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RASTER IMAGE FILE FORMATS

TIFF (Tagged Image File Format)

TIFF is a widely used raster image format. TIFF version 6.0 with Intel (Windows) byte order is considered the ''de facto'' standard for digital masters as its properties suit long-term preservation requirements. TIFF supports very high resolutions and bi-tonal color, black-and-white, greyscale, and high color-depth images. Beyond this, TIFF may act as a container for different compressed (e.g. JPEG) or uncompressed (e.g. PackBits) images and can also support layers and multiple pages within a single file. TIFF can additionally accommodate internal technical metadata in its header. Finally, the format can be used with or without compression and supports both lossless and lossy compression, which allows it to be used for access and distribution as well as preservation. Since TIFF can support quite a few special features, a given TIFF file may not be fully readable in a particular viewer. To counter this, TIFF comes in two types: baseline TIFF and TIFF with extensions. Baseline TIFF limits the number of features and thus lacks support for multiple layers and LZW and JPEG compression. It does, however, support uncompressed files, ITU.T4 and PackBits compression, and multiple pages. All viewers currently support baseline TIFF. Since 2009 TIFF has been under the control of Adobe. The format has not had a major upgrade since the release of TIFF 6.0 in 1992. TIFF is not an official standard.

Characteristics of TIFF

Simple encoded raster-based format;
Supports large file sizes;
Accommodates a wide range of color profiles and spaces, including independent color space (CIE L*a*b);
High-bit compatible;
Can support layers and alpha channels;
May be used uncompressed or with lossless compression;
Supports multiple compression types;
Accommodates internal technical metadata, including Adobe‘s XMP (Extensible Metadata Platform), through extensible and customizable header tags.

Advantages: TIFF is widely used and has a strong track record as a preservation format. It supports lossless compression and embedded metadata. TIFF is a better as source format for OCR then PDF.

Disadvantages: The size of uncompressed TIFF files can get quite large. It cannot be used as an access file as there is no native support in current web browsers. Unlike PDF, TIFF supports no text-layer so free text search of scanned reproductions is not possible.

Viewers: OpenOffice.org Draw, Artweaver, Adobe Premiere Pro, PhotoOnWeb, Nero Multimedia Suite, Adobe Creative Suite, ACDSee Photo Manager, XnView, FastPictureViewer Professional.

MIME Types: image/x-tif, image/tiff, image/x-tiff, application/tif, application/x-tif, application/tiff, application/x-tiff, image/tif

PDF (Portable Document Format)

PDF was created as a highly structured page description language that can contain embedded objects, such as raster images, in their respective formats. As such, it works well as a container for multiple logical objects that make up a coherent whole or composite document. PDF supports multiple compression schemes. As PDF can hold multiple objects, it is also possible to use different compression on different parts of a composite document. Metadata can be stored in a PDF file using Adobe's XMP specification for embedding metadata in XML. On the other side, PDF supports only a limited number of color spaces. PDF was developed by Adobe and since PDF 1.7 has been an official standard, ISO 32000-1:2008. It is considered a complex format as it supports embedded/externally linked objects. However, for the same reason it is also considered a risk for long-term preservation. PDF/A was created to confront this problem.

PDF/A has been published as a separate standard series, ISO 19005, starting from 2005. ISO chose Adobe PDF Reference 1.4, implemented in Acrobat 5, as the basis for the standard. Certain functionality allowed in PDF 1.4 has been specifically excluded from PDF/A, for example transparency and sound and movie actions. There are also elements that are not mandatory in PDF Reference 1.4, but which PDF/A requires. PDF/A comes in three 'flavours': PDF/A-1a, PDF/A-1b, and PDF/A-2. PDF/A-1a is more strict that PDF/A-1b; the most important additional requirement is the need for the file to be structured or 'tagged'. As it is near impossible to tag a PDF created from a scanned image, PDF/A-1a is essentially conceived for digitally born text files and not for digitized documents. PDF/A-1b is intended to be used for images and scans. PDF/A-2 is a new addition to the PDF/A standard; it is not fully clear yet which features and requirements will be added. Though PDF/A is less complex and more self-contained than simple PDF, even PDF/A poses obstacles to the long term sustainability of a file.'''explain''' It may however be suitable for use as a high quality or access derivative. PDF has double-layer functionality which enables full-text search. This offers a clear advantage over other formats.

