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Digital Audio

Digital Audio

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Digital Audio

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  1. Digital Audio

  2. Sound:Multimedia • Sound (and its derivatives; speech, music, etc., generally referred to, if audible to humans, as audio) has a significant part to play in multimedia applications • From interacting through a multi-modal user interface (e.g., Surfing the Web by voice) and text-to-speech systems (Apple Speech Technologies), to software agents capable of expressing themselves in natural language (e.g., VirtualFriend); Internet-based radio and TV (e.g, RealAudio, and Internet Radio and TV sites); video-conferencing (e.g., Cu-SeeMe) and Internet telephony (e.g., VocalTec Communications); generating computer music, sounds for games, and computer-controlled musical instruments (e.g., MIDI); to personalised elevator music and refrigerators that hum along to your mood, audio is essential.

  3. THE NATURE OF SOUND • ‘acoustics’ – the study of the behaviour of sound in enclosed spaces • 3 conditions necessary for sound to exist: • A source – a vibrating physical body or object. • A medium – any physical substance capable of carrying the vibrations of a sound source from one place to another • A receiver – any physical object, which is in contact with the sound medium and is capable of vibrating in the same way as the sound source.

  4. Sound properties & perception • A vibrating source pushes particles in air back and forth, creating areas of compression and rarefactions. • When a molecule moves, it collides with the next one and make it moves too and may eventually reach the ears of a listener. • When we hear sound, air vibrates against our ear-drums causing them to vibrate too. These vibrations are detected and analyzed by our brains.

  5. Sound Receivers • The perception of sound involves a great more deal than its reception • Any physical object capable of vibrating like a sound source can be described as a sound receiver • Ear to convert sound vibrations into their electrical equivalent and then encode the information in the form of a stream of pulses suitable for transmission to the brain through nerve fibres

  6. Sound Receivers • Microphone represents the ‘ears’ of modern sound recording and for that reason alone it is almost certainly the most important single item of equipment in sound production technology • Designed to convert sound energy into electrical energy, the microphone is called ‘an electro-acoustic transducer’

  7. Robert Boyle’s Experiment

  8. Properties of Sound • Frequency • Amplitude • The shape of waveform

  9. Frequency • As the wave travels, the disturbance of particles is in the direction of the wave travel. The particles vibrate about their normal positions. Each complete vibration of a particle is called a cycle (i.e. from its starting position, to a maximum distance in one direction, back through the starting position, then to a maximum displacement in the opposite direction and back to the starting place). . The frequency of a wave is measured as the number of complete back-and-forth vibrations of a particle of the medium per unit of time. If a particle of air undergoes 1000 longitudinal vibrations in 2 seconds, then the frequency of the wave would be 500 vibrations per second. A commonly used unit for frequency is the Hertz (abbreviated Hz), where 1 Hertz = 1 vibration/second.

  10. Frequency

  11. Frequency • The frequency of a sound is the number of periods in a second and is measured in hertz (Hz). 1000 Hz = 1 kiloHertz (kHz). Audible (to humans) frequency occurs in the 20Hz to 20kHz range (normally in range 1 to 5 KHz). Babies can hear up to 20KHz!!!. Other frequency ranges are: • Infra-sound 0 - 20Hz • Ultrasound 20kHz - 1GHz • Hypersound 1GHz - 10THz

  12. Amplitude & The shape of waveform • The shape of the waveform that repeats at regular intervals is called a period, and sounds musical (e.g., a bird singing). A waveform that is not periodic sounds like noise (drums, coughing, sneezing, etc.) • The amplitude of a sound is a property subjectively heard as loudness or volume • Measuring unit used to deviate the pressure wave from its mean value (idle state) – unit Decibel (dB) • Spoken language 60dB • Heavy Road Traffic 80 dB • Rock band 120dB

  13. Digitizing Audio • An Analog-to-Digital Convervter (ADC) measures the amplitude of pressure waves at regular time intervals (called samples) to generate a digital representation of the sound. The reverse conversion, to play digital sound through an analog device (such as speakers) is performed by a Digital-to-Analog Converter (DAC).

  14. Digitizing Audio: Sampling • The number of samples taken per second is called the sampling rate. CD quality sound is sampled at 44,100 Hz, which means that it is sampled 44,100 times per second. This appears to be well above the frequency range of the human ear. However, the Nyquist sampling theorem states that "For lossless digitization, the sampling rate should be at least twice the maximum frequency responses." • The human ear can hear sound in the range 20Hz to 20KHz, and the bandwidth (19980Hz) is slightly less than half the CD standard sampling rate. Following the Nyquist theorem, this means that CD quality sound can represent frequencies only up to 22,050Hz, which is much closer to that of human hearing.

  15. Digitizing Audio: Sampling • Natural sound is a continuous phenomena and is converted to digital form by sampling techniques

  16. Digitizing Audio: Sampling

  17. Digitizing Audio: Sampling Rate • No of samples per cycle too low • Monotonous sound • Nyquist sampling theorem states that "For lossless digitization, the sampling rate should be at least twice the maximum frequency responses."

  18. Digitizing Audio: Quantizing • Resolution • The accuracy with which we present each sampled amplitude • 8 bit – 255 varieties of sound • Higher bit, better quality • CD quality – 16 bit, recorded using 20 or 24 bit quality

  19. Digitizing Audio: Quantizing

  20. Digitizing Audio: Dynamic Range • Dynamic range • The total range of amplitude values that can be stored • Amplitudes outside the range will be given the highest or lowest values in the range (whichever applies) – the sound wave will be effectively chopped off at the limits of the range. This effect is called clipping.

  21. Size of Digital Audio • Length of sound (seconds) • Sampling rate (kHz/s) • Resolution bit • Whether using stereo or mono • Mono is suitable for narration, single sound without background sound

  22. Example • Calculate the size of this digital audio file: • 1 minute, 44.1 kHz, 16 bit of resolution, using stereo. Storage size = 60 seconds * 44100 Hz * 16 bits * 2 (stereo) = 84672000 bits = 1058400 bytes = 10335.937 KB = 10.3359 MB

  23. Quality of Digital Sound • Quality of Source file • Editing process will decrease the quality of original sound. Better record using high quality • Capture and playback device • More expensive, better functionality offer • Depends on cost of device

  24. Quality of Digital Sound • Properties used when capturing a sound; frequency, sampling rate, channel no (mono/stereo) • Trade-off with size of file and processing time