AAC (Advanced Audio Coding) – An up-and-coming compression format for digital audio. In terms of sound quality and data efficiency, AAC solidly beats the still-popular MP3 format – not surprising, since AAC is a newer, more advanced form of compression. According to some listening tests, AAC files encoded at lower bitrates (like 96 Kbps) sound as good or better than MP3s encoded at higher bitrates (like 128 Kbps) despite their notably smaller size.
The current version of the AAC codec was developed as part of the MPEG4 standard. AAC is the audio file format used by Apple in their popular iTunes Music Store. Files may appear on your system with the “.M4A” filename extension.
AIFF (Audio Interchange File Format) – An audio format for Macintosh operating systems commonly used for storing uncompressed, CD-quality sound (similar to WAV files for Windows-based PCs).
Apple Lossless – Apple Lossless Encoding (also known as Apple Lossless, Apple Lossless Audio Codec or ALE) is an audio codec developed by Apple Computer that provides full, uncompressed CD-quality audio in about half the space of the original file.
ATRACTM – Developed by Sony engineers in the early 90’s, ATRAC is an audio codec which offers near-CD sound quality. The MiniDisc format uses ATRAC to fit a whole CD’s worth of music on a 2-1/2″ disc.
ATRAC3TM is a newer version of this codec that squeezes music into even smaller files. It’s used for MDLP recording with some MiniDisc recorders, for music storage in some portable memory players, and in other Internet music applications like Liquid Audio and RealAudio.
A MiniDisc recorder with MDLP gives you a range of compression options (in order of increasing compression):
- regular recording mode: standard ATRAC codec (292 Kbps encoding bitrate)
- LP2 mode: ATRAC3 codec (132 Kbps encoding bitrate)
- LP4 mode: ATRAC3 codec (66 Kbps encoding bitrate)
ATRAC3plusTM, the newest version of the ATRAC codec, is found on Sony’s Hi-MD portable recorders and offers even better sound quality at lower bitrates than earlier versions.
AU – An audio format commonly used for posting sound clips on the Internet. AU files can be played back on Windows, Macintosh, and other operating systems.
AVI (Audio/Video Interleaved) – A file format for storing and playing back movie clips with sound on Windows-based PCs. An AVI file is organized into alternating (“interleaved”) chunks of audio data and video data. AVI is a “container format,” meaning that it specifies how the data will be organized, but it is not itself a form of audio or video compression.
Digital video fans may be familiar with AVI as the type of file that’s created when DV clips are imported from a digital camcorder to a PC. (These clips are often referred to as “DV-AVIs” because they contain full-quality DV content.)
Bitrate – With audio compression, the average amount of data required to store one second of music (expressed in kilobits per second, or Kbps). Some codecs like MP3, WMA, and AAC allow files to be encoded at different bitrates. Generally, as bitrate decreases, so does the sound quality of the resulting file (and also the amount of memory required to store it).
DV (Digital Video) – DV is the format used by most digital camcorders for capturing crisp, colorful movies and CD-quality sound, usually on Mini DV cassettes. Though the DV format employs a form of video compression (applied in real-time as you record with your camera), it’s still memory intensive. When transferred to a computer, a DV clip requires roughly 1 GB of storage per 5 minutes of video. (Clips are usually stored on the computer as QuickTime or .AVI files.)
Despite its use of compression, DV boasts an incredible picture with up to 520 lines of resolution. DV uses a type of compression known as “intraframe” – that is, it encodes video at the full standard frame rate of 30 frames per second. This not only makes for high-quality video, but also allows frame-by-frame editing. In contrast, video codecs like MPEG1 or MPEG2, which can “squeeze” clips into smaller sizes, tend to handle a video sequence by reducing the number of full frames per second and encoding the differences between frames. These are known as “interframe” forms of compression.
GIF (Graphic Interchange Format) – A format for storing digital images, commonly used for bullets, icons, and other graphics on the Web. The GIF format is limited to 256 colors, so it’s not as commonly used as JPEG for storing digital photos. A single GIF file can combine several frames together, for basic animated motion.
JPEG – Named after the Joint Photographic Experts Group, JPEG is a codec for storing and transferring full-color digital images, often used to post photography and artwork on the Web. JPEG compression takes advantage of the human eye’s inability to see minute color changes, removing portions of data from the original picture file. When creating a JPEG file, varying amounts of compression can be selected, depending on the desired file size and image quality.
A form of this codec known as Motion JPEG is used by some digital cameras and camcorders for storing video clips of relatively small file size. With Motion JPEG, each frame of video is captured separately and reduced in size using JPEG compression.
Liquid Audio – More than just a digital audio format, Liquid Audio offers software for music management and playback (Liquid Player), and a network of affiliated web sites (Liquid Music Network) that sell downloadable, copy-protected music. Liquid Audio files usually appear with the “.LQT” or “.LA1” extension. They can utilize several different types of compression, and can come in streaming or downloadable form.
