Dynamic range compression, also called DRC (often seen in DVD and car CD player settings) or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or “compressing” an audio signal’s dynamic range. Compression is commonly used in sound recording and reproduction and broadcasting and on instrument amplifiers.
The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. Compressors often have attack and release controls that vary the rate at which compression is applied and smooth the effect.
Downward compression reduces loud sounds over a certain threshold while quiet sounds remain unaffected. Upward compression increases the loudness of sounds below a threshold while leaving louder passages unchanged. Both downward and upward compression reduce the dynamic range of an audio signal.
An expander performs the opposite function, increasing the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal that falls below a set threshold level. A noise gate is a type of expander.
The signal entering a compressor is split, one copy sent to a variable-gain amplifier and the other to a side-chain where a circuit controlled by the signal level applies the required gain to an amplifier stage. This design, known as a “feed-forward” type, is used today in most compressors. Earlier designs were based on a “feedback” layout where the signal feeding the control circuit was taken after the amplifier.
There are a number of technologies used for variable gain amplification, each having different advantages and disadvantages. Vacuum tubes are used in a configuration called ‘variable-mu’: the grid-to-cathode voltage changes to alter the gain. Also used is a voltage controlled amplifier (VCA), 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) 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.
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.