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12_CM0340_MPEG_Audio - Compression Audio Compression(MPEG...

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517 JJ II J I Back Close Compression: Audio Compression (MPEG and others) As with video a number of compression techniques have been applied to audio. Simple Audio Compression Methods RECAP (Already Studied) Traditional lossless compression methods (Huffman, LZW, etc.) usually don’t work well on audio compression For the same reason as in image and video compression: Too much change variation in data over a short time
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518 JJ II J I Back Close Some Simple But Limited Practical Methods Silence Compression - detect the ”silence”, similar to run-length encoding ( seen examples before ) Differential Pulse Code Modulation (DPCM) Relies on the fact that difference in amplitude in successive samples is small then we can used reduced bits to store the difference ( seen examples before )
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519 JJ II J I Back Close Simple But Limited Practical Methods (Cont.) Adaptive Differential Pulse Code Modulation (ADPCM) e.g., in CCITT G.721 – 16 or 32 Kbits/sec. (a) Encodes the difference between two consecutive signals but a refinement on DPCM, (b) Adapts at quantisation so fewer bits are used when the value is smaller. It is necessary to predict where the waveform is heading > difficult Apple had a proprietary scheme called ACE (Audio Compression/Expansion)/MACE. Lossy scheme that tries to predict where wave will go in next sample. About 2:1 compression.
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520 JJ II J I Back Close Simple But Limited Practical Methods (Cont.) Adaptive Predictive Coding (APC) typically used on Speech. Input signal is divided into fixed segments ( windows ) For each segment, some sample characteristics are computed, e.g. pitch, period, loudness . These characteristics are used to predict the signal Computerised talking (Speech Synthesisers use such methods) but low bandwidth: Acceptable quality at 8 kbits/sec
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521 JJ II J I Back Close Simple But Limited Practical Methods (Cont.) Linear Predictive Coding (LPC) fits signal to speech model and then transmits parameters of model as in APC. Speech Model: Speech Model: Pitch, period, loudness, vocal tract parameters (voiced and unvoiced sounds). Synthesised speech More prediction coefficients than APC – lower sampling rate Still sounds like a computer talking, Bandwidth as low as 2.4 kbits/sec.
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522 JJ II J I Back Close Simple But Limited Practical Methods (Cont.) Code Excited Linear Predictor (CELP) does LPC, but also transmits error term. Based on more sophisticated model of vocal tract than LPC Better perceived speech quality Audio conferencing quality at 4.8–9.6kbits/sec.
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523 JJ II J I Back Close Psychoacoustics or Perceptual Coding Basic Idea : Exploit areas where the human ear is less sensitive to sound to achieve compression E.g. MPEG audio, Dolby AC. Perceptual Audio Demos
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524 JJ II J I Back Close Sensitivity of the Ear Range is about 20 Hz to 20 kHz, most sensitive at 2 to 4 KHz.
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