Characteristics of PDF/A

Device independent; can be reliably and consistently rendered without regard to the hardware or software platform;
Contains all resources necessary for rendering;
Contains its own description;
Unfettered by technical file protection mechanisms;
Must contain: embedded fonts, device-independent color, and XMP metadata;
Must not contain: encryption, LZW compression (but can contain ZIP- and JPEG compression), embedded files, external content references, PDF transparency, multimedia, JavaScript.

Advantages: PDF can contain multiple pages in a logical order. Double-layered PDF provides a convenient solution for OCRed material.

Disadvantages: PDF poses obstacles to long-term preservation. When applying OCR, a PDF file must be converted to the TIFF format and then back to PDF; this adversely affects the quality of the file and the OCR result.

Viewers: Adobe Acrobat Reader, Adobe Acrobat, Adobe Illustrator, Foxit Reader, ACDSee Photo Manager.

MIME Types: application/pdf, application/x-pdf, application/acrobat, applications/vnd.pdf, text/pdf, text/x-pdf

Another flaw for the use of PDF long term preservation is that it is being implemented differently in software, there are many different profiles of the format.

JPEG (Joint Picture Expert Group

JPEG is commonly known as the most used file format for images on the web. However JPEG on its own is not a file format but rather an image compression algorithm developed by the Joint Photographic Experts Group in 1990; the actual file format used for data exchange—what is generally referred as the JPEG format—is called JFIF (JPEG File Interchange Format). JPEG is mostly used as a lossy format for web images, however a lossless extension to JPEG is also available. '''why don't people use it?''' Since it is mostly used with lossy compression it is not advised for masters. JPEG is recommended for derivatives, though not for text or line drawings.

Characteristics of JPEG

Lossy compression, but most software allows for adjustable level of compression;
Presence of compression artifacts;
Smaller files;
High-bit compatible;
Longer decompression time;
Supports a limited set of internal technical metadata;
Supports a limited number of color spaces;
Not suitable format for editing image files—saving, processing, and re-saving results in degradation of image quality.

Advantages: JPEG produces reasonable quality in a much smaller file size. It is currently supported by all browsers.

Disadvantages: JPEG uses lossy compression and is therefore not suited for long-term preservation.

Viewers: Adobe Photoshop XnView, ACDSee Photo Manager, Adobe Illustrator, Corel PaintShop Photo Prob, GIMP.

MIME Types: image/jpeg, image/pjpeg, image/jpg, image/jp_, application/jpg, application/x-jpg, image/pjpeg, image/vnd.swiftview-jpeg, image/x-xbitmap

JPEG 2000

JPEG 2000 was created in 2000 with the intention of superseding the original discrete cosine transform-based JPEG standard with a newly designed, wavelet-based method. In addition to lossy compression, JPEG 2000 also supports lossless compression and can provide better image quality at smaller file sizes than classic JPEG. It enables extremely efficient storage as it can be compressed with lossless compression to about fifty percent of the size uncompressed TIFFs. (TIFF with lossless compression only reduces by about thirty percent from the original size.) Nevertheless, there are a few discrepancies in the standard that are left open to interpretation by software developers. This is problematic for long term sustainability of files. JPEG 2000 has been published as an ISO standard, ISO/IEC 15444. JPEG 2000 is currently not widely supported in web browsers.

Characteristics of JPEG 2000

Complex model for encoding data (content is not saved as raster data);
Supports multiple resolutions;
Extended version supports color profiles;
Extended version supports layers;
Includes additional compression algorithms to JPEG (wavelet, lossless);
Includes support for extensive metadata encoded in XML 'boxes', particularly technical, descriptive, and rights metadata; supports IPTC information and mapping to Dublin Core.

Advantages: JPEG 2000 is an open standard which uses extremely efficient compression algorithms, resulting in smaller, high quality files. It has extensive support for metadata in the file and the possibility to add XML, e.g. EAD, MODS, METS. Theoretically, there is no need for multiple versions of a file, as JPEG 2000 masters can act as derivatives.

Disadvantages: JPEG 2000 has limited browser support, and thus is practically unusable for derivative files. The support for color spaces is unclear, which makes it problematic as a digital photograph format. The image resolution can be stored in two different fields, which has a negative impact on display. There are open source software libraries available.

Viewers: IrfanView, Apple QuickTime, Nero Multimedia Suite, Adobe Illustrator, Adobe Photoshop, Adobe Creative Suite, CorelDraw Graphics Suite.

