lecture3 - CSE 471 Media Processing and Multimedia Sampling...

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1 CSE 471 Media Processing and Multimedia Sampling and Quantization Capturing sounds electronically Sampling sounds Aliasing Quantization
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2 CSE 471 Media Processing and Multimedia Electronic capture of sounds N S
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3 CSE 471 Media Processing and Multimedia Sampling Sampling - simply taking voltage readings at fixed points in time Uniform sampling – sampling at regular intervals
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4 CSE 471 Media Processing and Multimedia Sampling Real values at fixed points in time Uses a “sample-and-hold” Values are called “samples” Analog signal Sample and Hold Sampled signal Sample
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5 CSE 471 Media Processing and Multimedia How might you measure waves on a beach? Or, even, sample the wave height
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6 CSE 471 Media Processing and Multimedia Is sampling okay? Question: Is a sampled representation the same thing? Yes, provided…
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7 CSE 471 Media Processing and Multimedia The Sampling Theorem Given a uniform sample rate of f s , the highest frequency that can be unambiguously represented is f s /2 f s /2 is the Nyquist Frequency CD Players use f s =44,100 samples/sec The Nyquist Frequency is 22,050 Hz Why don’t CD’s sample at 40KHz?
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8 CSE 471 Media Processing and Multimedia Sound in the Real World Recall: All sounds can be expressed as a combination of sinusoids String instruments generate harmonics C8 on piano = 4186 Hz 6 th Harmonic is 25,116 Hz Above the Nyquist Frequency!! Many instruments generate content above 20 KHz ( 29 = + = 1 sin ) ( h h h t h c t f φ ϖ
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9 CSE 471 Media Processing and Multimedia What happens above Nyquist Frequency? For now, we’ll use the term Aliasing We’ll come back to it with better tools What does this mean for sampling?
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