BOOT CAMP 460 (23/01/07)

Multimedia Formats and Codecs, part 1


For many newcomers to computing one of the most confusing aspects of using a PC is the seemingly endless succession of file formats.  It’s bad enough when all you have to deal with are documents and emails but things can start to get really complicated when you add multimedia technologies, like digital cameras, music players and camcorders, into the mix.


Don’t worry, it is nowhere near as bad as it seems and if you break it down into bite-sized chunks it even starts to make some sense. We begin this week with most people’s first brush with PC multimedia, which is the sounds computers make and in the coming weeks we’ll be picking the bones out of still image and video file formats.


Computer audio is a relatively recent innovation and in the early 1990s sound cards were still an optional extra on many PCs. That’s not to say they were totally silent but most machines were only capable of the occasional warning bleep from a small built-in speaker (and before anyone writes in, yes, I know Apple Macs had stereo sound almost since day-one).


Until the late 1990s most Windows PC users were only likely to come across two types of audio files. Virtually all of the sounds made by computer hardware and software come from ‘wave’ or .wav files. Thanks to the growing popularity of CD-ROM drives most PCs could also play audio CDs, where music is in the form of .cda files. In practice, though, audio CD playback had little to do with the PC as the drives tended to do all the hard work; the PC simply played back the analogue sound signal coming from the drive through its speakers.


In fact wav, cda and aiff (Audio Interchange File Format, used on Apple Macs) are all very closely related, being digital pulse code modulation (PCM) systems. These break sounds down into a stream of numbers; the numbers represent slices or ‘samples’ of the original sound, usually taken 44,100 times each second though higher and lower sample rates are also used.


The key feature of PCM audio file formats is that they are ‘lossless’ or ‘uncompressed’ and usually no attempt is made to reduce the amount of data these files contain. In other words a five second blast of heavy metal music requires the same amount of data as a five seconds of silence, which clearly isn’t very efficient.


That brings us to audio codecs or compression-decompression algorithms. These are not files formats as such but systems for processing audio files and they are the basis of most current digital audio technologies, whether it’s your favourite tunes on a personal music player, the multi-channel surround soundtrack on a DVD movie, digital television and radio and even your mobile phone.


In essence a codec’s job is to reduce the amount of data in an audio file by stripping out all of the parts of a sound that you cannot hear, either because the frequency is too high or too low, or it is masked by louder sounds. Since some information is discarded audio codecs are referred to as ‘lossy’ systems.


The first widely used and still the most popular codec is Motion Picture Experts Group Audio Layer 3 or just ‘MP3’. It was developed by a group of German engineers in 1987 and became an ISO technical standard in 1991. MP3 files can be compressed by a factor of ten or more without significant reduction in quality – depending on your musical sensibilities -- and by the mid 1990s it had become the de facto standard for sending audio files over the Internet. As a matter of interest the first portable MP3 players appeared in 1988, some six years before the launch of the ubiquitous iPod.


MP3 is an open standard, anyone can use it and not surprisingly piracy became rife. It remains so to this day, despite the music industry’s belated and clumsy attempts to outlaw file sharing. Nevertheless, within the past two or three years legitimate music downloading has started to stem the tide. This is mainly due to the music industry finally coming to terms with the technology and the development of audio codecs that contain embedded copy protection and digital rights management (DRM) features.


The two most popular second generation codecs are Windows Media Audio or .wma and Advanced Audio Coding or .aac, which is used by Apple iPods and on downloads from the iTunes online music store. Most other Internet music download sites use a mixture of MP3 and AAC, ‘Podcasts’, which are mostly speech-based audio recordings are normally in MP3 format.


Playing back audio files and codecs isn’t anything like as complicated as it used to be and the Windows Media Player (version 10 and above), supplied as standard with Windows, plays just about anything, with the exception of AAC files (though unofficial ‘plugins’ are now available). This is not a problem, however, and in addition to the free iTunes software available from the Apple download site there are numerous freeware media players that between them can handle virtually every format and codec, either directly or by using readily available plugins.



Next Week -- Multimedia Formats part 2





Digital Rights Management - embedded data in digital multimedia files, designed to protect copyright, prevent piracy and control or restrict playback



International Standards Organisation, International body responsible for establishing and maintaining technical standards



A data file or program that extends the capability or adds extra features to an application




Two other audio systems that you are likely to encounter are Real Audio and MIDI. Real Audio is a proprietary ‘streaming’ media codec, used by, amongst others, the BBC for its ‘Listen Again’ services. It is designed to send audio over the Internet that can be listened to in real time, as it is being downloaded. Real Audio (.ra) files can normally only be heard through the Real Audio Player program, though a number of third-party players are now available.  


MIDI (Musical Instrument Digital Interface) is not a file format but a set of commands that allows electronic musical instruments to communicate directly with computers




 © R. Maybury 2006, 1701

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