MIME Types: image/jp2, image/jpx, image/jpm

PNG (Portable Network Graphics)

PNG is a bitmap image format and video codec that employs lossless data compression. PNG was created to improve upon and replace GIF (Graphics Interchange Format) as an image format not requiring a patent license. PNG supports palette-based images (with palettes of 24-bit RGB or 32-bit RGBA colors), grayscale images (with or without alpha channel), and RGB[A] images (with or without alpha channel). PNG was designed for transferring images on the internet, not for print graphics, and therefore does not support non-RGB color spaces such as CMYK. PNG may be used for master files but JPEG 2000 is the preferable lightweight option. (While PNG has lossless compression of about forty percent, JPEG 2000 offers fifty percent.) PNG is suitable as a derivative 1 format.

Characteristics of PNG

Simple raster format;
High-bit compatible;
Lossless compression.

Advantages: PNG is natively supported in later web browsers.

Disadvantages: PNG only supports lossless compression, and the compression algorithm is optimized for images with monochrome color fields. It likewise has no support for Exif metadata. PNG has not been widely adopted by imaging community.

Viewers: Adobe Photoshop, Corel Photo-Paint, Corel Paint Shop Pro, GIMP, GraphicConverter, Helicon Filter, ImageMagick, Inkscape, IrfanView, Pixel image editor, Paint.NET, Xara Photo and Graphic Designer, MS Paint, iPhoto, Preview, Adobe Fireworks.

MIME Type: image/png

GIF (Graphics Interchange Format)

GIF was developed in 1987 by CompuServe Incorporated, primarily for use on the internet. The current version, GIF89a, was released in 1990. It supports colour depths from 1-bit (monochrome) to 8-bit (256 colour) and always stores images in compressed form, using lossless LZW compression. Other supported features include interlacing and transparency. GIF is a proprietary format. In addition, the patent for the LZW algorithm is owned by Unisys Corporation, which has licensed its use by CompuServe. Despite the controversy which surrounded the application of license fees to GIF readers and writers, this format remains one of the most widespread in use on the internet. The US patent expired in June 2003 and the UK patent expired on 18th June 2004.

Characteristics of GIF

Lossy (high color) and lossless compression;
Limited color palette;
8-bit maximum, color images are dithered;
Short decompression time.

Advantage: GIF produces small file sizes.

Disadvantage: The bit depth of GIF files is very low as is the overall quality.

Viewers: Every browser and most graphics editor and viewers support gif images. It was one of the first image formats widely used on the world wide web.

MIME Type: image/gif

VIDEO FILE FORMATS

MPEG family (Motion Picture Expert Group family)

The MPEG compression methodology is considered asymmetric as the encoder is more complex than the decoder. The encoder needs to be 'smart' (algorithmic or adaptive) whereas the decoder is 'dumb' and carries out fixed actions. This is considered advantageous in applications such as broadcasting where the number of expensive complex encoders is small but the number of simple inexpensive decoders is large. The algorithms developed by the MPEG group are all lossy. During compression, the video stream is sampled, and reduced into segments. These segments are processed to extract key information; information is then encoded with lossless compression algorithms. The MPEG group does not define specifications on how the encoder should be realized. The group releases specifications on how the MPEG file format should be composed and decoded, so that each manufacturer can implement its own encoder, while maintaining the file format defined by the MPEG consortium and readable by every compliant decoder.

MPEG has standardized the following compression formats and ancillary standards:

