Detailed explanation of the expander and noise of the signal processing equipment of the mixer
1. The basic concept and principle of the expander is a special amplifier. It has an expansion threshold. For input signals smaller than this threshold, it is expanded by a certain expansion ratio. For input signals larger than this threshold, it is not expanded. 1: 1 output.
(1) Expansion ratio: dB number of dynamic change of the input signal of the expander / dB number of dynamic change of the output of the expander, for example: 1: 2, 1: 3, 1: 4, ... when the expansion ratio is 1: ∞ , The expander becomes a noise gate.
(2) Starting time of the expander (Attack
TIme): the time it takes for an input signal less than the expansion threshold to enter the expansion state to the specified expansion ratio.
(3) Recovery time of the expander (Release TIme): the time required for the input signal to return from the expanded state to the original non-expanded state.
The working principle of the expander is opposite to the compressor. It has a detector, called a gate threshold detector, which detects the input acoustic signal. If the detected input signal is higher than the threshold level, the voltage-controlled amplifier does not perform the input signal. The processing is carried out 1: 1. If the detected input signal is lower than the threshold level, the gain of the voltage-controlled amplifier is controlled. The control method depends on the gate threshold detector, which transforms the series impedance according to the set ratio, attenuates the input signal, and at the same time, expands the signal dynamics. Variety. Therefore, the detector actually attenuates signals below the threshold of the gate, and the speed of attenuation depends on the adjustment of the expansion ratio. Some machines engrave the expansion ratio as 2: 1, 3: 1, 4: 1, ...... This is the dynamic change dB number of the output signal / the dynamic change dB number of the input signal. This kind of representation makes it easy to see the proportion of the dynamic change of the output signal. Regardless of the method of presentation, it does not affect the operation and function of the machine.
2. Working characteristics of the expander The expander works according to the adjustment value of the expansion threshold (gate threshold), the expansion ratio, and the related start-up time and recovery time. The adjustment range of the gate threshold is usually from -60dB to + 20dB, and the expansion ratio is 1: 2, 1: 3, ..., 1: ∞.
It can be seen from the working characteristic curve of the expander that the expander works on two curves, namely the expansion section and the unit gain section. With the increase of the expansion ratio, the inflection point becomes more obvious, and the abruptness of the processed sound signal is more obvious. The sound is more natural, and some expanders use the asymptotic method to clear the obvious inflection point. In addition, the shorter the startup time (ms) and recovery time, the more obvious the abruptness of sound signal processing. Therefore, under normal circumstances, the startup time and recovery time should be adjusted appropriately. The change process of the sound signal is usually from small to large, and the sound signal will become short. If the expansion ratio is adjusted to 1: ∞, the expander becomes a noise gate, and the noise below the gate threshold is attenuated sharply, up to 90dB.
3. The application of expander and noise gate (1) The side chain circuit of the expander can be used to create music with vibrato. The method is: amplify the 4Hz ～ 6Hz very low frequency signal and send it to the input terminal of the side chain circuit (Side Chain In ), To control the expansion time of the expander, the music signal enters from the input terminal of the expander (input), and the vibrato music appears at the output terminal of the expander (Output).
(2) Use the microphone to pick up the drum rhythm and create rhythm music with drum sound. The drum signal picked up by the microphone is amplified, on the one hand, the expander is controlled, and on the one hand, the output signal is mixed with the output signal of the expander. When the drum sound appears, the drum sound signal controls the expander to work, the drum sound stops, the side chain circuit has no input, the music signal is not controlled by the expander, and the original music is output.
(3) Adjust the expansion ratio of the expander at 1: ∞ to become a noise gate. Any noise less than this threshold level is quickly attenuated, the expansion system is in a mute state, and music signals greater than this threshold are sent out in a 1: 1 manner. Played the purpose of cutting off the system noise. Threshold adjustment actually determines the level of noise removal.
(4) The spreader can be used to eliminate the interference of two adjacent microphones, and the microphone sound below the threshold will be cut off.
(5) Use the noise gate to cut off the mechanical noise outside the music, for example: the noise of the pulsating string, the pedal noise of the foot drum, etc.
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