digital-audio.org
Information centre for all about digital audioDigital Audio Formats
There are many formats in which Digital Audio can exist, relating to storage, transfer, or manipulation of data. We describe them below as well as our opinion of them. When discussing formats it important to distinguish between whether the data is being transferred or stored. This is because several formats can be applied to each. For example transferring a file from a computer to a player, the file may by stored as an MP3 file but transferred as PCM. See the explanation of each below.
PCM
PCM stands for Pulse Coded Modulation. It is a streaming format, and is used for transferring digital audio data from one place to another. It applies to live transfers and can also be used for storage of files when they are on a medium that plays back a live stream. Examples of this are Compact Discs and Digital Audio Tape. As well as this you will find PCM used in digital transfers between hi-fi equipment using SPDIF connections. PCM is able to be used for any bit depth and sampling rate, and is transparent to the data being transferred. PCM can carry encoded data or pure audio streams.
FLAC
Free Lossless Audio Codec. This is a compressed file format for storage of digital audio files developed in open source. It is quite recent in its development and is gaining use rapidly. Unlike other formats such as MP3 and WMA, the FLAC format is only lossless. That means that the original audio can always be recreated from the compressed file. One of the key features of the FLAC format is that it is not limited to CD quality audio. It has therefore gained popularity as a format for downloading and transferring digital audio where a sampling rate or resolution above CD is required. It can support 24-bit file formats as well as sampling rates up to 192kHz. It also has the ability to store multi channel audio too
MP3
MP3 is an abbreviation for MPEG3. MPEG is the Movie Pictures Experts Group, who define various standards relating to audio and video storage formats. MPEG3 is a widely used audio only format, with variable compression levels, often used on portable players. The format applies to digital audio data files stored on a computer or portable player. The specification of the format allows various levels of compression to be applied to reduce the size of the file. The compression is lossy, which means some data is lost in the compression process so that the original data cannot be re-created in the decompression process. The level of compression and data loss can be adjusted according to the file size needed and is usually described by the bit-rate of the resulting MP3 file. Many codecs are available to create MP3 files and generally the bit-rate varies from the best quality around 320 kbps down to the lowest quality around 64 kbps. The lowest bit rates are only really suitable for speech, and for music at least 128kbps is used. There are many opinions on which bit rates are the best but the generally accepted rule is that at least 192kbps is needed and preferably 256kbps for acceptable quality music reproduction. MP3 files are almost universally compatible with players and storage devices.
WMA
WMA is an abbreviation for Windows Media Audio. As the name suggests this is Microsoft's proprietary format for audio file storage. Just like MP3 there are various levels of compression, but in this case there is a lossless format as well as lossy. The lossless format can be expanded to re-create the original file if required. In its lossy format the compression can produce files of low bit rates comparable to MP3, though generally the quality is accepted to be slightly better. In lossless mode the bit rates of the files vary from around 600 kbps to nearly 1000 kbps depending on the content. WMA files are widely compatible within the windows pc environment and most portable players, though not Apple iPODs.