''MPEG-1 (1993):'' Standard for lossy compression of video and audio. It was designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s. Also known as ISO/IEC 11172, it was the first MPEG compression standard for audio and video. It was basically designed to allow moving pictures and sound to be encoded into the bitrate of a Compact Disc. It is used on Video CD and SVCD and can be used for low quality video on DVD Video. It was used in digital satellite/cable TV services before MPEG-2 became widespread. It includes the popular MPEG-1 Audio Layer III (MP3) audio compression format, that 'virtually all digital audio devices' can play back.
''MPEG-2 (1995):'' Generic coding of moving pictures and associated audio information. It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission bandwidth. MPEG-2 has been chosen as the standard of choice for broadcast quality television, including over-the-air (terrestrial) digital television, DVB and ISDB, digital satellite TV services, digital cable television signals, SVCD, and DVD Video. It is also used on Blu-ray Discs, though these normally use MPEG-4 Part 10 or SMPTE VC-1 for high-definition content. MPEG-2 is formally known as ISO/IEC 13818 and as ITU-T Rec. H.262.
''MPEG-4 (1998):'' Coding of audio-visual objects. MPEG-4 (ISO/IEC 14496) was initially aimed primarily at low bitrate video communications; however, its scope as a multimedia coding standard was later expanded. MPEG-4 is efficient across a variety of bitrates ranging from a few kilobits per second to tens of megabits per second. MPEG-4 provides improved coding efficiency over MPEG-2 (it can achieve higher compression factors) as well as the possibility to encode mixed media data (video, audio, speech). In addition to more efficient coding of video, MPEG-4 moves closer to computer graphics applications. In more complex profiles, the MPEG-4 decoder effectively becomes a rendering processor and the compressed bitstream describes three-dimensional shapes and surface texture. MPEG-4 supports also Intellectual Property Management and Protection (IPMP), which provides the facility to use proprietary technologies to manage and protect content not unlike digital rights management. It also supports MPEG-J, a fully programmatic solution for creation of custom interactive multimedia applications (Java application environment with a Java API) and many other features.<br>In MPEG-4 several new higher-efficiency video standards (newer than MPEG-2 Video) are included, notably MPEG-4 Part 2 (or Simple and Advanced Simple Profile) used by codecs such as DivX, Xvid, Nero Digital and 3ivx and by Quicktime 6, and H.264/MPEG-4 AVC (or MPEG-4 Part 10 or H.264) used on HD-DVD and Blu-ray Discs, and Quicktime 7. H.264 is also the default video encoding format used by YouTube.
The MPEG-7 (2002) and MPEG-21 (2001) standards were published over the same period. Though neither directly related with audio/video encoding, both support the management of content so produced. MPEG-7 defines a standard for representing semantic information about the content, while MPEG-21 is a multimedia framework for intellectual property management and protection, including a 'Rights Expression Language'.

Advantages: MPEG-1, 2, and 4 are international standards, developed and used for over fifteen years. They are widely used in digital television broadcasting, DVD and Blu-ray discs, with about 200 million decoders manufactured each year. Since only the decoder is standardized, competitive manufacturers can continue to improve their encoder products, without affecting compliant decoders. Users can also choose or replace vendors without changing formats or their encoded content. MPEG-4 (particularly MPEG-4 AVC/H.264) provides very good video quality at relatively low bitrates, making it ideal for internet streaming.

Disadvantages: The MPEG family standards use lossy compression algorithms, which—depending on the bitrate used to encode the video— can affect the image quality. The results can be imperceptibly different from the original or badly flawed with visible artifacts. This characteristic primarily affects the use of MPEG formats for long-term preservation purposes. Furthermore, the licensing agreements (especially for MPEG-2 and MPEG-4) are complicated, since both contain patented technologies that require licensing in certain countries. Patents covering MPEG-4 are claimed by over two dozen companies. In the case of free software, such as VLC media player (which uses the ffmpeg library and in which the software is not sold), the end-user bears the royalty.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Windows Media Player (Windows/partial support in Mac OS), QuickTime Player (Windows/Mac OS), ALLPlayer (Windows), KMPlayer (Windows).

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux, Free), HandBrake (Windows/MacOS/Linux, Free), Avidemux (Windows/MacOS/Linux, Free), MPEG Streamclip (Windows/MacOS, Free), StaxRip (Windows, Free), x264 (Windows, Free).

MIME Types: video/mpeg, video/mpg

MJ2 (Motion JPEG 2000)

Motion JPEG 2000 is defined in ISO/IEC 15444-3 and in ITU-T T.802. It specifies the use of JPEG 2000 format for timed sequences of images (motion sequences), possibly combined with audio, and composed into an overall presentation. It also defines a file format, based on ISO base media file format (ISO/IEC 15444-12). Motion JPEG 2000, often referenced as MJ2 or MJP2, is an advancement on MJPEG (or MJ), which was itself based on the legacy JPEG format. Unlike common video formats, such as MPEG-4 Part 2 and H.264, MJ2 does not employ temporal or inter-frame compression. Instead, each frame is an independent entity encoded by either a lossy or lossless variant of JPEG 2000. This characteristic potentially makes MJ2 a truly lossless compressed format, ideal for long-term video preservation. For this reason, MJ2 is also under consideration as a digital archival format by the Library of Congress. The very high resolutions supported (up to 8192x8192) and the lossless video encoding makes MJ2 also the format of choice for Digital Cinema.

Advantages: MJ2 is an ISO standard. It gives the possibility to archive videos without quality loss, which makes it ideal for long-term video archiving.

