MP3 in the context of Variable bit rate

A graphical user interface, or GUI, is a form of user interface that allows users to interact with electronic devices through graphical icons and visual indicators such as secondary notation. In many applications, GUIs are used instead of text-based UIs, which are based on typed command labels or text navigation. GUIs were introduced in reaction to the perceived steep learning curve of command-line interfaces (CLIs), which require commands to be typed on a computer keyboard.

The actions in a GUI are usually performed through direct manipulation of the graphical elements. Beyond computers, GUIs are used in many handheld mobile devices such as MP3 players, portable media players, gaming devices, smartphones and smaller household, office and industrial controls. The term GUI tends not to be applied to other lower-display resolution types of interfaces, such as video games (where head-up displays (HUDs) are preferred), or not including flat screens like volumetric displays because the term is restricted to the scope of 2D display screens able to describe generic information, in the tradition of the computer science research at the Xerox Palo Alto Research Center.

An audiobook (or a talking book) is a recording of a book or other work being read out loud. A reading of the complete text is described as "unabridged", while readings of shorter versions are abridgements.

Spoken audio has been available in schools and public libraries and to a lesser extent in music shops since the 1930s. Many spoken word albums were made prior to the age of cassettes, compact discs, and downloadable audio, often of poetry and plays rather than books. It was not until the 1980s that the medium began to attract book retailers, and then book retailers started displaying audiobooks on bookshelves rather than in separate displays.

An album is a collection of audio recordings (e.g., music) issued on a medium such as compact disc (CD), vinyl (record), audio tape (like 8-track or cassette), or digital. Albums of recorded sound were developed in the early 20th century as individual 78 rpm records (78s) collected in a bound book resembling a photo album; this format evolved after 1948 into single vinyl long-playing (LP) records played at 33+1⁄3 rpm.

The album was the dominant form of recorded music expression and consumption from the mid-1960s to the early 21st century, a period known as the album era. Vinyl LPs are still issued, though album sales in the 21st-century have mostly focused on CD and MP3 formats. The 8-track tape was the first tape format widely used alongside vinyl from 1965 until being phased out by 1983, being gradually supplanted by the cassette tape throughout the 1970s and early 1980s; the popularity of the cassette reached its peak during the late 1980s before sharply declining during the 1990s. The cassette had largely disappeared by the first decade of the 2000s.

A portable media player (PMP) or digital audio player (DAP) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. Normally, they refer to small, battery-powered devices utilising flash memory or a hard disk for storing various media files. MP3 players has been a popular alternative name used for such devices, even if they also support other file formats and media types other than MP3 (for example AAC, FLAC, WMA).

Generally speaking, PMPs are equipped with a 3.5 mm headphone jack which can be used for headphones or to connect to a boombox, home audio system, or connect to car audio and home stereos wired or via a wireless connection such as Bluetooth, and some may include radio tuners, voice recording and other features. In contrast, analogue portable audio players play music from non-digital media that use analogue media, such as cassette tapes or vinyl records. As devices became more advanced, the PMP term was later introduced to describe players with additional capabilities such as video playback (they used to also be called "MP4 players"). The PMP term has also been used as an umbrella name to describe any portable device for multimedia, including physical formats (such as portable CD players) or handheld game consoles with such capabilities.

An audio coding format (or sometimes audio compression format) is a encoded format of digital audio, such as in digital television, digital radio and in audio and video files. Examples of audio coding formats include MP3, AAC, Vorbis, FLAC, and Opus. A specific software or hardware implementation capable of audio compression and decompression to/from a specific audio coding format is called an audio codec; an example of an audio codec is LAME, which is one of several different codecs which implements encoding and decoding audio in the MP3 audio coding format in software.

Some audio coding formats are documented by a detailed technical specification document known as an audio coding specification. Some such specifications are written and approved by standardization organizations as technical standards, and are thus known as an audio coding standard. The term "standard" is also sometimes used for de facto standards as well as formal standards.

A discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequencies. The DCT, first proposed by Nasir Ahmed in 1972, is a widely used transformation technique in signal processing and data compression. It is used in most digital media, including digital images (such as JPEG and HEIF), digital video (such as MPEG and H.26x), digital audio (such as Dolby Digital, MP3 and AAC), digital television (such as SDTV, HDTV and VOD), digital radio (such as AAC+ and DAB+), and speech coding (such as AAC-LD, Siren and Opus). DCTs are also important to numerous other applications in science and engineering, such as digital signal processing, telecommunication devices, reducing network bandwidth usage, and spectral methods for the numerical solution of partial differential equations.

A DCT is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. The DCTs are generally related to Fourier series coefficients of a periodically and symmetrically extended sequence whereas DFTs are related to Fourier series coefficients of only periodically extended sequences. DCTs are equivalent to DFTs of roughly twice the length, operating on real data with even symmetry (since the Fourier transform of a real and even function is real and even), whereas in some variants the input or output data are shifted by half a sample.

A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or audiobooks. CD players may be part of home stereo systems, car audio systems, personal computers, or portable CD players such as CD boomboxes. Most CD players produce an output signal via a headphone jack or RCA jacks. To use a CD player in a home stereo system, the user connects an RCA cable from the RCA jacks to a hi-fi (or other amplifier) and loudspeakers for listening to music. To listen to music using a CD player with a headphone output jack, the user plugs headphones or earphones into the headphone jack.

