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Digital Audio

Digital Audio. Teppo Räisänen LIIKE/OAMK. General Information. 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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Digital Audio

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  1. Digital Audio Teppo Räisänen LIIKE/OAMK

  2. General Information • 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)

  3. General Information • The human ear’s hearing range is somewhat between 20 – 20 000 Hz • The energy unit of a sound is Decibel (dB) • 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

  4. Digital Audio • 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

  5. Digital Audio

  6. Sampling Rate • = amount of samples from the analog signal / one second • The lower the sampling rate, the lower the quality of the signal • CD records: 44100 Hz • GSM phone call: 8000 Hz • The greater the sampling rate, the bigger the audio file size

  7. Resolution • = the amount of digital signal levels / one sample • Declared in bit value, e.g. 16 bit results in 65536 possible signal levels • Examples: • CD record: 16bit • GSM phone call: 8bit

  8. Sound Quality & File Sizes • 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. • 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.

  9. Sound Quality & File Sizes • It is often contrasted with monophonic, or "mono" sound, where audio is in the form of one channel • 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.

  10. Sound Quality & File Sizes • Sound files of good quality are often quite large: • 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

  11. Compressed File Formats • There are both compressed and uncompressed audio file formats • Compression can be either • 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

  12. Codecs • There must be one algorithm for compressing and one for de-compressing (e.g. before playback) • Many codecs have to be installed as plugins for player/editing applications • There are many dedicated sites for downloading codecs

  13. Wave/WAV • 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

  14. UNIX/NeXT audio (.au) • ”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

  15. AIFF • 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

  16. MPEG (mp3) • 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

  17. RealAudio • .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

  18. Other Audio Formats • Besides the aforementioned there are may audio formats for different uses: • WMA • Ogg • Monkey’s Audio (.ape) • FLAC • In addition the audio editors produce files which include project information + actual audio data

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