Disadvantages: Lossless compression and high image quality exact a cost in disk space. Encoding and playback of lossless MJ2 videos is also very demanding from a computational point of view, and there are still few implementations of encoders and decoders. The format is not suited for internet streaming.

Free Encoders/Converters: OpenJPEG 2000 (Windows/MacOS/Linux, free), Kakadu software toolkit (Windows/MacOS/Linux, free), Morgan Multimedia MJ2 encoder (Windows, commercial).

MIME Type: video/x-motion-jpeg

Theora

Theora is a free and open video compression format from the Xiph.Org Foundation. It is distributed without licensing fees alongside their other free and open media projects, including the Vorbis audio format and the Ogg container. Theora is a lossy video compression format derived from the proprietary VP3 codec and released into the public domain by On2 Technologies. It is broadly comparable in design and bitrate efficiency to MPEG-4 Part 2, Windows Media Video, RealVideo, and similar low bitrate codecs. It is comparable in open standards philosophy to the BBC's Dirac codec. On the technological side Theora video codec is well engineered for low bitrate streaming. Theora video streams are multiplexed along with audio streams (usually in Vorbis or FLAC formats) by the open container format Ogg, which also acts as a transport layer. As the Ogg format has gained acceptance, components have become available to play Ogg files on practically all of the major media players. The open source Firefox, Chrome, and Opera web browsers have native Theora playback support as part of their media element (<video>) element, following a recommendation in the original HTML5 specifications. This recommendation was then retracted in June 2009, leaving HTML5 codec neutral.

Advantages: Theora is an open source standard which is completely free for commercial or noncommercial use. It is also suited for internet streaming due to its fast decoding.

Disadvantages: Theora uses lossy compression algorithms and is therefore not suited for long-term digital preservation. The codec is less efficient and performant than H.264 and VP8 and doesn't support hardware acceleration. Thus it is also not suited for use on mobile devices.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Xiph Plugins for Real Player/Producer (Windows/Mac OS/Linux), QuickTime plug-ins (Windows/MacOS), Xine (MacOS/Linux), Cortado streaming applet (Windows/MacOS/Linux), Mozilla Firefox (Windows/Mac OS/Linux), Opera Web browser (Windows/Mac OS/Linux), Google Chrome (Windows/Mac OS/Linux).

Free Encoders/Converters: Theora Encoder (Winodws/MacOS/Linux, encoder supported by various third party applications), ffmpeg (Windows/MacOS/Linux, can convert to Theora format), HandBrake (Windows/MacOS/Linux, can convert to Theora format only with the Matroska container), LiVES (Linux).

MIME Type: video/ogg</blockquote>

Dirac

Dirac is an open and royalty-free video compression format, specification, and system developed by BBC Research at the BBC. Schrödinger and Dirac-Research (formerly just called Dirac) are open and royalty-free software implementations (video codecs) of Dirac. The Dirac format aims to provide high quality video compression for web video up to Ultra HDTV and beyond and as such competes with existing formats such as H.264 and VC-1. The specification was finalized in January 2008. In September of that year version 1.0.0 of an I-frame only subset known as Dirac Pro was released and has since been standardized by the SMPTE as VC-2, aimed for professional and studio use in high bitrate applications. Version 2.2.3 of the full Dirac specification, including motion compensation and inter-frame coding, was issued a few days later. Dirac Pro was used internally by the BBC to transmit HDTV pictures at the Beijing Olympics in 2008.

Dirac supports resolutions up to HDTV (1920x1080) and greater, and it is claimed to provide significant savings in data rate and improvements in quality over video compression formats such as MPEG-2 Part 2, MPEG-4 Part 2, Theora, and WMV. Dirac's implementers make the claim that it offers, "a two-fold reduction in bitrate over MPEG-2 for high definition video". This makes it comparable to the latest generation standards such as H.264/MPEG-4 AVC and VC-1. Dirac supports both constant bitrate and variable bitrate operation and lossy and lossless compression modes. Dirac employs wavelet compression, instead of the discrete cosine transforms used in most older compression formats. Dirac is one of several projects attempting to apply wavelets to video compression. Others include Rududu, Snow, cineform, REDCODE and the now-obsolete Tarkin. Wavelet compression is also used in the JPEG 2000 and PGF compression standards for photographic images. Dirac can be used in AVI, Ogg, and Matroska (MKV) container formats and is also registered for the use in the MPEG-4 file format and MPEG-2 transport streams.

Advantages: It is an open source and royalty-free standard with support for HDTV resolutions and higher. Dirac Pro also has support for lossless compression.

