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CSc 461/561 Multimedia Systems Part A: 1. Audio

Explore the basics of sound, sampling, and quantization in multimedia audio systems. Learn about Nyquist theorem, audio filtering, and various audio coding techniques like PCM, DPCM, DM, and ADPCM.

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CSc 461/561 Multimedia Systems Part A: 1. Audio

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  1. CSc 461/561Multimedia SystemsPart A: 1. Audio

  2. Summary • Audio ABC: 1.1 What is sound? 1.2 Sampling and Quantization 1.3 Nyquist theorem 1.4 Quantization 1.5 Audio filtering/ Synthetic sounds • Audio coding 2.1 PCM 2.2 DPCM 2.3 DM 2.4 ADPCM

  3. 1.1 What is sound: it is a wave (1) • How we hear • mouth / transmitter • air / cord • ear / receiver • Continuous in both • time • amplitude

  4. 1.1 a wave of many waves (2) • Frequency • pitch • Amplitude • loudness • Fourier Transform

  5. 1.1 Not all waves are equal (3) • For human • frequency • volume

  6. 1.2 Sampling and quantization (1) ? ?

  7. 1.2 Sampling rate (2) • If sampling_freq = true_freq • in this example, a constant signal • If sampling_freq = 1.5x true_freq • an alias_freq of 0.5x true_freq

  8. 1.3 Nyquist Theorem (1) • If a signal is bounded by frequency (f1,f2), then the sampling rate should be at least 2(f2-f1) to reconstruct the signal • Human can hear 20Hz - 20KHz • CD sampling rate: 44.1KHz • Human voice 300Hz - 4KHz • telephone systems sampling rate: 8KHz

  9. 1.3 Alias frequency (2) • If sampling_freq >= 2x true_freq • true_freq is recovered • If sampling_freq in [true_freq, 2x true_freq) • alias_freq = sampling_freq - true_freq • If sampling_freq < true_freq • true_freq’ = true_freq mod sampling_freq • follow the first two approaches

  10. 1.4 Quantization depth (1) • How many bits per sample • In this example: 2 bits • telephone systems: 8 bits; CD: 16 bits

  11. 1.4 Quantization noise (2) • Quantization is lossy • More bits, less information loss • Signal-to-quantization-noise ratio • defined as a (log) ratio of signal/noise power • one more bit can add 6 db to SQNR

  12. 1.4 Not all quantizer are linear (3) • Nonlinear/non-uniform quantization • humans are more sensitive to small changes from small values • u-law or A-law (more bits for small values)

  13. 1.5 Audio Filtering/Synthetic Sounds • Demo with Audacity

  14. 2.1 Pulse Code Modulation (PCM) • Band-limit filter • sampling rate: 8KHz • Quantizer • sample size: 8-bit (output) with u-law or A-law • Data rate: 64Kbps (output) • Reconstruction: low-pass filter • G.711 Filter A/D 64kbps Filter D/A

  15. 2.2 Differential PCM (DPCM) • Predictor • Predict the current based on the history • Quantizer • quantize the difference Quantizer + - + Predictor

  16. 2.3 Delta Modulation (DM) • Prediction • If current > last, output 1 (to increase) • if current < last, output 0 (to decrease) • slope overload • change too fast • granular noise • change too slow

  17. 2.4 Adaptive DPCM (ADPCM) • Adaptive quantizer • adapt quantization step to the difference • G.723: 8KHz, 4-bit, 32Kbps Adaptive Quantizer + + Predictor

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