Audio encoding format
Encoding efficiency comparison of popular audio formats.
An audio coding format (or sometimes an audio compression format) is a content representation format used to store or transmit digital audio, such as in digital television, digital radio, and audio and video files. Examples of audio encoding formats include MP3, AAC, Vorbis, FLAC, and Opus. A specific software or hardware implementation capable of compressing and decompressing audio of a specific audio encoding format is called an audio codec; An example of an audio codec is LAME, which is one of several different codecs that implement audio encoding and decoding in MP3 audio encoding software formatting.
Certain audio encoding formats are defined by detailed technical specification documents known as Audio Encoding Specifications. Some of these specifications are written and approved as technical standards by standards bodies and are therefore called Audio Coding Standards. The term “standard” is also sometimes used for the fact that norms and formal standards.
Audio content encoded in a specific audio encoding format is usually encapsulated in a container format. So instead of raw AAC files, users often have .m4a audio files, which are MPEG-4 Part 14 containers that contain AAC-encoded audio. The container also contains metadata such as titles and other tags, and possibly an index for quick searches. One notable exception is MP3 files, which are raw audio encodings and do not have a container format. The de facto standard for adding metadata tags like title and artist to MP3s as ID3s is a hack that works by adding the tag to the MP3 and then relying on the MP3 player to recognize the snippets as malformed audio encoding, so skip the block. In a video with audio file, the encoded audio content is included with the video (in the video encoded format) within the media container format.
An audio encoding format does not specify all of the algorithms used by the codecs that implement the format. According to psychoacoustic models, an important part of how lossy audio compression works is to remove data in a way that humans cannot hear. The encoder implementer is free to choose which data to remove (depending on their psychoacoustic model).
Lossless audio encoding formats reduce the total data needed to represent the sound, but can decode it back to its original uncompressed form. Lossy audio coding formats also reduce the bit resolution of the sound in addition to compression, resulting in much less data, but at the cost of irrecoverable loss of information.
Consumer audio is often compressed using lossy audio codecs because smaller sizes are easier to distribute. The most widely used audio coding formats are MP3 and Advanced Audio Coding (AAC), both of which are lossy formats based on modified discrete cosine transform (MDCT) and perceptual coding algorithms.
Lossless audio encoding formats like FLAC and Apple Lossless are sometimes available, but at the cost of larger files.
Uncompressed audio formats such as pulse code modulation (PCM or .wav) are also sometimes used. PCM is the standard format for Compact Disc Digital Audio (CDDA), and after the introduction of MP3, lossy compression eventually became the standard.
