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Multimedia Systems

Multimedia Systems. Lecture - 8. Lecture Outline. Nature of Sound Properties of Sound waves Digitization of Sound Quality vs. File size of Digitized Sound Audio File Formats. Nature of Sound. Sound is a physical phenomenon produced by the vibration of matter.

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Multimedia Systems

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  1. Multimedia Systems Lecture - 8

  2. Lecture Outline • Nature of Sound • Properties of Sound waves • Digitization of Sound • Quality vs. File size of Digitized Sound • Audio File Formats

  3. Nature of Sound • Sound is a physical phenomenon produced by the vibration of matter. • When the object vibrates, it sends out a series of waves which can be interpreted as sound. • The perception of sound by human beings is a very complex process. It involves three systems: • the source which emits sound; • the medium*through which the sound propagates; • the detector which receives and interprets the sound. * Medium = Solid, liquid or Gas. Sound can not travel through vacuum

  4. Nature of Sound • Range of audible sound waves • 20 Hz to 20 kHz (according to following references) • Cutnell, John D. and Kenneth W. Johnson. Physics. 4th ed. New York: Wiley, 1998: 466 • Acoustics National Physical Laboratory (NPL), 2003. • Caldarelli, David D. and Ruth S. Campanella. Ear. World Book Online Americas Edition. 26 May 2003

  5. Sound waves properties • Sound waves are generally characterized by • Period • Frequency • Pitch • Amplitude • Loudness • Bandwidth • Intensity

  6. Sound waves properties • Period • Period is the interval at which a periodic signal repeats regularly. • Periodicity of sound waves is their occurrence at regular intervals (in time or space) • Frequency • Frequency is the number of occurrences of a repeating event per unit time. • It is the reciprocal value of period (f=1/P) and is measured in Hertz. • Pitch • It is the perception of sound by human beings. • It measures how ‘high’ is the sound as it is perceived by a listener.

  7. Sound waves properties • Amplitude • Amplitude is the magnitude of change in the oscillating variable with each oscillation within the oscillating system. • Sound waves create an oscillation of pressure in the transmission medium. With sound waves, amplitude measures the amount of positive or negative change in atmospheric pressure caused by the wave. • Loudness • It is the volume of sound. • Higher the amplitude, higher is the energy level carried by the wave; thus a high amplitude sound wave will be louder.

  8. Sound waves properties • Bandwidth • It is the range of frequencies that a device can produce or a human can hear • FM radio 50Hz – 15kHz • AM radio 80Hz – 5kHz • CD player 20Hz – 20kHz • Inexpensive microphone 80Hz – 12kHz • Telephone 300Hz – 3kHz • Children’s ears 20Hz – 20kHz • Older ears 50Hz – 10kHz • Male voice 120Hz – 7kHz • Female voice 200Hz – 9kHz

  9. Sound waves properties • Intensity • The intensity of sound is the amount of power transmitted through an area of 1m2 oriented perpendicular to the propagation direction of the sound. • At an intensity of 10-12watt/m2, we may be able to just hear a sound. This is known as threshold of hearing. • At an intensity of 1watt/m2 the ear may be damaged (it is called threshold of feeling). • The relative intensity of two different sounds is measured using the unit Bel or more commonly deciBel (dB).

  10. Computer Representation of Sound (Digitization) • Sound waves are continuous while computers handle digital data. • The method for analog to digital conversion (digitizing) of sound is known as pulse code modulation (PCM). • To basic idea is to measure the signal amplitude at equally spaced time intervals (sampling) and represent it with one of finite digital values (quantization)

  11. Computer Representation of Sound (Digitization) • Sampling • In this step, samples of the continuous signal are taken. • The no. of samples taken during a time period is called sampling rate. • According to Nyquist sampling theorem, in order to capture all audible frequencies of a sound, the sampling rate must not be less than twice the maximum frequency of the original signal, e.g., if the maximum frequency is 3kHz, the sampling rate must not be less than 6kHz. • If we undersample, i.e., taking less samples than as required by Nyquist sampling theorem, some of the frequency components will be mistakenly converted into other frequencies. This is known as aliasing.

