This content is brought to you by Audiocourses dot com
The term compression is often heard in discussions about audio, but for some the different types of compression can cause confusion.
There are 2 forms of compression commonly used in audio; Dynamic and Data/File. They are used for very different purposes. This article will give you an brief overview of the 2 different types.
Dynamic
Dynamic compression will reduce the dynamic range of a signal. By dynamic range we mean the difference between the quietest and loudest signals. This is useful for taming the output of signals with very intense attacks, and also for smoothing out the average level of signals such as bass guitars. Dynamic compression can also be used on the stereo file during the mastering process to make the track appear 'louder' and to allow more gain to be added to the file before distortion takes place.
There are a few controls you will find on practically all compressors;
Threshold, Ratio, Attack, Release and Make-Up Gain.
The 'threshold' is expressed in dB's and determines at what point the compressor will 'turn on' and begin compressing. Setting a high threshold means that only the loudest pieces of audio trigger the compressor - a lower threshold means that a lot of the signal will be compressed.
'Ratio' determines how much the audio will be compressed by and is expressed as a ratio. 1:1 means that no compression will take place, whereas a ration of 2:1 means that all audio over the threshold will be compressed in level by 50%. An easy way to think of 'ratio' is to look at the 2 numbers involved. A ratio of 2:1 means that is the signal is 2dB over the threshold, it will be compressed so it is only 1dB over the threshold. Similarly a ratio of 4:1 means that if the signal exceeds the threshold by 4dB, it will be compressed so that it is only 1dB over.
The 'attack' control determines how quickly the compressor takes to turn on and is expressed in msecs. With an attack time of 0ms, compression begins as soon as the signal exceeds the threshold. An attack time of 50ms means that the compressor 'waits' 50ms before turning on. This is useful for preserving the initial transients on sources such as percussion.
'Release' is also expressed in msecs and this dictates how long the compressor takes to return the gain to normal after compression.
Finally, the 'Make-Up Gain' allows you to turn up the gain of the signal to compensate for any lost volume due to the compression of the dynamics.
Data/File
Data/File compression is used to reduce the file size of audio - often this is for storage purposes (i.e. to store on an iPod) or for bandwidth purposes (i.e. to stream audio over the internet). By now everyone has heard of formats such as MP3, Ogg and WMA, and these are the most popular forms of audio file compression.
We can further split data compression into 2 distinct types - Lossy and Lossless.
Lossy compression reduces the file size by removing audio information that is not deemed 'important'. This is done via a process called Psychoacoustic modeling. This is a complex subject which will be featured in a later article. To put it at its simplest, the software examines the audio and predicts which information can be discarded without noticeably degrading the audio quality. Different methods can yield varying results, although in recent years the results are becoming more and more impressive.
Lossless compression works in a very different way. This method of compression suffers absolutely no loss of quality and works by replacing common sequences of code with a 'shorthand' code.
A simplified way to think of this is to imagine a 60 second audio file that was just pure silence. Silence would be represented in a 8 bit audio file as 00000000. 60 sec's of silence would involve a lot of 0s! Rather than sending lots of groups of 0, a shorthand would be used whereby 00000000 could be represented by, for arguments sake, 08. When the file is decompressed for playback the decompression process would replace every instance of the digits 08 with 00000000. As you can see this would make a 60sec file much smaller in size. In reality lossless compression is more complex than this but this should give you an idea. As well as specific audio lossless compression formats such as FLAC, WavPack and Monkey's Audio , common non-audio compression formats such as WinZip and WinRAR operate in the same fashion.
