III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC - PowerPoint PPT Presentation

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III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC PowerPoint Presentation
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III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC

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III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC
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III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC

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  1. III Digital Audio III.6 (Fr Oct 24) The MP3 algorithm with PAC

  2. The MP3 encoder chain 5. HuffmanCompression Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel ExternalCheck 2. FFT with Filter Bank 4. Quantization 3. Psychoacoustical Model (Perceptual-Audio-Coding Model PAC) 6. Frame OutputstreamFormatting 1. Digital Datastream

  3. The MP3 encoder chain Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel ExternalCheck 1. Digital Datastream 2 ~ stereo 768 kbit/s ~ 48 000 × 16 b/s

  4. The MP3 encoder chain Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel Important: Since # sample rate = # Fourier coefficients, speak of “Fourier samplesper second” ExternalCheck 2. FFT with Filter Bank 2.1 Cut spectrum 0 – 20 kHz into 32 subbands of 625 Hz each (32 × 625 = 20 000) for 1/40 sec windows. 2.2 Use MDCT (Modified Discrete Cosine Transformation ~ variant of FFT)to split each 625 Hz band into 18 subbands with variable widths, according to psychoacoustical criteria. Get 576 = 18 × 32 “lines”.

  5. The MP3 encoder chain 5. Huffmanlossless Compression Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel ExternalCheck 4. lossy Quantization Already discussed, Ok!!!! 40% of compression

  6. The MP3 encoder chain Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel ExternalCheck 3. Psychoacoustical Model (Perceptual-Audio-Coding Model PAC)

  7. The MP3 encoder chain Psychoacoustical Model (Perceptual-Audio-Coding Model PAC) = core features of MP3, it covers 60 % of MP3 compression The PAC Model is based upon threelimitations of human audio-perception: PAC 1: hearing thresholds PAC 2: auditory masking PAC 3: temporary masking All three PAC components generate lossy compression

  8. The MP3 encoder chain you don’t hear sinusoidal sounds belowthis threshold of loudness PAC 1: hearing thresholds loudness frequency (kHz)

  9. The MP3 encoder chain PAC 2: auditory masking loudness frequency For every sinusoidal frequency component of frequency f and loudness l,there is a surrounding masking surface, where other frequency/loudnesscomponents cannot be heard together with the given one. Example: the 4 kHz/40 dB component (red) masks the blue one.

  10. The MP3 encoder chain PAC 3: temporary masking loudness time For every sinusoidal frequency component of frequency f and loudness l (red)another subsequent component (blue) cannot be heard below the given curve of loudness in time, because the ear needs some time to “recover” from thatfirst component’s perception. This is even true for sounds before the givenone (red curve), because the perception needs to be built up!

  11. The MP3 encoder chain Line Audio Data Subbands Data Stream2*16 to DatastreamFormatting toFrames etc. QuantizationandEncoding(Check of Quantizationloop) Filter Bank32 Subbands Encoding Encoding ofAdditionalInformation Additional Data PsychoacousticalModel ExternalCheck 6. Frame OutputstreamFormatting