U of I CS 414 - Digital Audio Representation (30 pages)

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Digital Audio Representation



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Digital Audio Representation

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Lecture Notes


Pages:
30
School:
University of Illinois
Course:
Cs 414 - Multimedia Systems
Multimedia Systems Documents

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CS 414 Multimedia Systems Design Lecture 3 Digital Audio Representation Klara Nahrstedt Spring 2011 CS 414 Spring 2011 Administrative Form Groups for MPs Deadline January 24 today to email TA CS 414 Spring 2011 Key Questions How can a continuous wave form be converted into discrete samples How can discrete samples be converted back into a continuous form CS 414 Spring 2011 Characteristics of Sound Amplitude Wavelength w Frequency Timbre Hearing 20Hz 20KHz Speech 200Hz 8KHz Digital Representation of Audio Must convert wave form to digital sample quantize compress Sampling in time Measure amplitude at regular intervals How many times should we sample CS 414 Spring 2011 Nyquist Theorem For lossless digitization the sampling rate should be at least twice the maximum frequency response In mathematical terms fs 2 fm where fs is sampling frequency and fm is the maximum frequency in the signal CS 414 Spring 2011 Nyquist Limit max data rate 2 H log2V bits second where H bandwidth in Hz V discrete levels bits per signal change Shows the maximum number of bits that can be sent per second on a noiseless channel with a bandwidth of H if V bits are sent per signal Example what is the maximum data rate for a 3kHz channel that transmits data using 2 levels binary Solution H 3kHz V 2 max data rate 2x3 000xln2 6 000bits second CS 414 Spring 2011 Limited Sampling But what if one cannot sample fast enough CS 414 Spring 2011 Limited Sampling Reduce signal frequency to half of maximum sampling frequency low pass filter removes higher frequencies e g if max sampling frequency is 22kHz must low pass filter a signal down to 11kHz CS 414 Spring 2011 Sampling Ranges Auditory range 20Hz to 22 05 kHz must sample up to to 44 1kHz common examples are 8 000 kHz 11 025 kHz 16 000 kHz 22 05 kHz and 44 1 KHz Speech frequency 200 Hz 8 kHz sample up to 16 kHz but typically 4 kHz to 11 kHz is used CS 414 Spring 2011 CS 414 Spring 2011 Quantization CS 414 Spring 2011 Quantization Typically use 8 bits 256



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