  12. Computer Representation of Sound (Digitization) • Quantization • It is the process of approximating a continuous range of values by a relatively-small set of discrete symbols/values. • The values of obtained samples are restricted to a fixed set of levels. • To reconstruct a signal that is as closed to the original signal as possible, we need to take sufficiently many samples, and we need to have as many levels to record the sample values in as possible. • If we use too few levels to represent each sample value, there will be large amount of error for each sample. • This is known as quantization error. These errors can be thought of as noise on the signal.

  13. Computer Representation of Sound (Digitization) • We measure the quality of a sample by its signal-to-noise ratio (SNR). The higher the resolution*, the smaller the noise, and the better the quality. • The unit of SNR is dB. This is defined by • where S is the strength of the signal and N is the noise • For 8-bit samples, the SNR is 10 log(256/0.5) 48dB. • For 16-bit samples, the SNR is 10 log(65536/0.5) 96dB. * no. of bits that represent a sample

  14. Sample and Hold (Digitising the time-axis) Sample held to allow the value to be read Voltage Voltage Time Time Sample-rate: the number of samples per second e.g. Telephone: 8000 samples/sec, CD: 44100 samples/sec

  15. Quantization (digitizing the voltage-axis) Voltage Time Precision: the number of bits used to represent the voltage e.g. Telephone: 8-bits (256 levels), CD: 16-bits (65536 levels)

  16. A closer view of quantization errors

  17. Quality Vs. File Size • The size of a digital recording depends on the sampling rate, resolution and number of channels. • where S = file size bytes R = sampling rate samples/second b = resolution no. of bits representing a sample C = channels 1-mono, 2-stereo D = recording duration seconds

  18. Quality Vs. File Size • Higher sampling rate, higher resolution gives higher quality but bigger file size. • For example, if we record 10 seconds of stereo music at 44.1kHz, 16 bits, the size will be: S= 44100 x (16/8) x 2 x 10 = 1,764,000 bytes OR 1722.7 kbytes OR 1.68 Mbytes Note: 1 kbytes = 1024 bytes 1 Mbytes = 1024 kbytes • High quality sound files are very big, however, the file size can be reduced by compression.

  19. Audio Hardware • Recording and Digitizing sound: • An ADC converts the analog sound signal into digital samples. • A digital signal processor (DSP) processes the sample, e.g. filtering, modulation, compression, and so on. • Play back sound: • A digital signal processorprocesses the sample, e.g. decompression, demodulation, and so on • A DAC converts the digital samples into sound signal.

  20. Audio Hardware All these hardware devices are integrated into a few chips on a sound card

  21. Audio Software • The audio software (device driver) controls the hardware device. • Many popular sound cards are Plug and Play, i.e. Windows has drivers for them and can recognize them automatically. • The cards for which Windows does not have drivers, can be obtained from the manufacturer.

  22. Various Audio File Formats Extension MIME Type Platform Use aif Audio/x-aiff Mac, SGI Audio aifc Audio/x-aiff Mac, SGI Audio(compressed) AIFF Audio/x-aiff Mac, SGI Audio aiff Audio/x-aiff Mac, SGI Audio au Audio/basic Sun, NeXT ULAW audio data mov Video/QuickTime Mac, Win QuickTime video mpe Video/mpeg All MPEG video mpeg Video/mpeg All MPEG video mpg Video/mpeg All MPEG video mp3 Audio/x-mpeg All MPEG audio qt Video/QuickTime Mac, Win QuickTime video ra,ram Audio/realaudio All RealAudio Sound snd Audio/basic Sun, NeXT ULAW Audio Data vox Audio All VoxWare Voice wav Audio/x-wav Win WAV Audio

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