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Manipulating Audio

Explore the advantages of digital audio over analogue, from manipulating amplitude to quantization and noise immunity. Learn about sampling rates, filters, quantization levels, aliasing, dithering, and more in the digital domain.

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Manipulating Audio

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  1. Manipulating Audio

  2. amplitude time Why Digital Audio • Analogue electronics are always prone to noise

  3. number time The Digital Domain • Based on numbers • Digital representation of analogue signal:

  4. Digital Electronics • Less expensive to design • Less expensive to manufacture • Offer high noise immunity • Age immunity • Temperature immunity • Increased reliability

  5. 7 7 6 6 5 5 Analogue signal 4 4 3 3 2 2 1 1 6 7 6 5 3 2 1 2 0 0 s Digital waveform s 6, 7, 6, 5, 3, 2, 1, 2 Digital stream 6.3 7.2 6.4 4.8 3.2 1.9 1.3 2.2 The Digital Domain

  6. Nyquist • The highest frequency which can be accurately represented is one-half of the sampling rate • CD: Sample Rate = 44,100 Hz • Nyquist Frequency = SR/2 = 22,050 Hz

  7. Aliasing

  8. Aliasing • 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)

  9. Low 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

  10. low-pass filter band-pass filter high-pass filter Filters White noise (unfiltered) Filtered white noise (low pass, band pass, high pass)

  11. Filters

  12. Oversampling

  13. Sampling reality • Critical sampling is not attempted • Lowpass filters can not attenuate the signal precisely at the Nyquist frequency • DVD audio – 96 KHz? • CD – 44.1KHz • Telephone – 8KHz

  14. 7 7 6 6 5 5 Analogue signal 4 4 3 3 2 2 1 1 6 7 6 5 3 2 1 2 0 0 s Digital waveform s 6, 7, 6, 5, 3, 2, 1, 2 Digital stream 6.3 7.2 6.4 4.8 3.2 1.9 1.3 2.2 Quantization

  15. Quantization • 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

  16. Quantization Error • Difference between the actual analogue value and the selected quantization interval value

  17. Dithering • A small amount of noise that is uncorrelated with the input signal is added

  18. Dithering

  19. Dithering

  20. Anti-aliasing filter Anti-aliasing filter Sample and Hold Sample and Hold A to D Converter A to D Converter A to D Principal elements Analogue input (L) Processing (error correction) Dither generator Multiplexer Analogue input (R)

  21. Summary • Sampling and Quantization are the two fundamental criteria for a digitisation system • Aliasing occurs when sampling theory is not observed • Sampling is lossless • Quantisation is lossy • Dither can substantially reduce quantization distortion

  22. Fin Fin

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