom, the sound output power is very large, the sound pressure level reaches 110dB, and the music signal fluctuates violently. You must put the room equalizer before the compressor, use the compressor as a limiter, and adjust the gain of each device to achieve The purpose of protecting the amplifier.

(6) Some special applications of the compressor (1) The compressor can be used for voice-over compression or for sports live broadcasting (see Figure 4-7). After the microphone input signal is amplified by the microphone amplifier, it is divided into two channels, one into the mixer, the other into the side chain input of the compressor, and the output level detector is used to control the voltage-controlled amplifier for compression. When the dialogue voice or the voice of the narrator appears, the sound signal is amplified and sent on the one hand, on the other hand to control the compressor to compress, and the background music sound or the venue game sound coming in from the compressor input is compressed and dropped. When the dialogue voice or the voice of the narrator does not appear, the compressor is not controlled, and ordinary amplification is performed to automatically increase the background music or the sound of the playing field. Using the live broadcast mixer, the connections in Figure 4-7 can be easily achieved. The microphone is inserted into the mixer, and the direct output on the path is used to connect to the input of the side chain of the compressor, the background music enters the compressor, and the output of the compressor is connected to the other line input of the mixer, and then the mixer Mix on stage and send it out through the mixer. The compression ratio is lowered, and the startup time and recovery time are adjusted longer to control background music or the sound of the playing field.
(2) Remove the hissing sound or the tooth sound of speech and singing in the sound reinforcement system. The connection method is shown in Figure 4-8. The input and output of the graphic equalizer and the output of the side chain jack of the compressor are used. Input docking. That is, the side chain output of the compressor enters the input terminal of the graph equalizer, and the output of the graphic equalizer enters the input terminal of the side chain jack of the compressor. The frequency of hiss will increase one by one from 2.5kHz to 10kHz. If there is still a hiss, it means that it is not the frequency and should be put back to 0dB. If a certain frequency is raised and the hiss disappears, then the frequency is the hiss frequency, which controls the hiss of the sound reinforcement system. Generally, the startup time and recovery time should be adjusted to be shorter, and the compression ratio should be adjusted to less than 8: 1.
(3) In singing performances, in order to prevent the distortion caused by the excessive singing voice of the actor or the distortion caused by the loud volume caused by the singer holding the microphone too close to the mouth, the compression threshold can be adjusted higher. The sound is compressed and the gain is reduced to keep the singing dynamic and complete.
(4) Use the opposite effect of the expansion door and the compressor. For example, in a conference room, multiple microphones are turned on, the gating threshold is adjusted, only the voice of the speaker is picked up, and the side noise or the sound of flipping the speech or excessive reverberation tail is attenuated and removed.
Many professional audio equipment, due to the diversity of their functions, leads to the flexibility of their use. There is no single stereotype of use, and the compressor is one of them. Only by fully understanding its working characteristics can it be used freely, according to different occasions, using different connection methods to achieve different uses.

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