Digital Media: Audio Acquisition Lecture Notes
Audio File Compression
Introduction
In these notes you will be introduced to some of the common audio file formats used within digital audio acquisition and also introduce you to the understanding of compressed audio.
What is Compression?
Compression simply means in general computing as a, “process by which data is compressed into a form that minimizes the space required to store or transmit it.” (http://www.thefreedictionary.com/compression). In layman’s terms, this means we make files smaller in size on a computer in order for the file to be transported to another location; Internet, USB or just utilising size on our computer.
As you know, computers have many different ways of working, http://wikibon.org and can theybe utilised in many ways, software engineers have developed ways of compression within data files in order to make things smaller. From this, there are two main methods of compressing data: Lossy compression and Lossless compression
Lossless Compression Definition
In lossless compression, every single bit of data that was originally in the file is restored after the file is uncompressed. All of the information is completely restored back to its original state.
So, you may find this confusing considering how a file can be made smaller when in fact there is no loss of any of the original data when the file is compressed. This is all down to the rules of the algorithm (a set of calculations done in a specific order by the computer) to rearrange the data into a more ordered manner; it’s a bit like rearranging items on a shelf so that they take up minimal space.
Lossless Compressed Audio Formats examples
The different types of lossless compression formats provide a compression ratio of about 2:1. These types include:
1. FLAC – Free Lossless Audio Codec is an audio codec for lossless audio data compression. It is suitable for everyday playback and archival and is well-supported by some software applications.
2. Monkey’s Audio – The filename extension used by Monkey’s Audio files is .ape for audio and .apl for track metadata. Digital Media: Audio Acquisition Lecture Notes
Audio File Compression
3. ALAC/ Apple Lossless – Apple Lossless data is stored with the filename extension .m4a. Currently, all iPod players can play Apple Lossless-encoded files.
Lossy Compression Definition
On the other hand, lossy compression reduces a file by permanently eliminating certain data, especially redundant data. When the file is uncompressed, only a part of the original information is still there (although the user may not notice it). Lossy compression is generally used in sound, where most users will not detect a certain amount of information loss. A perfect example of this would be in you average .mp3 file. Lossy compression typically achieves far greater compression than lossless compression (data of 5 percent to 20 percent of the original stream, rather than 50 percent to 60 percent), by discarding less-critical data.
Psychoacoustics
The innovation of lossy audio compression was to use psychoacoustics (the study of sound perception) to recognise that not all data in an audio stream can be perceived by the human auditory system.
Lossy compression identifies sounds that are considered very hard to hear in your average audio. Typical examples include high frequencies at the far reaches of our hearing range, or sounds that occur at the same time as louder sounds. Those quitter sounds are coded with decreased accuracy or not coded at all.
The main point to remember about lossy compression is that data removed during lossy compression cannot be recovered after.
Using lossy compression repetitively to the same file results in increasing degradation of quality if data; it’s a bit like a photocopy of a photocopy
1. MP3 - MPEG-2 Audio Layer III, more commonly referred to as MP3 is a lossy compression format, designed to remove parts of the music that the human ear find hard to hear. It uses the .mp3 file extension
2. Mp4 - MPEG-4 files with audio and video generally use the standard .mp4 extension. 3. M4a Advanced Audio Coding (AAC) is a standardised, lossy compression and encoding scheme for digital audio. It regards itself as the main successor to mp3.