DYNAMIC SIGNAL PROCESSING – To alter a signal based on its frequency and amplitude level; hence the term “dynamics” defines the continuous change and is program dependent. The term “Dynamic Range” refers to the level difference between the loudest musical passage and the softest musical passage.
Dynamic processing is one of the most effective tools that a sound engineer has to make sure all of those important parts will actually be heard in the final product. Dynamic Processing with the use of Compression, Limiting, Expansion & Gating. It is also used for raising apparent volume, reducing noise and leakage, preventing overload, modifying attack or sustain, reducing sibilance or pops, keeping a consistent level, reducing the need for other processing.
TYPES OF PROCESSING
Compression – The function of any signal that exceed a set “threshold” level will not be allowed to increase as much. This reduces or limits the loud passages so they won’t be as loud and is used to even out the volume of the music so that all passages are more even. It might be explained as the smoothing out of the peaks and spikes. This processing is designed to manage the dynamic range of an audio signal. Compression is accomplished by the reduction of the gain a signal that is above a defined level (threshold) by a defined amount (ratio). The typical selection range for this ratio is between 1:1 and 20:1.
Expansion – This function allows for a signal the below the threshold level will be decrease in level. This provides for the elimination of that background noise that is induced into or produced by your electronics. This would make low level sounds (like mechanical sounds of instruments) less noticeable. The word expansion is just the opposite of compression. One would use expansion is “undo” the over compression.
Limiting – To limit would not allow a signal to exceed the threshold level at all. Limiters are used to prevent distortion by setting the threshold control just below the distortion level. The limiter then prevents any peak from getting to the level where it would distort. Some system design criteria specify limiters for component protection and gain structure control. The use of a compressor with an infinite ratio is typically referred to as a limiter.
Gate (Noise Gates) – The use of a “gate” refers to allowing a signal to pass if that signal is above the threshold level. The range control that is included with this gate only reduces the signal rather than trun completely off. Gates are used to get rid of noise and leakage when the instrument is not playing. A gate function is in a sense the opposite to that of a compressor. This function is sometimes called “noise gates” and are useful for microphones which will pick up noise which is not relevant to the program, such as the hum of a miked electric guitar amplifier or the rustling of papers on a minister’s podium. Noise gates are also used to process the microphones placed near the drums of a drum kit in many hard rock and metal bands. Without a noise gate, the microphone for a specific instrument such as the floor tom will also pick up signal from nearby drums or cymbals. With a noise gate, the threshold of sensitivity for each microphone on the drum kit can be set so that only the direct strike and subsequent decay of the drum will be heard, not the nearby sounds.
Effects – The alteration of an input signal known as “effects” include reverberation and delay that are widely used in sound reinforcement systems to enhance the mix relative to the desired artistic impact of the program material. Modulation type effects are applied to some instruments to alter the sound. These include a flanger, phaser, and chorus. Reverb can give the effect of a massive stadium which performing in a small room. The use of effects in the reproduction of modern music is often in an attempt to mimic the sound of the studio version of artist’s music. Another effect call an exciter can “livens up” the sound of audio signals by applying dynamic equalization, phase manipulation and harmonic synthesis of typically high frequency signals.
DYNAMICS PROCESSING CONTROLS
Threshold – The threshold level is where a dynamic processor will start to work. With a compressor or limiter, loud passages that exceed the threshold level will be reduced. With an expander or gate, the unit will turn down any signals that are below the threshold level. Reducing the threshold level means lower the point at where the trigger will be activated with a compressor; increasing the threshold level means that an expander or gate will turn allow more low-level signals to pass.
Ratio – Ratio control determines how much of the signal will be compressed or expanded. For a compressor ratio of 2:1, a turn down of 1db for a 2dB value above the threshold level and 2db turn down for 4db above. At a 4:1 ratio, a turn down of 1db for a 4dB value above the threshold level and 2db turn down for 8db above. When the ratio is set to 10:1 or more, a compressor will be considered a limiter.
When using an expanders ratio of 1:2, for every 1 dB below threshold will reduce the output 2 dB. At a 1:4 ratio, every 1 dB below threshold will reduce the output 4 dB. If an expander has a very high expansion ratio, it will be considered a gate.
Gain Reduction Meters – You can determine the amount of compression or expansion by reading the gain reduction meter. The meter will read the number of dB that is being turned down by the compression or expansion. If the reading is “-6” the output is being turned down dB or to half the level of that signal.
Attack – Attack time determines how fast the processor will react to a signal crossing the threshold. For a compressor it is the time it will take for the compressor to reduce gain of that signal. On an expander, it is the time it will takes to restore full gain after the signal comes up after a low signal.
Release – Release time determines how fast the processor will react to a signal crossing the threshold. When using a compressor, it is the time to restore gain after a high-level signal. For an expander, it is the time an expander takes to turn down a low level signal. Generally speaking, fast attack times are good. Release times will be determined by the frequency of the signal.
Makeup Gain (Output Level) – Makeup gain is the level control for the output of the compressor. This gain control allows the compressed signal to be adjusted so that the peaks are as high as before compression.
Knee – A knee determines the intense the compression ratio is applied to a signal when above the threshold level. A Hard Knee applies the full compression ratio to the signal above the threshold. A Soft Knee applies a mild compression ratio to the signals approaching the threshold and the ratio is increased as the signal rises above the threshold with full compression above threshold level. A soft knee makes gain reduction less noticeable and is used where you are using the compressor to even volume changes for an instrument.
Sidechain – The sidechain function is the control circuit of a compressor. This is an input signal that is used to key the compression function. When a signal is put into the sidechain, the compressor will be activated according to the signal fed to the sidechain rather than to the main input. The main signal will be changed by the level of the signal plugged into the sidechain input.
Auto Release – Auto Release is a variable release time function in a compressor that helps make a volume even to the ear. With auto release activated, a compressor’s release time is longer for peaks well above the threshold and shorter for peaks slightly above the threshold.
Range – Range Control is a expander or gate function. It will set the maximum dB that the expander will reduce gain on lower level signals. Breathing is hearing the gain restoring after a compressor works, hearing the noise or leakage increase between notes. Breathing generally will be caused by the release time too long when with a large amounts of gain reduction used.
Links on Signal Processing:
Introduction to Signal Processing
Rane’s Dynamics Processors — Technology & Applications
Dynamic Processing Fundamentals
MDX2600 – 2-Ch Expander/Gate/ Compressor/Limiter with De-Esser, Enhancer and Tube Simulation
MXD4600 – 4-Ch Expander/Gate/Compressor/Peak Limiter with Dynamic Enhancer and Low Contour Filter
Pinterest – Signal Processing
PerkAudio - Live Sound Consulting 