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 The human ear’s hearing range is somewhat between 20 – 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

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

Digital Audio

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: Hz GSM phone call: 8000 Hz The greater the sampling rate, the bigger the audio file size

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

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.audioloudspeakers Thus the term "stereophonic" applies to so-called "quadraphonic" and "surround-sound" systems as well as the more common 2-channel, 2-speaker systems.quadraphonic

Sound Quality & File Sizes It is often contrasted with monophonic, or "mono" sound, where audio is in the form of one channelmonophonic often centered in the sound field (analogous to a visual field).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 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

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 Lossless 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 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 codecsdedicated sites

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

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

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

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 formatsettings

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

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