Dynamic range compression
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or compressing an audio signal's dynamic range. Compression is commonly used in sound recording and reproduction, broadcasting,[1] live sound reinforcement and some instrument amplifiers.
This article is about a process that intentionally reduces the dynamic range of audio signals. For similar reductions caused by circuit imperfections, see Gain compression. For processes that reduce the size of digital audio files, see Audio compression (data).
A dedicated electronic hardware unit or audio software that applies compression is called a compressor. In the 2000s, compressors became available as software plugins that run in digital audio workstation software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect. A limiter is a compressor with a high ratio and, generally, a short attack time.
Compression is used to improve performance and clarity in public address systems, as an effect and to improve consistency in mixing and mastering. It is used on voice to reduce sibilance and in broadcasting and advertising to make an audio program stand out. It is an integral technology in some noise reduction systems.
Uses[edit]
Public spaces[edit]
Compression is often applied in audio systems for restaurants, retail, and similar public environments that play background music at a relatively low volume and need it compressed, not just to keep the volume fairly constant, but also to make quiet parts of the music audible over ambient noise.
Compression can increase average output gain of a power amplifier by 50 to 100% with a reduced dynamic range. For paging and evacuation systems, this adds clarity under noisy circumstances and saves on the number of amplifiers required.
Parallel compression[edit]
Inserting the compressor in a parallel signal path is known as parallel compression. It is a form of upward compression that facilitates dynamic control without significant audible side effects so long as the ratio is relatively low and the compressor's sound is relatively neutral. On the other hand, a high compression ratio with significant audible artifacts can be chosen in one of the two parallel signal paths. This is used by some concert mixers and recording engineers as an artistic effect called New York compression or Motown compression. Combining a linear signal with a compressor and then reducing the output gain of the compression chain results in low-level detail enhancement without any peak reduction; The compressor significantly adds to the combined gain at low levels only.
Multiband compression[edit]
Multiband compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth compression is that problems related to a specific frequency range can be fixed without unnecessary compression in the other, unrelated frequencies. The downside is that frequency-specific compression is more complex and requires more processing capacity than full-bandwidth compression and can introduce phase issues.[29]
Multiband compressors work by first splitting the signal through some number of band-pass filters, crossover filters or filter banks. Each split signal then passes through its own compressor and is independently adjustable for threshold, ratio, attack, and release. The signals are then recombined and an additional limiting circuit may be employed to ensure that the combined signals do not create unwanted peak levels.
In music production, multiband compressors are primarily an audio mastering tool, but their inclusion in digital audio workstation plug-in sets is increasing their use among mix engineers.
The TC Electronic Finalizer included a three band compressor and was a popular audio mastering tool around year 2000.[30]
On-air signal chains of radio stations commonly use multiband compressors to increase loudness while avoiding overmodulation. Having a louder sound is often considered an advantage in commercial broadcasting.
Serial compression[edit]
Serial compression is a technique used in sound recording and mixing. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor generally stabilizes the dynamic range while the other aggressively compresses stronger peaks. This is the normal internal signal routing in common combination devices marketed as compressor-limiters, where an RMS compressor (for general gain control) is followed by a fast peak-sensing limiter (for overload protection). Done properly, even heavy serial compression can sound natural in a way not possible with a single compressor. It is most often used to even out erratic vocals and guitars.
Software audio players[edit]
Some software audio players support plugins that implement compression. These can increase loudness of audio tracks, or level out the volume of highly-variable music (such as classical music, or a playlist that spans multiple music types). This improves the listenability of audio played through poor-quality speakers, or when played in noisy environments (such as in a car or during a party).