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COSC 109 Ch.4 Notes.doc

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Computer Science
COSC 109
Mona Tavakalon

COSC 109 Chapter 4: Fundamentals of DigitalAudio Sound is a wave that is made by vibrating objects in a medium like air Ex. Vocal cords The frequency of a sound wave refers to the complete number of cycles of vibrational motion of the medium particles per unit of time • Frequency is measured in Hertz ( Hz) • 1 Hz = 1 cycle/second Ex. The following image has 3 cycles therefore it has a frequency of 3Hz Pitch is a sound frequency, the higher the frequency the higher the pitch. Human ears can hear sounds ranging from 20Hz to 20,000Hz The difference between sound intensity and loudness is that sound intensity can be : • heard in Decibles (Db) • measured by auditory devices • Has objective measurement Loudness is: • Subjective perception • measured by human listeners • human ears have different sensitivity In general higher sound intensity means louder sound. Most audio editing programs use decibels for audio amplitude. 0dB does not mean there is no sound, it just means that you cant hear it. 0dB is the threshold of hearing. 120dB is the threshold of pain. When you add sound: Asingle sine wave form =Asingle tone Asecond single sine wave form =Asecond single tone Which end up equalling a more complex wave form=A more complex sound Ex. Sampling: The sound wave is sampled at a specific rate into discrete samples of amplitude values. Suppose we sample the waveform 10 times a second, Sampling rate= 10Hz We get 10 samples per second Reconstructing the wave form using discrete sample points Effects of Sampling Rate Original wave form Sampling rate= 10Hz Sampling rate = 20Hz With higher sampling rate the reconstructed wave looks closer to the original wave. When there is more sample points the file size is larger Quantization: • Each of the discrete samples of amplitude values obtained from the sampling step are mapped and rounded to the nearest value on a scale of discrete levels. • The number of levels in the scale is expressed in bit depth--the power of 2. • An 8-bit audio allo8s 2 = 256 possible levels in the scale • CD-quality audio is 16-bit16i= 65,536 possible levels) • Ex. Suppose we are quantizing the samples3using 3 bits = 2 = 8 levels • Now round each sample to the nearest level Now reconstruct the waveform using the quantized samples After quantization: • Data with different original amplitudes may be quantized onto the same level • ⇒ loss of subtle differences of samples • With lower bit depth, samples with larger differences may also be quantized onto the same
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