this article is reprinted from the Fraunhofer website
Basics about MPEG Perceptual Audio Coding
The purpose of audio compression
There is a lot of confusion surrounding
the terms audio compression, audio encoding, and audio
decoding. This section will give you an overview what audio coding
(another one of these terms...) is all about.
44.100 samples/s * 2 channels * 2 bytes/sample * 60 s/min = around 10 MBytes
of storage space on your harddisk. If you wanted to download that over the internet, given an average 28.8 modem, it would take you
10.000.000 bytes * 8 bits/byte / (28.800 bits/s * 60 s/min) = around
Digital audio coding, which - in this
context - is synonymously called digital audio compression as well, is
the art of minimizing storage space (or channel bandwidth) requirements
for audio data. Modern perceptual audio coding techniques (like MPEG Layer-3
or MPEG-2 AAC) exploit the properties of the human ear (the perception
of sound) to achieve a size reduction by a factor of 12 with little or
no perceptible loss of quality.
Compression ratios, bitrate and quality
It has not been explicitly mentioned
up to now: What you end up with after encoding and decoding is not the
same sound file anymore: All superfluous information has been squeezed
out, so to say. (More precisely: the redundant and irrelevant parts of
the sound signal.) The reconstructed WAVE file differs from the original
WAVE file, but it will sound the same - more or less, depending on how
much compression had been performed on it.
How does it work?
Audio compression really consists of
two parts. The first part, called encoding, transforms the digital
audio data that resides, say, in a WAVE file, into a highly compressed
form called bitstream (or coded audio data). To play the bitstream
on your soundcard, you need the second part, called decoding. Decoding
takes the bitstream and reconstruct it to a WAVE file.
The term psychoacoustics describes
the characteristics of the human auditory system on which modern audio
coding technology is based.
inaudible elements of an audio signal are irrelevant for the human perception
and thus can be eliminated by the encoder.
For the audio quality of a coded and decoded audio signal the quality of the psychoacoustics model used by an audio encoder is of prime importance. The audio coding schemes developed by Fraunhofer engineers belong to the best worldwide.
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