MIDI (Musical Instrument Digital Interface) – A MIDI file doesn’t contain actual audio data, but rather contains commands that let MIDI-capable synthesizers re-create a specific musical passage. The MIDI protocol has been used for years as a way for electronic musical instruments (like digital keyboards and sequencers) to communicate with each other.
Computer sound cards typically feature the ability to interpret MIDI files into music. Since they don’t actually contain the music itself, but rather the commands used to re-create music, MIDI files are a lot smaller than audio files like MP3s, WMAs, or WAVs. MIDI files are small and manageable enough that it’s not uncommon to find them embedded in web pages, adding a sonic element to the surfing experience. MIDI files usually appear with the “.MID” filename extension.
MPEG – MPEG stands for Moving Picture Experts Group – a committee that sets international standards for the digital encoding of movies and sound. There are several audio/video formats which bear this group’s name. In addition to their popularity on the Internet, several MPEG formats are used with different kinds of A/V gear:
- MPEG1. This format is often used in digital cameras and camcorders to capture small, easily transferable motion video clips. It’s also the compression format used to create Video CDs. Commonly used for posting clips on the Internet.
- MPEG2. Commercially produced DVD movies, home-recorded DVD discs, and digital satellite TV broadcasts all employ MPEG2 video compression to deliver their high-quality picture. MPEG2 is also the form of compression used by TiVo-based hard disk video recorders. MPEG2 can rival, or even slightly outperform, the DV format when it comes to picture quality – despite the fact that MPEG2 is a “heavier” form of compression (i.e., it removes a larger portion of the original video signal). The MPEG2 codec allows for selectable amounts of compression to be applied, which is how home DVD recorders and hard disk video recorders can offer a range of recording “speeds.”
- MPEG4. This newer, very flexible MPEG codec is used for both streaming and downloadable Web content, and is also the video format employed by a growing number of portable video recorders.
The well-known MP3 audio format (see definition below) is part of the MPEG1 codec.
MP3 (MPEG1, Audio Layer 3) – The most popular codec for storing and transferring music. Though it employs a “lossy” compression system which removes frequencies judged to be essentially inaudible, MP3 still manages to deliver near-CD sound quality in a file that’s only about a tenth or twelfth the size of a corresponding uncompressed WAV file. When creating an MP3 file, varying amounts of compression can be selected, depending on the desired file size and sound quality.
MP3 Pro – An updated version of the original MP3 codec, MP3 Pro allows small, low-bitrate files to contain much more high-frequency detail than standard MP3 files encoded at similar low bitrates. The high-frequency portion of the audio signal is handled by an advanced and extremely efficient coding process known as Spectral Band Replication (SBR), while the rest of the signal is encoded using the same process as in regular MP3. This enables older MP3 “player” software applications to play files encoded in MP3 Pro – though older software can only play the standard, non-SBR-encoded portions of the signal (which means you lose the highs altogether). However, when encoded and played back using a fully compatible audio program, MP3 Pro files deliver very good sound quality using low bitrates.
Ogg Vorbis = Vorbis – Vorbis is an “open-source” digital audio compression format – that is, it exists in the public domain and is completely free for commercial or non-commercial use. Because Vorbis is most often used in conjunction with a digital A/V container format known as “Ogg,” it’s usually referred to as “Ogg Vorbis.”
QuickTime – QuickTime is a file format for storing and playing back movies with sound. Though developed and supported primarily by Apple Computer, Inc., this flexible format isn’t limited to Macintosh operating systems – it’s also commonly used in Windows systems, and other types of computing platforms. In Windows, QuickTime files usually appear with the “.MOV” filename extension.
RealMedia – One of the most popular formats for streaming content on the Internet, RealMedia includes the RealAudio codec for sound clips and RealVideo codec for movies. RealAudio and RealVideo files are often given the common RealMedia “.RM” file extension. RealMedia files are often heavily compressed so they can stream over dial-up Internet connections.
SDII (Sound Designer II) – An audio format for Macintosh operating systems which is often employed by pro-quality sound editing software applications. SDII files, like AIFF and WAV files, are capable of storing uncompressed CD-quality audio.
SDMI (Secure Digital Music Initiative) – SDMI isn’t a type of file format; rather, it’s a copyright protection system for digital music files. SDMI-compliant hardware and software will enable you to download and play music distributed on the Internet – not only free MP3 files, but also copy-protected music from major-label artists.
SHN (Shorten) – Shorten is a “lossless” form of compression for digital audio. A SHN file is only about 1/2 the size of its original WAV or AIFF source. Unlike “lossy” audio codecs (like MP3, WMA, etc.), SHN is capable of reproducing the original audio signal in its entirety, without removing frequencies. Because of this, SHN offers significantly better sound quality than MP3. However, since SHN files require significantly more storage space than MP3 files, this format isn’t nearly as convenient when it comes to storage or download time.
