An audio compressor can be a piece of software or hardware that takes a digital audio signal and applies an algorithm to the data to reduce the size of the signal either for storage or for transmission. The term "audio compressor" also describes hardware or software that is used to modify audio channels to remove or normalize frequencies outside a defined threshold, usually to improve perceived sound quality. When describing data compression, an audio compressor can compact digital data in a lossless format, so all the audio data is kept intact, or in a lossy format, so some of the data are changed or removed to increase the efficiency of the compression algorithm. Almost all audio data that is digitally transmitted employs some type of compression, leading to the development of hardware dedicated to compressing audio in real time. Similarly, many forms of digital audio recording, such as a compact disc (CD) or the sound on a digital video disc (DVD), use compression to save space and allow more information to be stored.
There are a number of common algorithms that are regularly used to compress different types of digital data. Audio signals, however, are very erratic and do not benefit from traditional compression in the same way that an image or other data would. Instead, an audio compressor can choose to create very large files that have a minimal compression ratio, or they can be designed to modify the audio data to make it more appropriate for compression, although this will result in lossy compression in which some elements of the original signal will be changed or lost.
One way an audio compressor can prepare audio data for compression is to remove sound elements whose presence is not necessarily identifiable in the first place. This includes frequencies that are outside the range of human hearing and sounds that generate data but are actually masked by louder sounds within the same timeframe. Additionally, an audio compressor sometimes will attempt to modify fast changes in a signal so it is smoother, more even or more predictable. All these methods allow different compression techniques to be used to ensure that an audio file or signal achieves a good amount of compression and still retains an acceptable quality.
When music is being recorded or edited, an audio compressor can be hardware or software that helps to even out the sound of an audio channel or track. This software ultimately will make faint noises seem louder and anomalously loud peaks of sound softer. The channel compressor also can simply remove these peaks from an audio channel, increasing the perceived sharpness of the remaining sounds.