Download - Manipulating Audio
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Manipulating Audio
Manipulating Audio
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Why Digital AudioWhy Digital Audio
Analogue electronics are always prone to noise
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The Digital DomainThe Digital Domain
Based on numbersDigital representation of analogue
signal:
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Digital ElectronicsDigital Electronics
Less expensive to designLess expensive to manufactureOffer high noise immunityAge immunityTemperature immunity Increased reliability
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The Digital DomainThe Digital Domain
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6.3 7.2 6.4 4.8 3.2 1.9 1.3 2.2
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0 6 7 6 5 3 2 1 2
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Analogue signal
6, 7, 6, 5, 3, 2, 1, 2
Digital stream
Digital waveform
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NyquistNyquist
The highest frequency which can be accurately represented is one-half of the sampling rate CD: Sample Rate =
44,100 HzNyquist Frequency =
SR/2 = 22,050 Hz
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AliasingAliasing
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AliasingAliasing
Bach trumpet
SR=22050 - highest frequency in music is about 7000 Hz (no aliasing)
SR=11025 - (some aliasing; adds a little dullness and a metallic quality)
SR=4410 - (lots of aliasing; sounds like bad video game)
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Low Pass FilteringLow Pass Filtering
The original signal must be low-pass filtered to remove signals higher than 0.5 of the sampling rate
CD: SR = 44,100Hz thus original signal must not exceed 22,050Hz to be completely sampled
Low-pass filtering avoids frequency fold-over
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FiltersFilters
low-pass filter band-pass filter high-pass filter
White noise (unfiltered)Filtered white noise (low pass, band pass, high pass)
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FiltersFilters
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OversamplingOversampling
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Sampling realitySampling reality
Critical sampling is not attempted
Lowpass filters can not attenuate the signal precisely at the Nyquist frequencyDVD audio – 96 KHz?CD – 44.1KHzTelephone – 8KHz
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QuantizationQuantization
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Analogue signal
6, 7, 6, 5, 3, 2, 1, 2
Digital stream
Digital waveform
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QuantizationQuantization
The number of quantization levels (N) is dependant on binary word length
N = 2n
n = number of bits• 28 = 256• 216 = 65536 (CD)• 224 = 16777216 (DVD)
The more bits the better the approximation
Worst error is 0.5 LSB
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Quantization ErrorQuantization Error
Difference between the actual analogue value and the selected quantization interval value
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DitheringDithering
A small amount of noise that is uncorrelated with the input signal is added
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DitheringDithering
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DitheringDithering
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A to D Principal elementsA to D Principal elements
MultiplexerProcessing
(error correction)
Dither generator
Sample and Hold
Anti-aliasing
filter
A to D Converter
Sample and Hold
Anti-aliasing
filter
A to D Converter
Analogue input (L)
Analogue input (R)
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SummarySummary
Sampling and Quantization are the two fundamental criteria for a digitisation system
Aliasing occurs when sampling theory is not observed
Sampling is losslessQuantisation is lossyDither can substantially reduce
quantization distortion
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FinFin
Fin