Variable Bitrate (VBR) – Many newer audio and video codecs employ a technology known as variable-bitrate encoding, which allows resulting files to look and sound better, while still retaining a compressed, convenient file size. Essentially, VBR encoding assigns more bits to complexly detailed passages in the original source, and fewer bits to the simpler passages.
By contrast, constant-bitrate (CBR) encoding uses about the same amount of memory for simple and complex passages – so the user is more likely to experience audible or visible loss of quality during complex parts, especially with lower-bitrate files.
Vorbis (Ogg Vorbis) – Vorbis is an “open-source” digital audio compression format – that is, it exists in the public domain and is completely free for commercial or non-commercial use. Because Vorbis is most often used in conjunction with a digital A/V container format known as “Ogg,” it’s usually referred to as “Ogg Vorbis.”
Vorbis, like MP3, is a “lossy” compression system, removing frequencies deemed inaudible. Both formats offer variable-bitrate encoding options, for better efficiency. But the algorithms Vorbis uses to decide which information to discard differ from those used by MP3. Proponents claim that the Vorbis format outperforms MP3, producing files that are significantly smaller than MP3s of similar sound quality (or files that sound better than similarly sized MP3s).
WAV – A standard audio format for Windows operating systems, often used for storing high-quality, uncompressed sound. WAV files can contain CD-quality (44.1 KHz/16-bit) audio signals. However, CD-quality WAV files require relatively large amounts of memory – roughly 10 MB per minute of music.
WMA (Windows Media Audio) – Developed by Microsoft, Windows Media Audio is one of today’s most popular ‘Net audio formats. Though not as popular as MP3, WMA tends to outperform MP3 in the area of sound quality, particularly with files encoded at lower bitrates like 64 or 96 Kbps. This performance advantage makes it handy for applications like portable digital audio players, where total play time is limited by a finite amount of internal memory.
The Windows Media Audio format features built-in copy protection abilities, unlike MP3. Windows XP, Microsoft’s current flagship operating system software, contains native support for WMA encoding, enabling users to create their own WMA music files.
WMV (Windows Media Video) – Microsoft’s proprietary compression format for motion video, Windows Media Video is used for both streaming and downloading content via the Internet. Microsoft’s Windows Media Player, an application bundled with Windows XP operating systems, lets you playback and manage a range of audio and video file types, including, of course, WMA and WMV.
AC3 = Dolby Digital – в общем случае система кодирования (сжатия) звука, разработанного инженерами фирмы Dolby Laboratories. Звуковая дорожка Dolby Digital может содержать моно звук, стерео звук, а также 5.1 звук, запись и воспроизведение которого выполняется для 6 каналов – левого, центрального и правого фронтальных каналов, низкочастотного LFE (общепринятое название subwoofer) а также левого и правого тыловых каналов.
Dolby Pro Logic – система объемного звучания, разработанного инженерами фирмы Dolby Laboratories. Запись и воспроизведение звука выполняется для пяти каналов – левого, центрального и правого фронтальных каналов, а также левого и правого тыловых каналов. Отличается от системы Dolby Surround большим разделением фронтальных и тыловых каналов.
Dolby Surround – одна из первых систем объемного звучания, разработанных инженерами фирмы Dolby Laboratories. Запись и воспроизведение звука выполняется для четырех каналов – левого и правого фронтальных каналов, а также левого и правого тыловых каналов. Сигнал для тыловых каналов кодируется вместе с сигналами для фронтальных каналов. При воспроизведении стерео сигнал требует декодирования для извлечения информации тыловых каналов.
DTS – Digital Theater Systems Digital Sound (Цифровой Звук для Цифрового Театра). Разработанный компанией DTS, Inc., DTS является форматом многоканального звука похожим на Dolby Digital и используется в DVD-video дисках, DVD-audio дисках, а также в некоторых кинотетрах. DTS отличается от Dolby Digital увеличенным битрейтом и по-мнению многих имеет лучшее качество.
MP3 – акроним от MPEG-1 (или MPEG-2) Layer 3 (не путать с MPEG3). Популярный формат кодирования аудио используемый в компьютерах или портативных устройствах. Компрессия MP3 использует “психоакустическую” модель, которая позволяет удалить часть аудиоинформации не воспринимаемой человеческим слухом и позволяет получить высококачественные аудиофайлы со значительной компрессией. Обычно MP3 файлы кодируются с битрейтом 128 kbit/s (12:1) близким к CD качеству. В последнее время MP3 аудио все чаще используется совместно с различными MPEG-4 видео кодеками, например DivX. Аудио данные могут быть закодированы с постоянным и переменным битрейтом.