| When you listen to a live musical
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| | COMPRESSION TYPES
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| instrument, the sound you hear is an
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| | There are two broad compression types:
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| analog sound wave. Such a wave can take
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| | 1. Lossless codecs. This kind of
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| an infinity number of values. This signal
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| | compression reduces the file size while
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| has to be converted into a sequence of
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| | retaining every scrap of information from
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| numbers (0s and 1s in native machine
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| | the original file. This means that the
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| language) for storage and playback on a
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| | original file can be reconstructed
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| computer. This process of digitisation
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| | exactly as it was before the compression.
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| creates a raw digital music file which
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| | Lossless compression does not
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| uses 10MB or more per minute (at least
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| | significantly reduce the size of an audio
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| 30MB for a 3 minute song).
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| | file. Examples of general lossless
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| On a PC, a .WAV is an example of a
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| | compression algorithms include zip or
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| default audio file, while AIFF(Audio
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| | gzip compressions.
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| Interchange File Format) is the default
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| | 2. Lossy codecs. These throw away some of
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| format on a Mac. On a Unix machine, Sun
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| | the information from the original music
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| Microsystems' AU(AUdio) format is the
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| | file. Ideally what's thrown away is what
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| default for audio. These file formats are
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| | the human ear cannot hear, namely sounds
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| an exact replica of the original sound,
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| | below 20Hz and above 20KHz. Thus, the
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| except for small sampling errors. (They
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| | most successful lossy codecs are based on
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| also support limited audio file
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| | an understanding of psychoacoustic
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| compression.)
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| | perception. Lossy compression can reduce
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| These files have to be compressed to the
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| | the size of the file to less than 10% of
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| more usual levels of 3-5MB per song.
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| | its original size. Most of the codecs you
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| Compression rearranges the sequence of
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| | will come across are of this type.
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| numbers, sometimes throwing away less
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| | Examples include MP3 and WMA codecs.
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| significant information, in order to
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| | CONSTANT V VARIABLE BIT RATE
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| reduce the file size. The way the
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| | Something else you might come across
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| compression is achieved leads to the many
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| | concerning compression is bit rate. This
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| different audio file formats available.
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| | is the number of bits (0s and 1s) per
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| Each compression algorithm requires a
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| | second in an audio stream. This kind of
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| decoder to make sense of the data during
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| | represents how much information is being
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| playback. The combination of a compressor
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| | conveyed per second. The bit rate within
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| and a decoder is called, surprisingly, a
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| | a single music file can be constant or
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| codec.
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| | variable.
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