Tuesday, June 05, 2001

Common movie/audio encoding formats defined

Tuesday, June 05, 2001

ASF is yet another video format to receive the MPEG4 codec treatment. It stands for Advanced Streaming Format because it was designed to be watched while being transferred from the internet (or to be 'streamed' if you want to use the correct lingo).

The quality is nowhere near as good as MPG, DivX or VCD, but is still acceptable for most purposes while maintaining a comparatively favourable size to quality ratio.

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AVI is an acronym for Audio Video Interleave. Technically it is a container rather than an encoding format in its own right in that it specifies how audio and video data is structured within the file.

The DivX codec is one notable example of an encoding format which makes use of the AVI wrapper.

AVI files contain a four digit code which is recognised by movie playback tools such as Media Player or VLC, allowing them to determine how to decode the movie.

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DivX is the name of the encoding process used to convert DVD, VHS etc movies into a very high quality AVI video format. This is carried out using a combination of the MP3 format for audio compression and a hacked version of Microsoft's MPEG4 codec for video compression.

A typical DivX movie will fill a standard 650mb CD-R and can be played back using nothing more than Microsoft's trusty (ha!) Media Player providing you have the DivX codec installed (see my movie links page).

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The MPEG format was devised by the Motion Picture Experts Group, hence the abbreviation, and its variations (MPEG-1, MPEG-2 and so on) form the basis for many other formats (see previous definitions). It combines very high quality video playback with large file sizes.

Once again keep the VLC Media Player handy.

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Musepack is a lossy audio compression format much like MP3 though distinguished by it's .mpc extension. The most notable benefits of the MPC format are:- - Superior quality audio at medium to high bit rates - Very fast encoding and decoding - 100% transparency i.e. MPC music tracks sound identical to their CD counterparts Despite the high quality audio output rendered by Musepack, the format is not widely used because most media players do not support it natively i.e. a plugin is required. Currently the only media player which can handle MPC files 'out of the box' is Foobar. Also to it's detriment, very few hardware audio players make provisions for MPC playback. In effect, to listen to MPC music on the go, the files first have to be converted. As this entails switching from one lossy format to another, inevitably the quality will deteriorate in the process. Ideally if you wish to use MPC files in such a way, you would be best advised to re-encode them directly to the destination format, though if this isn't possible you might like to let either the Alive MP3 Wav Converter or 4U WMA MP3 Converter go to work on them.

Burning MPC files directly to CD enabling you to play them in the usual manner is less problematic. To do this using Nero you will need to install the MPC plugin available from Mausau's audio plugins site. Alternatively you could use the freeware audio CD writing tool, Burrrn, which supports the MPC format by default.

More detailed information relating to the Musepack audio compression format can be found at the official home page.

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nAVI is a non-streamable hybrid of the ASF format, which boasts better quality playback due to its adoption of much improved frame rates.

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Ogg Vorbis is a completely free, streamable, open source audio encoding format. Compared to the more widely known MP3 format, Ogg Vorbis files provide superior sound quality and a smaller file size. Since the format is patent-free, software vendors do not need to pay licensing fees to implement the encoding of Ogg Vorbis files into their software. Effectively this means that a greater variety of encoding software will be available for the Ogg Vorbis format.

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QuickTime format was conceived by Apple and therefore it is not surprising that these type of movies first appeared on Macintosh computers before being ported to the PC. They comprise high quality playback and the ability to be streamed over the internet. Many Mac-friendly web sites favour this encoding method, largely due to its association with Apple I don't doubt.

What is most annoying about proprietary codecs is that the authors will have you believe you need to use a specialist tool to play them back - in this case Apple's QuickTime Player (who'd have thunk it?). A waste of perfectly good hard disk space if you ask me! Use VLC instead.

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Real Media is a proprietary, MPEG-2 based streamable format used to encode audio and video data. Small file sizes mean you won't have long to wait for content to begin streaming, or finish downloading if you are transferring the complete .ra or .rm files before attempting to play them back, but quality can leave a lot to be desired. Real Media movies and audio can be played back using Real Network's free RealPlayer application or Real Alternative. Real Media movie clips are very common on mainstream news sites that don't know any better, bless 'em.

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VCD stands for Video Compact Disc. These are CD-Roms containing full motion video encoded using the MPEG1 codec. Because they consist of high quality stereo sound coupled with reasonable quality video playback (equivalent to the VHS format) you will often find that they occupy two CDs.

VCDs can be played back with Microsoft's Media Player, amongst a host of other more specialist programs simply by locating and opening their '.dat' files.

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WMA is an acronym of Windows Media Audio; Microsoft's propriety audio format. WMA files provide CD quality sound and are slightly more compact than their MP3 equivalents. Like MP3 files they are streamable.

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WMV is an acronym of Windows Media Video; again one of Microsoft's propriety formats. WMV is used to encode movie data, which can contain both video and audio streams. Such files offer near broadcast quality video, and as an added bonus, can be streamed over the internet.

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The observant readers among you may have noticed that XviD spells DivX backwards. I would imagine this reflects the wish of its authors to highlight the similarities between the two codecs. Both are MPEG-4 based and are derived from source code written under Project Mayo.

The principal fork in the path can be traced back to their respective software licenses; DivX is commercial and closed source, whereas XviD is open source and released under the GPL license.

It's difficult to definitively answer the question, "which codec is better?" because there are so many factors which can affect the performance and quality of movies encoded using either codec. See www.xvid.org for downloads and further information.

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