Disadvantages: The Dirac codec performs a bit worse than H.264 and adoption and support is still low compared to formats like MPEG, Theora, and VP8.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), ALLPlayer (Windows).

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux), LiVES (Linux); MediaCoder (Windows).

MIME Type: video/ogg

VP8

VP8 is an open video compression format released by Google and originally created by On2 Technologies. Google acquired On2 in 2010 and immediately released the VP8 source code, following an open letter issued from the Free Software Foundation asking Google to gradually replace Adobe Flash Player and H.264 on YouTube with a mixture of HTML5 and a freed VP8. Google released the VP8 codec software under a BSD-like license, and the VP8 bitstream format specification under an irrevocable free patent license. This made VP8 the second product from On2 Technologies to be open sourced. The older VP3 codec, released in 2001, was later donated under the BSD license to the Xiph.Org Foundation as the Theora codec.

At the same time the WebM Project was launched, featuring contributions from Mozilla, Opera, Google, and more than forty other publishers, software, and hardware vendors in a major effort to use VP8 as the codec for HTML5. In the WebM container format, the VP8 video is used with Vorbis audio. Internet Explorer 9 will support VP8 video playback if the proper codec is installed. Android is WebM-enabled from version 2.3 - Gingerbread. And Adobe has also announced that the Flash Player will support VP8 playback in a future release. YouTube now offers WebM videos as part of its HTML5 player experiment. All uploaded files with resolutions from 720p and above are encoded to WebM in 480p and 720p, and other resolutions will follow. YouTube has committed to encode their entire portfolio of videos to WebM. Skype has likewise implemented the VP8 codec into the Skype 5.0 software. The WebM hardware development team has recently released specs for chip companies to develop hardware decoding for VP8.

Advantages: VP8 is an open source, royalty-free standard, which performs better than Theora and Dirac. Hardware support for encoding/decoding is building and adoption will quickly increase after Google's recent to switch YouTube encoding from H.264 to VP8.

Disadvantages: Claims that it is patent free still are questionable, particularly given its similarity to H.264. The specifications also remain weak. And the format should still be optimized.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), XBMC (Windows/Mac OS/Linux), Winamp (Windows), Mozilla Firefox 4 beta 1 and later (Windows/Mac OS/Linux), Opera 10.60 and later (Windows/Mac OS/Linux), Chrome 6 and later (Windows/Mac OS/Linux).

Free Encoders: ffmpeg (Windows/Mac OS/Linux), GStreamer (Windows/Linux).

MIME Type: video/webm

AUDIO FILE FORMATS

LPCM (Linear Pulse-Code Modulation)

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It was invented by Alec Reeves in 1937 and is currently the standard form for digital audio in computers and various Blu-ray, Compact Disc, and DVD formats. It is also employed in other technologies such as digital telephone systems. Linear PCM (LPCM) is a particular method of pulse-code modulation which implements linear quantization. LPCM is used for the uncompressed encoding of audio data in the compact disc ''Red Book'' standard (informally also known as 'Audio CD'), which was introduced in 1982. Since LPCM uses an uncompressed storage method that keeps all the samples of an audio track, professional users or audio experts may use this format for maximum audio quality.

LPCM audio is coded using a combination of various parameters: resolution/sample size (e.g., 8, 16, 20, 24 bit, etc); frequency/sample rate (e.g., 8,000, 11,025, 16,000, 22,050, 24,000, 32,000, 44,100, and 48,000 Hz / samples per second, etc); sign (signed or unsigned); number of channels (monaural, stereo, quadrophonic, etc) and interleaving of channels; byte order (little endian, big endian). One minute of LPCM-encoded sound can occupy as little as 644 kilobytes (KB) or as much as 27 megabytes (MB) of storage. The size of the storage space depends on the sampling frequency, the number of channels (mono or stereo), and the number of bits that are used for the sample. The standard audio file format for CDs, for example, is LPCM-encoded, containing two channels of 44,100 samples per second, 16 bits per sample.

LPCM is the encoding format usually used in WAVE/WAV and AIFF audio container formats. Windows and MacOS systems use these formats to store sounds as waveforms in a raw and typically uncompressed way. WAVE, short for Waveform Audio File Format, is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs. It is an application of the RIFF bitstream format method for storing data in 'chunks', and thus is also close to the AIFF format used on Macintosh computers. Uncompressed LPCM-encoded WAV files are quite large, so the file sharing of WAV files has declined in popularity. However, it is still a commonly used file type suitable for retaining first generation archived files of high quality, for use on a system where disk space is not a constraint, or for use in applications such as audio editing, where the time involved in compressing and decompressing data is a concern.

