The principle and role classification of Heng Aide's new limiter
Limiter (Limiter) is a circuit that can flatten the amplitude of a signal voltage within a defined range, also known as a clipper. The function of the limiter circuit is to limit the amplitude of the output signal to a certain range, that is, when the input voltage exceeds or falls below a certain reference value, the output voltage is limited to a certain level (called a slice level). And no longer change with the input voltage.
effect
1 Shaping, such as cutting off the interference at the top or bottom of the output waveform.
2 Waveform transformation, such as cutting off the positive pulse in the output signal, leaving only the negative pulse.
3 Overvoltage protection, such as strong output signal or interference may damage a component, you can access the limiter circuit in front of this component
classification
First, the limiter circuit is divided into three types: upper limit limiter circuit, lower limit limiter circuit and two-way limiter circuit.
1. In the upper limit limiting circuit, when the input signal voltage is lower than a pre-designed upper limit voltage, the output voltage will increase or decrease with the input voltage; but when the input voltage reaches or exceeds the upper limit voltage, the output voltage will remain. It is a fixed value that does not change with the input voltage, so the signal amplitude is limited at the output.
2. The lower limit limiting circuit generates a limiting effect when the input voltage is lower than a certain lower limit level.
3. The bidirectional limiting circuit produces a limiting effect in both directions where the input voltage is too high or too low.
Second, the limiter is divided into a diode limiting circuit and a triode limiting circuit according to the circuit composition.
1. The diode limiting circuit utilizes the clamping action of the diode to achieve clipping limiting.
2. The triode limiter uses the cut-off and saturation characteristics of the triode to form a limiting circuit. This type of circuit also has amplifying effect. In order to meet some of the higher technical requirements, it is also possible to form a limiting circuit with an integrated operational amplifier.
The ideal limiter is a non-memory nonlinear circuit. The ideal limiter should have the dual functions of amplification and limiting, and the amplification is required to be infinite and the limiting is instantaneous. Usually, the limiter is composed of a nonlinear limiting device and a band-pass filter. When the FM wave passes through it, the nonlinear device first cuts the amplitude of the portion exceeding the clipping level E, and then passes the band pass filter. The filter filters out its fundamental component so that the frequency of the output voltage remains the same as the input frequency. In the actual design, we can improve the weakening ratio by adding an appropriate amount of positive feedback to a limiter with an approximate IF bandwidth. It can act as a limiter with several positive feedbacks but other structures with the same structure.
principle
Compression / limiter adjustment and application
The range of human hearing dynamics can withstand the maximum loudness and the most quiet sound can be felt in the range of up to 1 million: 1 (ie 106 times) the dynamic range of the sense of hearing is 120dB. The dynamic range of the sound reproduction of the sound reinforcement system is much smaller than the dynamic range of the human ear due to the limitation of the electronic device. The lowest sound response is limited by the uncorrelated noise in the system, so that the small sound signal is submerged in the noise and cannot be heard; the loudness of the maximum sound is limited by the signal clipping, so that the extra peak in the music signal is "beheaded" (Clip) not only produces severe clipping distortion, but also generates a large amount of harmonics after signal clipping, damaging the speaker unit (especially the tweeter). For this reason, signal dynamic processing devices have been widely used in the dynamic range of control signals. Simply put, it is an electronic device that limits the dynamic range of the audio signal, making quiet small signals louder, making high-amplitude spikes smaller, and not generating signal clipping, protecting speakers and amplifiers. Protect from shock and damage.
Compressor/limiters and expanders make the actual benefits of sound reinforcement systems include:
(1) Limits the extreme peak signal in the music signal to protect the speaker system and amplifier system from damage.
(2) A larger sound gain can be obtained because the signal peak is depressed, and other smaller amplitude signals in the program signal are sufficiently boosted.
(3) The small signal in the program will not fall into the noise of the mixer, and the signal-to-noise ratio of the program signal is improved.
Their negative effect is: if the four basic parameters (threshold level, compression ratio, start-up time and release time) of these devices are adjusted inappropriately, “pump soundâ€, “breath sound†or fundamental It is impossible to control the dynamic range of the signal.
