Dynamic range compression is not data compression (i.e. the process in which a lossless audio codec such as WAV orAJFF is converted to a lower quality, lossy audio codec such as MP3orM4AJ.
Attenuation is the reduction of the volume or amplitude of a signal, electric current, or other oscillation.
"Compression can be dangerous in the wrong hands."2
Chris Lord-Alge, mix engineer for Green Day's American Idiot
INTRODUCTION
Dynamic range compression is achieved by reducing the volume of loud sounds, or increasing the volume of quiet sounds, thereby "compressing" the dynamic range of an audio signal. The process of compression has many applications in audio recording, mixing, and mastering. Audio compression is accomplished by utilizing hardware or software signal processors. The hardware or software designed for compressing audio signals is called a compressor.
Essentially, compression adjusts the volume of audio relative to a threshold level set by a control on the compressor. Audio signals louder than the threshold are attenuated, while sounds below the threshold level are unaffected.
Compressors can improve an audio signal's perceived quality by taming irregular volumes and improving vocal intelligibility. Harmonics and tone can also be enhanced. When audio is compressed correctly, the result should be free from noticeable fluctuations and sound "clearer" with more depth and dimension. If compression is not applied correctly (or at all), the listener might be distracted by variations in volume and need to adjust the loudness of their playback system.
Compared to other audio effects (such as reverb), it can be challenging to set up compressors effectively, especially for beginners, because the impact of the compressor's settings is not apparent to the untrained ear. A wrongly configured compressor will introduce numerous unpleasant artifacts to an audio signal.
Methodologies
The audio compression process can bebroadly categorized into two principal methodologies: downward and upward.
Fig. 1. Gain computer
log| out
log|tftr --
1:1 ratio
(Normal)
2:1 ratio
4:1 ratio
Infinity:! (Limiting)
log\thr
log|m
Both techniques effectively reduce an audio signal's overall dynamic range. Downward compression reduces sounds that exceed the threshold level, while upward compression boosts the volume of audio signals below the threshold level. Nearly all modern compressors employ the downward compression methodology.
The exact opposite of a compressor is called an expander. It increases or "expands" the dynamic range of an audio signal, working like a compressor in reverse.
An expander increases the dynamic range by loweringthe volume level of audio below the threshold. When the ratio of an expander is set to "negative infinity," the effect is called a noise gate. When a signal goes below the threshold of a noise gate, its amplitude is turned to negative infinity, effectively muting the signal. Noise gates are commonly used in live sound mixing to avoid microphone feedback.
The gain computer is the port of the compressor that generates the control voltoge (CV) and calculates the amount of compression applied to art audio signal. Once the input signal exceeds the threshold point, if is attenuated according to the ratio setting. Ratios of 2:1 to 8:1 are generally considered normal compression, while ratios of 10:7 or higher are called limiting.
Expanders and gates "expand" the dynamic range of an audio signal effectively reducing the volume of noise.
I Compression Techniquesßr Modern Mixing ami Muttering
AUDIO DYNAMICS |