Modern units can play audio formats other than the original CD PCM audio coding, such as MP3, AAC and WMA. DJs playing dance music at clubs often use specialized players with an adjustable playback speed to alter the pitch and tempo of the music. Audio engineers using CD players to play music for an event through a sound reinforcement system use professional audio-grade CD players. CD playback functionality is also available on CD-ROM/DVD-ROM drive-equipped computers as well as on DVD players and most optical disc-based home video game consoles.

A keep case or poly-box is a type of packaging, most commonly used with DVDs and Blu-ray videos (and sometimes CDs).

Besides DVD-Video films, keep cases are very common with most disc-based video games since the PlayStation 2, and they are also found on many PC titles and MP3-CD audiobooks (all use discs that are the same basic dimensions as a DVD).

A music download is the digital transfer of music via the Internet into a device capable of decoding and playing it, such as a personal computer, portable media player, MP3 player or smartphone. This term encompasses both legal downloads and downloads of copyrighted material without permission or legal payment. Music downloads are typically encoded with the MP3 audio coding format. or using the modified discrete cosine transform (MDCT) audio data compression, particularly the Advanced Audio Coding (AAC) format used by iTunes.

Since the advent of streaming, downloads as a mode of music distribution has seen a steady decline from its peak in the early 2010s. According to a Nielsen report, downloadable music accounted for 55.9 percent of all music sales in the US in 2012. By the beginning of 2011, Apple's iTunes Store alone made US$1.1 billion of revenue in the first quarter of its fiscal year. According to the RIAA, music downloads peaked at 43% of industry revenue in the US in 2012, and has since fallen to 3% in 2022.

Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. It was developed by Dolby, AT&T, Fraunhofer and Sony, originally as part of the MPEG-2 specification but later improved under MPEG-4. AAC was designed to be the successor of the MP3 format (MPEG-2 Audio Layer III) and generally achieves higher sound quality than MP3 at the same bit rate. AAC encoded audio files are typically packaged in an MP4 container most commonly using the filename extension .m4a.

The basic profile of AAC (both MPEG-4 and MPEG-2) is called AAC-LC (Low Complexity). It is widely supported in the industry and has been adopted as the default or standard audio format on products including Apple's iTunes Store, Nintendo's Wii, DSi and 3DS and Sony's PlayStation 3. It is also further supported on various other devices and software such as iPhone, iPod, PlayStation Portable and Vita, PlayStation 5, Android and older cell phones, digital audio players like Sony Walkman and SanDisk Clip, media players such as VLC, Winamp and Windows Media Player, various in-dash car audio systems, and is used on Spotify, Google Nest, Amazon Alexa. Apple Music, YouTube and also YouTube Music streaming services. AAC has been further extended into HE-AAC (High Efficiency, or AAC+), which improves efficiency over AAC-LC. Another variant is AAC-LD (Low Delay).

Windows Media Audio (WMA) is a series of audio codecs and their corresponding audio coding formats developed by Microsoft. It is a proprietary technology that forms part of the Windows Media framework. Audio encoded in WMA is stored in a digital container format called Advanced Systems Format (ASF).

WMA consists of four distinct codecs. The original WMA codec, known simply as WMA, was conceived as a competitor to the popular MP3 and RealAudio codecs. WMA Pro, a newer and more advanced codec, supports multichannel and high-resolution audio. A lossless codec, WMA Lossless, compresses audio data without loss of audio fidelity (the regular WMA format is lossy). WMA Voice, targeted at voice content, applies compression using a range of low bit rates.

MP4 (formally MPEG-4 Part 14) is a digital multimedia container format most commonly used to store video and audio, but it can also be used to store other data such as subtitles and still images. Like most modern container formats, it allows streaming over the Internet. The only filename extension for MPEG-4 Part 14 files as defined by the specification is .mp4.

MPEG-4 Part 14 is a standard specified as a part of the MPEG-4 specifications, formally as ISO/IEC 14496-14:2003. Unlike the audio-only compression formats MP3 and MP2, MP4 is a container format that can hold various types of media from various codecs. During the 2000s, portable media players were sometimes erroneously advertised as "MP4 players", even if they may play a different format like AMV video and not necessarily the MPEG-4 Part 14 format.

Transcoding is the direct digital-to-digital conversion of one encoding to another, such as for video data files, audio files (e.g., MP3, WAV), or character encoding (e.g., UTF-8, ISO/IEC 8859). This is usually done in cases where a target device (or workflow) does not support the format or has limited storage capacity that mandates a reduced file size, or to convert incompatible or obsolete data to a better-supported or modern format.

In the analog video world, transcoding can be performed just while files are being searched, as well as for presentation. For example, Cineon and DPX files have been widely used as a common format for digital cinema, but the data size of a two-hour movie is about 8 terabytes (TB). That large size can increase the cost and difficulty of handling movie files. However, transcoding into a JPEG2000 lossless format has better data compression performance than other lossless coding technologies; in many cases, JPEG2000 can compress images to half their original size.

A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the media's data so that it becomes small enough to be stored within the desired disk space or transmitted (streamed) within the available bandwidth (known as the data rate or bit rate). If the compressor cannot store enough data in the compressed version, the result is a loss of quality, or introduction of artifacts. The compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the user.

The most common digital compression artifacts are DCT blocks, caused by the discrete cosine transform (DCT) compression algorithm used in many digital media standards, such as JPEG, MP3, and MPEG video file formats. These compression artifacts appear when heavy compression is applied, and occur often in common digital media, such as DVDs, common computer file formats such as JPEG, MP3 and MPEG files, and some alternatives to the compact disc, such as Sony's MiniDisc format. Uncompressed media (such as on Laserdiscs, Audio CDs, and WAV files) or losslessly compressed media (such as FLAC or PNG) do not suffer from compression artifacts.