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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
JARGON FILTER
DRM
Digital Rights Management
- embedded data in digital multimedia files, designed to protect
copyright, prevent piracy and control or restrict playback
ISO
International Standards Organisation, International body responsible for
establishing and maintaining technical standards
PLUG-IN
A data
file or program that extends the capability or adds extra features to an
application
TOP TIP
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
---end---
© R. Maybury 2006, 1701
Part 2 3 4
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