Transcript 17Digital Audio

Digital Audio
Teppo Räisänen
LIIKE/OAMK
General Information
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Auditive information is transmitted by
vibrations of air molecules
The speed of sound waves is around
340 m/s
The basic unit of the sound is Hertz
(Hz)
General Information
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The human ear’s hearing range is
somewhat between 20 – 20 000 Hz
The energy unit of a sound is Decibel
(dB)
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Human speech ~ 50 dB
Pain treshold ~ 130 dB
Nuclear explosion ~ 250 dB
The heard result of a sound is also
affected by the harmonic qualities of
the sound wave
Digital Audio
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By nature all the sound waves are of
analog kind
For computer to process audio an
analog-to-digital conversion is needed
(A/D)
For producing a heard result a vice
versa operation (D/A) is needed
Digital Audio
Sampling Rate
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= amount of samples from the analog
signal / one second
The lower the sampling rate, the lower
the quality of the signal
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CD records: 44100 Hz
GSM phone call: 8000 Hz
The greater the sampling rate, the
bigger the audio file size
Resolution
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= the amount of digital signal levels /
one sample
Declared in bit value, e.g. 16 bit results
in 65536 possible signal levels
Examples:
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CD record: 16bit
GSM phone call: 8bit
Sound Quality & File Sizes
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Stereophonic (stereo) sound refers to any method
of sound reproduction in which an attempt is made to
create an illusion of directionality and audible
perspective.
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This is usually achieved by using two or more
independent audio channels through a configuration of
two or more loudspeakers in such a way as to create
the impression of sound heard from various directions,
as in natural hearing.
Thus the term "stereophonic" applies to so-called
"quadraphonic" and "surround-sound" systems as well
as the more common 2-channel, 2-speaker systems.
Sound Quality & File Sizes
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It is often contrasted with monophonic,
or "mono" sound, where audio is in the
form of one channel
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often centered in the sound field
(analogous to a visual field).
Stereo sound is now common in
entertainment systems such as
broadcast radio and TV, recorded music
and the cinema.
Sound Quality & File Sizes
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Sound files of good quality are often
quite large:
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One minute of 44100hz, 16-bit stereo
audio = 10,1 Mb
One minute of 22050hz, 8-bit mono audio
= 1,3 Mb
One minute of 11025hz, 8-bit mono audio
= 0,6 Mb
Compressed File Formats
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There are both compressed and
uncompressed audio file formats
Compression can be either
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Lossless = The quality of uncompressed audio
equals the quality of original audio file
Lossy = The quality of uncompressed audio is
lower than the original (e.g. mp3, wma)
For compression/uncompression matching
algorighm pairs (codecs) are used
Codecs
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There must be one algorithm for
compressing and one for decompressing (e.g. before playback)
Many codecs have to be installed as
plugins for player/editing applications
There are many dedicated sites for
downloading codecs
Wave/WAV
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The uncompressed audio format
developed by Microsoft
Wav files can be either mono or stereo
files and of different sample rates and
resolutions
Many platforms support wav files
UNIX/NeXT audio (.au)
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”Original” format of networked audio
Many early versions of browsers and
programming languages (e.g. Java)
support au-files
Au was originally developed by Sun
AIFF
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Audio Interchange File Format
Originally the sound format of Amiga &
Apple computers
AIFF file consists of so-called chunks in
which the qualities of audio + the
actual audio data are stored
MPEG (mp3)
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Nowadays very popular compressed
audio format
Mp3 is one of the subcategories of
MPEG standard
Mpeg 1 Layer 3 (mp3) was originally
targeted for transmitting speech over
networks
Several settings can be adjusted when
packaging audio into mp3 format
RealAudio
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.ra/.rm files
The first audio format to use streaming
technology
The sound file can be listened while it is
been streamed into the harddrive (The
complete file doesn’t need to be loaded)
Many internet radio stations use
ReadAudio as the broadcasting format
Other Audio Formats
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Besides the aforementioned there are
may audio formats for different uses:
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WMA
Ogg
Monkey’s Audio (.ape)
FLAC
In addition the audio editors produce
files which include project information
+ actual audio data