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1、电声设计培训教材之四音频电路设计传声器放大电路设计语音通讯传声器放大电路设计自动电平控制（ALC：Automatic Level Control）为什么需要ALC？在听阈与痛阈之间声压级的差异达140dB，理想情况下，转换成电信号幅度的差异也是140dB。而语音通讯电路的动态范围一般达不到140dB，如果不采用ALC，声压级高的声音转换后的电信号就会出现削波失真，而严重影响语音的清晰度。ALC的原理Noise gateA Noise Gate or gate is an electronic device or software logic that is used to control th
2、e volume of an audio signal. In its most simple form, a noise gate allows a signal to pass through only when it is above a set threshold: the gate is open. If the signal falls below the threshold no signal is allowed to pass (or the signal is substantially attenuated): the gate is closed.A noise gat
3、e is used when the level of the signal is above the level of the noise. The threshold is set above the level of the noise and so when there is no signal the gate is closed. A noise gate does not remove noise from the signal. When the gate is open both the signal and the noise will pass through.They
4、are commonly used in the recording studio and sound reinforcement. Rock musicians may also use small portable units to control unwanted noise from their amplification systems. Band-limited noise gates are also used to eliminate background noise from audio recordings by eliminating frequency bands th
5、at contain only static.Noise Gates have a Threshold control to set the level at which the gate will open. More advanced noise gates have more features. The Release sets the amount of time for the gate to go from open to fully closed. A fast release abruptly cuts off the sound once it has fallen belo
6、w the threshold, a slower release smoothly changes from open to closed, much like a slow fade out. If the release time is too short a click can be heard when the gate re-opens.The Attack, Hold, and Release functions of a noise gateRelease is the most common control to find on a gate, after Threshold
7、.The Attack control sets the time for the gate to change from closed to open, much like a fade-in. The Hold control allows you to define the amount of time the gate will stay open after the signal falls below the threshold. This is useful during short pauses between words or sentences in a speech si
8、gnalThe amount of attenuation when the gate is closed can be set by the Range control. Often there will be complete attenuation, that is no signal will pass when the gate is closed. In some circumstances complete attenuation is not desired and the range can be changed.Advanced gates have a sidechain
9、. This is an additional input that allows the gate to be triggered by another audio signal.A variation of a sidechained noise gate used in electronic music production is a trigger gate, trance gate or just simply gate, where the noise gate is not controlled by audio signal but a preprogrammed patter
10、n resulting in a precisely controlled chopping of a sustained sound.Noise gates often implement hysteresis, that is, they have two thresholds. One to open the gate and another, set a few dB below, to close the gate. This means that once a signal has dropped below the close threshold, it has to rise
11、to the open threshold for the gate to open, so that a signal that crosses over the close threshold regularly does not open the gate and cause chattering. A longer hold time as described above also helps avoid chattering.A noise gate without hysteresis can open and close undesirably with a fluctuatin
12、g signal (top). With hysteresis the noise gate does not chatterDynamic range compression, also called DRC (often seen in DVD and car CD player settings) or simply compression, is a process that reduces the dynamic range of an audio signal. Compression is used during sound recording, live sound reinf
13、orcement, and broadcasting to control the level of audio. A compressor is the device used to apply compression.BasicsThe relationship between input level, output level, and gain reduction in a compressorIn simple terms, a compressor is an automatic volume control. Loud sounds over a certain threshol
14、d are reduced in level while quiet sounds remain untreated (this is known as downward compression, while the less common upward compression involves making sounds below the threshold louder while the louder passages remain unchanged). In this way it reduces the dynamic range of an audio signal. This
15、 may be done for aesthetic reasons, to deal with technical limitations of audio equipment, or to improve audibility of audio in noisy environments.The relationship between input level, output level, and gain reduction in a compressorIn a noisy environment, background noise can overpower quiet sounds
16、 (such as listening to a car stereo while driving). A comfortable listening level for loud sounds makes the quiet sounds inaudible below the noise; a comfortable listening level for quiet sounds makes the loud sounds too loud. Compression is used in order to make both the soft and loud parts of a sound more tolerable at the same volume setting.Compression reduces the level of the loud sounds, but not the quiet