Advantages: It is an uncompressed format ideal for long-term digital preservation. It has been widely adopted, particularly in professional environments (e.g. radio and broadcasting).

Disadvantages: Uncompressed high quality audio needs more disk space. The format is not suited for internet streaming and distribution.

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux, free).

MIME Types: audio/wav, audio/wave, audio/x-wav

FLAC (Free Lossless Audio Codec)

Free Lossless Audio Codec (FLAC) is an open source audio codec that employs a lossless data compression algorithm; this means that a digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to fifty or sixty percent of their original size. FLAC is an open and royalty-free format with a free software implementation made available. FLAC has support for tagging, cover art, and fast seeking. Though FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats like MP3, FLAC is supported by more hardware devices than competing lossless formats like WavPack. The European Broadcasting Union (EBU) has adopted the FLAC format for the distribution of high quality audio over its Euroradio network.

FLAC is specifically designed for efficient packing of audio data, unlike general purpose lossless algorithms such as DEFLATE, which is used in ZIP and gzip. While ZIP may compress a CD-quality audio file by ten to twenty percent, FLAC achieves compression rates of thirty to fifty percent for most music, with significantly greater compression for voice recordings. By contrast, lossy codecs can achieve ratios of eighty percent or more by discarding data from the original stream. As a lossless scheme, FLAC is also a suitable archive format for digital preservation of audio collections. In fact if the original media is lost, damaged, or worn out, a FLAC copy of the audio tracks ensures that an exact duplicate of the original data can be recovered at any time. An exact restoration from a lossy archive (e.g. MP3) of the same data is impossible.

Advantages: FLAC is an open and royalty-free lossless format ideal for long-term digital preservation. Compression can save up to fifty percent of storage space.

Disadvantages: Compared to MP3 or WAV-LPCM, FLAC is still not widely supported in hardware devices nor is it suited for internet streaming and distribution.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Songbird (Windows/Mac OS/Linux), Windows Media Player (Windows/partial support in Mac OS), QuickTime Player with the Xiph QT component (Windows/Mac OS), Winamp (Windows).

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux, Free), Flake (Windows/Linux, Free), Audacity (Windows/MacOS/Linux, Free).

MIME Type: audio/x-flac

MP3 (MPEG-1 or MPEG-2 Audio Layer 3)

MPEG-1 or MPEG-2 Audio Layer 3 (or III), more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression. It is a common audio format for consumer audio storage, as well as a ''de facto'' standard of digital audio compression for the transfer and playback of music on digital audio players. MP3 is an audio-specific format that was designed by the Moving Picture Experts Group as part of its MPEG-1 standard and later extended in the MPEG-2 standard. MPEG-1 Audio (MPEG-1 Part 3), which included MPEG-1 Audio Layer I, II and III was finalized in 1992 and published in 1993 as ISO/IEC 11172-3:1993. Backwards compatible standard MPEG-2 Audio (MPEG-2 Part 3) with additional bitrates and sample rates was published in 1995 as ISO/IEC 13818-3:1995.

The use in MP3 of a lossy compression algorithm is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners. An MP3 file that is created using the setting of 128 kbit/s will result in a file that is about 11 times smaller than the CD file created from the original audio source. An MP3 file can also be constructed at higher or lower bitrates, with higher or lower resulting quality. The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory resolution ability of most people. This method is commonly referred to as perceptual coding. It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.

When performing lossy audio encoding there is a trade-off between the amount of space used and the sound quality of the result. Typically, the creator is allowed to set a bitrate, which specifies how many kilobits the file may use per second of audio. The higher the bitrate, the larger the compressed file will be, and generally the closer it will sound to the original file. With too low a bitrate, compression artifacts (i.e. sounds that were not present in the original recording) may be audible in the reproduction. Besides the bitrate of an encoded piece of audio, the quality of MP3 files also depends on the quality of the encoder itself, and the difficulty of the signal being encoded. As the MP3 standard allows quite a bit of freedom with encoding algorithms, different encoders may feature quite different quality, even with identical bitrates. Therefore, the quality of an MP3 file is dependent on the choice of encoder and encoding parameters.

Advantages: MP3 is an international standard and has been widely adopted; it is also supported by a range of portable players and devices. It can provide relatively good audio quality with high compression rates.

