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MPEG Audio Compression

MPEG Audio Compression. by V. Loumos. Introduction. Motion Picture Experts Group (MPEG) International Standards Organization (ISO) First High Fidelity Audio standard Part of a multiple standard for Video compression Audio compression

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MPEG Audio Compression

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  1. MPEG Audio Compression by V. Loumos

  2. Introduction • Motion Picture Experts Group (MPEG) • International Standards Organization (ISO) • First High Fidelity Audio standard • Part of a multiple standard for • Video compression • Audio compression • Audio, Video and Data synchronization at an aggregate rate of1.5 Mbit/sec

  3. MPEG Audio • Physically Lossy compression algorithm • Perceptually lossless, transparent algorithm • Exploits perceptual properties of human ear • Psychoacoustic modeling

  4. Medium quality audio compression • Code Excited Linear Prediction • for speech coding • μ-law • Adaptive Differential Pulse Code Modulation

  5. The MPEG Audio standard • Ensures inter-operability • Defines coded bitstream syntax • Defines decoding process • Guarantees decoder’s accuracy

  6. MPEG audio acceptance • Wide acceptance • Large number of MPEG audio codecs produced • Stand-alone, Mobile phone add-ons etc

  7. MPEG audio features • No assumptions about the nature of the audio source • Exploitation of human auditory system perceptual limitations • Removal of perceptually irrelevant parts of audio signal

  8. MPEG audio sampling rates • 32 kHz • 44.1 kHz • 48 kHz

  9. MPEG audio supports • One or two audio channels in • a monophonic mode for a single audio channel • a dual monophonic mode for two independent audio channels • a stereo mode with sharing of bits • a joint stereo mode based on the correlation or the phase difference between channels

  10. MPEG audio supports • Several predefined fixed bit rates ranging from 32 to 224 kbits/sec per channel • Free bit rate other than the predefined rates

  11. MPEG audio offers • Three independent layers of compression • A wide range of tradeoffs between codec complexity and compressed audio quality

  12. MPEG Audio Layer I • Simplest coding • Suitable for bit rates above 128 kbits/sec per channel • Philips Digital Compact Cassette

  13. MPEG Audio Layer II • Intermediate complexity • Bit rates around 128 kbits/sec per channel • Digital Audio Broadcasting (DAB) • Synchronized Video and Audio on CD-ROM • Full motion CD-I • Video-CD

  14. MPEG Audio Layer III • Most complex coding • Best audio quality • Bit rates around 64 kbits/sec per channel • Suitable for audio over ISDN

  15. MPEG Audio extras • All three layers allow single chip real-time decoder implementation • Optional Cyclic Redundancy Check (CRC) error detection • Ancillary data may be included in the bit stream

  16. Overview • Quantization, the key to MPEG audio compression • Transparent, perceptually lossless compression • No distinction between original and 6-to-1 compressed audio clips • stereo, 16 bit/sample, sampled at 48 kHz, compressed at 256 kbits/sec

  17. The Polyphase Filter Bank • Key component common to all layers • Divides the audio signal into 32 equal-width frequency subbands • The filters provide good time and reasonable frequency resolution • Critical bands associated with psychoacoustic models

  18. Psychoacoustics • The aim is to remove acoustically irrelevant parts of the audio signal • The human auditory system is unable to hear quantization noise under conditions of auditory masking • Masking occurs whenever a strong signal makes a neighborhood of weaker audio signals imperceptible

  19. Critical bands • The human auditory system has a limited, frequency dependent resolution • This frequency dependence is expressed in the form of critical band widths, less then 100 Hz for low and more then 4kHz for high frequencies • The human ear blurs the various signal components inside a critical band

  20. Noise masking threshold • Human ear resolving power is frequency dependent • Noise masking threshold, at any frequency, depends only on the signal energy within a limited bandwidth neighborhood that frequency

  21. The Psychoacoustic Model • Analyzes the audio signal and computes the amount of noise masking as a function of frequency • The encoder decides how best to represent the input signal with a minimum number of bits

  22. Basic Steps • Time align audio data • Convert audio to frequency domain representation • Process spectral values into tonal and non-tonal components • Apply a spreading function • Set a lower bound for threshold values • Find the threshold values for each subband • Calculate the signal to mask ratio

  23. MPEG Layer III coding • Based on Layer I&II filter banks • Compensation of filter deficiencies by processing outputs with a Modified Discrete Cosine Transform

  24. Layer III enhancements • Alias reduction • Non uniform quantization • Scalefactor bands • Entropy coding of data values • Use of a “bit reservoir”

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