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Different Types
of Audio Compression
When you speak of audio compressions, it refers to a type of
data compression that reduces the size of audio files. It is
not advisable to use generic data compression algorithms for
reducing audio file sizes as it performs poorly with audio
data. With generic data compression the file size is reduced
not more than 87% of the original file size and is basically
not designed for real time use.
In the case of audio compressions, the algorithms are found in
the computer software as audio codecs. There are basically two
types of audio compression used today; lossy and lossless
comparisons where lossy algorithms is the most commonly used
algorithm that provides better compression ratios which are
used in mainstream consumer audio devices.
Information redundancy is reduced in both forms of algorithms
through coding, linear prediction and pattern recognition to
reduce the amount of information used in describing data.
Lossless audio compression formats like Monkey’s Audio, FLAC
and Shorten have grown in popularity because of less expensive
file storage and communications bandwidth which is easily
available. With lossless audio compression, it is possible to
maintain a permanent archive of audio files.
Audio engineers, audiophiles and people who want preservation
of exact copies of their audio files use lossless audio
compression. This is in contrast to the irreversible changes
that arise from lossy compression techniques like MP3 and
Vorbis. High definition DVD formats are also being introduced
with lossless formats like Dolby True HD.
It is basically difficult to maintain data in audio streams
while achieving substantial compression because most sound
recordings are complex and recorded from the real world. Even
computer generated sounds have complicated waveforms that form
a challenge to compression algorithms. This is mainly due to
the fact that audio waveforms are difficult to simplify
without any conversion to frequency information as picked by
the human ear.
Audio data conversion is also difficult to perform as the
value of audio samples change quickly making it impossible to
use generic data compression algorithms for audio data
conversion. In addition to this, strings of consecutive bytes
don’t tend to appear often. However filter convolution
slightly whitens the spectrum wherein traditional lossless
compression found at the encoder helps the decoder restore the
original signal with integration.
FLAC, TTA and Shorten all estimate the spectrum of signal with
linear prediction. The estimator’s inverse is used for
whitening the signal at the encoder by removing spectral
peaks. However the estimator is used for reconstruction of the
original signal found at the decoder.
Today you find lossy audio compression being used in numerous
applications. Not only direct applications like computers and
MP3 players use lossy audio compression, it is also used in
digital television, most video DVDs, satellite and cable
radio, streaming media on the internet and in terrestrial
radio broadcasts. You find greater compression rates with
lossy compression than lossless compression by the discarding
of less-critical data.
Lossy audio compression uses psychoacoustics to prove that not
all data found in audio stream can be picked up by the human
auditory system. This is why perceptual redundancy is reduced
by first identifying sounds that are hard to hear like high
frequencies and sounds that occur together. These sounds are
either coded with decreased accuracy or no coding at all.
With the removal of these ‘unhearable’ sounds only a small
percentage of bits are saved in lossy compression. If this is
not sufficient in achieving compression for an application,
more lossy compression is required. However sometimes this too
may not produce perceptible differences as speech is usually
compressed more than music.
The bit rate is usually used to express the target rate of
data that has to be achieved through lossy compression
schemes. Depending on the bandwidth and storage requirements,
lossy compression may lead to a reduction to the audio quality
which ranges from none to severe which is usually unacceptable
to listeners. With the removal of data in lossy compression
which cannot be recovered by decompression, many people don’t
prefer lossy compression for archival storage.
Source:
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