Lec8 - IEG 4190 Lecture Notes 8; Lecturer: Jianzhuang Liu...

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1 IEG 4190 Lecture Notes 8 ; Lecturer: Jianzhuang Liu Outline: 1. Psycho-acoustics 2. Subband Coding 3. Dictionary Coding for Text Document 4. LZ77 5. LZ78 6. LZW
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2 Psycho-acoustics Basic structure of human hearing system: Sound propagates through air as pressure waves Sound velocity = 344 meters/second in air at 20 degree The hearing range of human beings: 20 Hz ~ 20k Hz Power level in decibels (dB): 10 12 sound power output PWL 10log ( ) 10 watts - = Human shout has PWL of 70 dB, large rocket motor 190 dB. The air pressure wave vibrates our eardrum. Then the pressure is transposed to the inner ear, which consists of fluid-filled structure. In this structure, mechanical pressure is transformed into electrochemical signals in frequency domain.
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3 Psycho-acoustics, cont’d The electrochemical signals are then transported to the auditory systems in the brain.
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4 Psycho-acoustics, cont’d Critical Bands They relate to the ability of our hearing system to distinguish and separate two signals which are present simultaneously. Our ears only respond to the strongest stimulation in a local frequency region. Critical bands are much narrower at low frequencies than at high frequencies. It can be approximated as, Critical bandwidth in hertz 24.7(4.37 1) F = + F is the center frequency in kHz. There are approximately 25 critical bands within the human hearing frequency range (see Table 11-1).
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5 Psycho-acoustics, cont’d Threshold of Hearing The hearing range of a human being: 20 Hz ~ 20k Hz. The sensitivity of our hearing system is not equal over the total frequency range. A sound at a given frequency has a minimum level (threshold) in order to be heard. 0.5 10 log f
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6 Psycho-acoustics, cont’d Amplitude (Frequency, Simultaneous) Masking One frequency, with higher level, can make another frequency with a lower level inaudible. 0.5 10 log f
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7 Psycho-acoustics, cont’d Temporal Masking Besides simultaneous masking, the forward-masking (pre-masking) and backward-masking (post-masking) mean that a higher level sound can mask a lower level sound which is preceding or following.
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8 Psycho-acoustics, cont’d
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9 Subband Coding A good coder should use the psychoacoustic models to adaptively quantize only the perceptually significant parts of the signal. Other parts of the signal masked by the significant parts are judged to be inaudible and are not coded.
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Subband Coding, cont’d Temporal masking is calculated by dividing the audio signal into blocks of samples (typically several milliseconds in length) and analyzing each block for transients which will act as temporal maskers. Most systems vary the length of the block to take advantage of both backwards and forwards masking. a long block
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Lec8 - IEG 4190 Lecture Notes 8; Lecturer: Jianzhuang Liu...

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