Disadvantages: MP3 uses a lossy compression algorithm and therefore not suited for long-term preservation. The format has a limited data rate of 320 kbps. It is also subject to patents.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Songbird (Windows/Mac OS/Linux), Windows Media Player (Windows/partial support in Mac OS), QuickTime Player (Windows/Mac OS), iTunes (Windows/Mac OS), ALLPlayer (Windows), Winamp (Windows).

Free Encoders/Converter: LAME

MIME Types: audio/mpeg, audio/MPA

AAC (Advanced Audio Coding)

Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bitrates. AAC has been standardized by ISO and IEC, as Part 7 of the MPEG-2 standard and Subpart 4 in Part 3 of the MPEG-4 standard. AAC is also the default or standard audio format for iPhone, iPod, iPad, Nintendo DSi, iTunes, DivX Plus Web Player and PlayStation 3. It is supported on PlayStation Portable, Wii, the latest generation of Sony Walkman, mobile phones made by Sony Ericsson and Nokia as well as Android-based mobile phones.

Blind tests have shown that AAC demonstrates greater sound quality and transparency than MP3 for files coded at the same bitrate. Overall, the AAC format allows developers more flexibility to design codecs than MP3 does and corrects many of the design choices made in the original MPEG-1 audio specification. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression. AAC and HE-AAC are better than MP3 at low bitrates (typically less than 128 kilobits per second). This is especially true at very low bitrates where the superior stereo coding, pure MDCT, and more optimal transform window sizes leave MP3 unable to compete. However, as bitrates increase the efficiency of an audio format becomes less important relative to the efficiency of the encoder's implementation, and the intrinsic advantage AAC holds over MP3 no longer dominates audio quality. In reality, the advantages of AAC over MP3 are not entirely clear cut, and the MP3 specification, although antiquated, has proven surprisingly robust in spite of considerable flaws.

Advantages: AAC is an international standard and has been widely adopted. It is supported in portable players and devices, though not as much as MP3. AAC can provide better audio quality than MP3 at the same bitrates.

Disadvantages: AAC uses a lossy compression algorithm and is therefore not suited to long-term preservation. The format is also subject to patents.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Songbird (Windows/Mac OS/Linux), Windows Media Player (Windows/partial support in Mac OS), QuickTime Player (Windows/Mac OS), iTunes (Windows/Mac OS), ALLPlayer (Windows), Winamp (Windows).

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux, free), Nero Digital Audio (Windows, free).

MIME Types: audio/aac, audio/aacp, audio/mp4, audio/mpeg4-generic, audio/3gpp

Vorbis

Vorbis is an open source project headed by the Xiph.Org Foundation, the same foundation that is maintaining the FLAC and Theora projects. The project produces an audio format specification and software implementation (codec) for lossy audio compression. Vorbis is most commonly used in conjunction with the Ogg container format, and it is therefore often referred to as Ogg Vorbis. Vorbis is a continuation of audio compression development started in 1993 by Chris Montgomery. Intensive development began following a September 1998 letter from the Fraunhofer Society announcing plans to charge licensing fees for the MP3 audio format.

Vorbis seems to perform quite well compared to the more widely supported MP3 and AAC codecs, and it has the advantage of being royalty free. As originally recommended in the HTML 5 specifications, web browsers like Mozilla Firefox, Google Chrome, Opera, and SeaMonkey natively support Vorbis audio (without a plug-in) using the <audio> element. Vorbis is also the audio codec (along with the VP8 video codec) used by Google in the open source WebM audiovisual format. The Spotify audio streaming service also uses Vorbis for its audio streams.

Advantages: Vorbis is an open and royalty-free format which is well suited for internet streaming and distribution.

Disadvantages: Vorbis uses a lossy compression algorithm and is therefore not suited for long-term preservation. Moreover, it is still not widely supported in hardware devices, especially when compared to MP3. However, support is growing.

Free Players: VLC Media Player (Windows/Mac OS/Linux), MPlayer (Windows/Mac OS/Linux), Songbird (Windows/Mac OS/Linux), QuickTime Player with the Xiph QT component (Windows/Mac OS), iTunes with the Xiph QT component (Windows/Mac OS), Winamp (Windows).

Free Encoders/Converters: ffmpeg (Windows/MacOS/Linux, free), aoTuV (Windows/MacOS/Linux, free).

MIME Types: audio/ogg, audio/vorbis, audio/vorbis-config