电声设计培训教材之四_音频电路设计

电声设计培训教材之四电声设计培训教材之四 音频电路设计音频电路设计 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 自动电平控制 ALC Automatic Level Control 为什么需要为什么需要 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 gate A Noise Gate or gate is an electronic device or software logic that is used to control the 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 gate 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 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 that 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 below 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 gate Release is the most common control to find on a gate after Threshold 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 signal The 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 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 pattern 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 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 fluctuating signal top With hysteresis the noise gate does not chatter Dynamic 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 reinforcement and broadcasting to control the level of audio A compressor is the device used to apply compression BasicsBasics The relationship between input level output level and gain reduction in a compressor In simple terms a compressor is an automatic volume control Loud sounds over a certain threshold are reduced in level while quiet sounds remain untreated this is known as downwarddownward compressioncompression while the less common upwardupward compressioncompression 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 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 compressor In a noisy environment background noise can overpower quiet sounds 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 sounds thus the level can be raised to a point where the quiet sounds are more audible without the loud sounds being too loud Contrast this with the complementary process of an expander which performs almost the exact opposite function of a compressor i e an expander increases the dynamic range of the audio signal 1 A compressor reduces the gain level of an audio signal if its amplitude exceeds a certain threshold The amount of gain reduction is determined by a ratio For example with a ratio of 4 1 when the time averaged input level is 4 dB over the threshold the output signal level will be 1 dB over the threshold The gain level has been reduced by 3 dB When the input level is 8 dB above the threshold the output level will be 2 dB a 6 dB gain reduction A more specific example for a 4 1 ratio Threshold 10 dB Input 6 dB 4 dB above the threshold Output 9 dB 1 dB above the threshold DesignDesign A feed forward compressor design left and feedback design right The signal entering a compressor is split with one copy sent to a variable gain amplifier and the other to a path called the side chain where a control circuit calculates the required amount of gain reduction The control circuit outputs the requested gain reduction amount to the amplifier This type of design is known as feed forward type and is used today on most compressors Early compressor designs were based on a feedback type layout where the signal feeding the control circuit was taken after the amplifier The variable gain amplifier is the component reducing the gain of the signal There are a number of technologies used for this purpose each having different advantages and disadvantages Vacuum tubes are used in configuration called variable where the grid to cathode voltage changes to alter the gain 2 Also used is a voltage controlled amplifier which has its gain reduced as the power of the input signal increases Optical compressors use a light sensitive resistor LDR and a small lamp LED or Electroluminescent panel 3 to create changes in signal gain This technique is believed by some to add smoother characteristics to the signal because the response times of the light and the resistor soften the attack and release Other Technologies used include Field Effect Transistors and a Diode Bridge 4 When working with digital audio digital signal processing techniques are commonly used to implement compression via digital audio editors or dedicated workstations Often the algorithms used emulate the above analog technologies CompressorCompressor featuresfeatures ThresholdThreshold Threshold is the level above which the signal is reduced It is commonly set in dB where a lower threshold e g 60 dB means a larger portion of the signal will be treated compared to a higher threshold of 5 dB RatioRatio Different compression ratios The ratio determines the input output ratio for signals above the threshold For example a 4 1 ratio means that a signal overshooting the threshold by 4 dB will leave the compressor 1 dB above the threshold The highest ratio of 1 is commonly achieved using a ratio of 60 1 and effectively denotes that any signal above the threshold will be brought down to the threshold level unless some attack is in force AttackAttack andand releaserelease A compressor might provide a degree of control over how quickly it acts The attack phase is the period when the compressor is increasing gain reduction to reach the level that is determined by the ratio The release phase is the period when the compressor is decreasing gain reduction to the level determined by the ratio or to zero once the level has fallen below the threshold The length of each period is determined by the rate of change and the required change gain reduction For more intuitive operation a compressor s attack and release controls are labeled as a unit of time often milliseconds This is the amount of time it will take for the gain to change a set amount of dB decided by the manufacturer very often 10 dB For example if the compressor s time constants are referenced to 10 dB and the attack time is set to 1 ms it will take 1 ms for the gain reduction to rise from 0 dB to 10 dB and 2 ms to rise from 0 dB to 20 dB 5 The attack and release phases in a compressor In many compressors the attack and release times are adjustable by the user Some compressors however have the attack and release times determined by the circuit design and these cannot be adjusted by the user Sometimes the attack and release times are automatic or program dependent meaning that the times change depending on the input signal Because the loudness pattern of the source material is modified by the compressor it may change the character of the signal in subtle to quite noticeable ways depending on the settings used SoftSoft andand hardhard kneesknees Hard Knee and Soft Knee compression Another control a compressor might offer is hard soft knee This controls whether the bend in the response curve is a sharp angle or has a rounded edge A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user A soft knee reduces the audible change from uncompressed to compressed especially for higher ratios where the changeover is more noticeable 6 PeakPeak vsvs RMSRMS sensingsensing A peak sensing compressor responds to the instantaneous level of the input signal While providing tighter peak control peak sensing might yield very quick changes in gain reduction more evident compression or sometimes even distortion Some compressors will apply an averaging function commonly RMS on the input signal before its level is compared to the threshold This allows a more relaxed compression that also more closely relates to our perception of loudness StereoStereo LinkingLinking A compressor in stereo linking mode applies the same amount of gain reduction to both the left and right channels This is done to prevent image shifting that could occur if each channel is compressed individually and content on one channel is louder than that on the other an example would be a tom hit in a drum mix for a tom panned extreme left Stereo linking can be achieved in two ways Either the compressor sums to mono the left and right channel at the input then only the left channel controls are functional or the compressor still calculates the required amount of gain reduction independently for each channel and then apply the highest amount of gain reduction to both in such case it could still make sense to dial different setting on the left and right channel as one might wish to have less compression for left side events 7 MakeMake upup gaingain Because the compressor is reducing the gain or level of the signal the ability to add a fixed amount of make up gain at the output is provided so that an optimum level can be used editedit Look aheadLook ahead The look ahead function is designed to overcome the problem of being forced to compromise between slow attack rates that produce smooth sounding gain changes and fast attack rates capable of catching transients Look ahead is a misnomer in that the future is not actually observed Instead the input signal is split and one side is delayed The non delayed signal is used to drive the compression of the delayed signal which then appears at the output This way a smooth sounding slower attack rate can be used to catch transients The cost of this solution is that the signal is delayed CompressorCompressor usageusage An audio engineer might use a compressor subtly in order to reduce the dynamic range of source material in order to allow it to be recorded optimally on a medium with a more limited dynamic range than the source signal or they might use a compressor in order to deliberately change the character of an instrument being processed Engineers wishing to achieve dynamic range reduction with few obvious effects might choose a relatively high threshold and low compression ratio so that the source material is being compressed very slightly most of the time To deliberately soften the attack of a snare drum they might choose a fast attack time and a moderately fast release time combined with a higher threshold To accentuate the attack of the snare they might choose a slower attack time to avoid affecting the initial transient It is easier to successfully apply these controls if the us