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

18. 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

22. 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

23. 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

24. 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

28. 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

29. 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

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

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