According to the special function of the compression/limiter, the four parameters of the device must be properly adjusted. The functions and meanings of these parameters are as follows:
1. Threshold: The threshold of the compressor is specified as the highest input signal level that is unaffected by compression. Input signals above the threshold level will be compressed by reduced system gain. This parameter is adjustable in the compression/limiter. If the threshold level is adjusted too high and exceeds the clipping level of the system, the compression/limiter does not control the signal because the signal has been clipped before the threshold level. The threshold level is generally adjustable from -40dBU to +20dBU. When the level is adjusted below -40dBu, it becomes a "level controller", which makes both large and small signals compressed and reduces the signal. Dynamic range reduces the average sound pressure level. This controller is the level at which the compression/limiter is set to act on the signal and is used in conjunction with the input boost control. The threshold level control of the compression/limiter can be adjusted to accommodate a wide range of signal dynamic levels. When used as a limiter to protect the speaker, the threshold level should be set higher, otherwise the dynamic range of the system should be greatly reduced, making the sound dry and tasteless.
2. Ratio: A measure of the reduction in gain. A 2:1 compression ratio means that when the input signal exceeds the threshold level, the input level is increased by only 1 dB when the input is increased by 2 dB. A 1:1 compression ratio means that the compressor is bypassed (not working), and the compression ratio is adjustable from 1.5:1 to ∞:1, where between 1.5:1 and 10:1 is called the compressor, 10: Between 1 and ∞1 is called a limiter.
According to the rate of change of the compression characteristics at the threshold of the compression/limiter, it can be divided into "hard" (inflection point) limiting.
And "soft" (inflection point) limit two. The limiter must have sufficient control rate to ensure that the signal is not clipped, which is usually a "hard" knee limit. If not properly adjusted, an audible "click" pump sound will be introduced. The setting of the compression ratio depends on the use of the compressor. A compression ratio of 1:1 means that no compression is applied. As a compression of most natural sounds, a compression ratio (soft compression) of less than 10:1 is used. A compression ratio (hard limit) greater than 10:1 acts as a limiter. ∞: The compression ratio of 1 does not allow the output signal to exceed the threshold level.
3. Attack Time: How long it takes for the compression/limiter to control the signal when a signal overload occurs. This time is called the start-up time. The control time adjustment range is between 500ms and 10ms. To prevent instantaneous overload, there must be a rise time and control time faster than the overload time. The fastest rise time can be divided by the highest frequency of the system. Ok, however, the fast start-up time introduces an audible “click†pump sound, and if the “soft knee†is used to mitigate the transition characteristics, the “click†can be minimized. Learn to eliminate the "clicks", usually prefer to adjust the action time to a larger, so that the extremely short transient peaks are not restricted (because the harmonic energy generated after the extremely short transient peak signal clipping is not enough to burn out Speaker unit). The short start-up time setting allows the compressor to quickly track short transients, but it can cause a “click, click†of pumping or breathing. A longer start-up time eliminates pumping and breathing, but it will miss the compression of transient transients, but it will make the output sound more natural. Therefore, when used as a protective speaker, it is usually set in the range of 10ms to 40ms.
Release Time: After the overload signal is over, how long it takes for the compression/limiter to clear the control, which is a very important parameter affecting the sound quality. The general adjustment range is (100ms to 3S, when the release time is too short) The action of the limiter makes the sound incoherent and the distortion is increased. If it is too long, the limiter will have a short "breathing sound". For speech and sound, the release time of 100ms to 500ms is more suitable for instrumental music. In other words, a longer release time is required. The longer the release time, the smaller the distortion caused by compression/limitation. The faster release time protects the original dynamic range of the signal, but it can cause pumping or breathing. Increasing the release time can effectively reduce the breathing sound and pumping sound, smooth the output of the compressor, and reduce the signal distortion, but it will affect the end of the instrument sound. The range is usually set from 200ms to 800ms. The release time includes The two parts of the hold time and the time when the control voltage disappears are not clearly defined in the compression/limiter, but it is still an effective parameter and long enough. The holding time can maintain the important tail of a musical instrument of the gongs and triangles. In addition, the longer the holding time, the smaller the dynamic distortion of the compression/limiter, and the operator can more accurately tailor and compress the dynamics of the program. It can be adjusted from 50ms to 1S. The talk show requires the shortest hold time, and the sonic instrument's vibrato source has 1-2 seconds to make the sound better.
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