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EE5359 Multimedia Processing Project Study and Comparison of AC3, AAC and HE-AAC Audio Codecs

EE5359 Multimedia Processing Project Study and Comparison of AC3, AAC and HE-AAC Audio Codecs. Dhatchaini Rajendran Student ID: 1000636681 Date : 10-07-2010. Introduction. Analog Audio Input. Analog Audio Output. CD Audio: 16 bit encoding 2 Channels (Stereo) 44.1 kHz sampling rate

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EE5359 Multimedia Processing Project Study and Comparison of AC3, AAC and HE-AAC Audio Codecs

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  1. EE5359 Multimedia Processing ProjectStudy and Comparison of AC3, AAC and HE-AAC Audio Codecs Dhatchaini Rajendran Student ID: 1000636681 Date : 10-07-2010

  2. Introduction Analog Audio Input Analog Audio Output CD Audio: • 16 bit encoding • 2 Channels (Stereo) • 44.1 kHz sampling rate • Bit rate = 4.32 Mb/s taking into consideration the overhead due to synchronization, error correction Analog to Digital Conversion Digital Signal Processing Digital to Analog Conversion

  3. Digital Audio Compression Need for Audio Compression Algorithms: • Most of the internet and wireless applications are not compatible with high data rates as that of CD audio. Thus, the data rate must be reduced without increasing the quantization error. Objective of Audio Compression Algorithms: • To minimize the number of bits representing the signal • Efficient reproduction of the input signal Types of Audio Coding Techniques: • Parametric Coding • Waveform Coding • Hybrid Coding

  4. Perceptual Audio Coding • Incorporates psychoacoustic knowledge of the auditory system to compress the audio signal. • Maximum compression is achieved by considering the following facts: • Insensitivity of the human ear to frequencies less than 20 kHz • Redundancy in the audio signal

  5. Perceptual audio coding Analysis Filter Bank Quantization and Coding Encoding of Bitstream Digital Audio Encoded Input Output Perceptual Model Masking Thresholds Block Diagram for Perceptual Audio Coding [2]

  6. Psychoacoustic parameters • Absolute hearing thresholds • Simultaneous masking • Critical band frequency analysis • Temporal masking • Spread of masking along the basilar membrane

  7. AC-3 • It is an audio codec developed by Dolby laboratories • Dolby AC-3 audio compression algorithm is a ATSC (Advanced Television Systems Committee) standard for digital audio compression • AC-3 is a lossy audio compression format and supports multi-channel format • It is used in a variety of applications including digital television and DVD

  8. AC-3 Contd. • There are 5 full range channels (3Hz- 20,000Hz). Three of them are in the front (left, right and centre) and the other two are surround channels. • The sixth channel ranges from 3Hz-120Hz and is also known as Low frequencies Enhancement Channel. • This set of channels is known as “5.1” channels

  9. AC3 – Encoder Block Diagram of AC-3 Encoder [3]

  10. Advanced Audio Coding • This scheme was a joint development by many companies mainly including Dolby, Fraunhofer, AT&T, Sony and Nokia. • Motion Pictures Experts Group officially made AAC an international standard in 1997. • It is a lossy digital compression scheme which improves quality at lower bit rates. • AAC is the default iTunes codec which makes it very popular

  11. AAC Profiles Four profiles which can be chosen based on factors like complexity of bit stream to be encoded, desired performance and output. • Low complexity (LC) • Main profile (MAIN) • Sample-rate scalable (SRS) • Long term prediction (LTP)

  12. AAC Block Diagram of MPEG 2 – AAC [4]

  13. AAC with SBR technology • Spectral band replication is an add on to the audio coder. • This scheme uses the fact that harmonic series in the higher band of frequencies is the same as that in the lower band. • The higher frequencies are reconstructed using the lower frequency components.

  14. AAC Codec with SBR technology Block Diagram of HE-AAC [5]

  15. References: • [1] K. Brandenburg and M. Bosi, “Overview of MPEG audio: current and future standards for low-bit-rate audio coding,” JAES, vol.45, pp.4-21, Jan./Feb. 1997. • [2] A/52 B ATSC Digital Audio Compression Standard: http://www.atsc.org/cms/standards/a_52b.pdf •  [3] D.Meares, K. Watanabe and E.Scheirer, “Report on the MPEG-2 AAC stereo verification tests”, ISO/IEC JTC1/SC29/WG11, Feb.1998.  • [4] M. Dietz, L. Liljeryd and K. Kjörling, “Spectral band replication, a novel approach in audio coding,” in 112th AES Convention, Munich, May 2002. •  [5] F. Henn , R. Böhm and S. Meltzer, “ Spectral band replication technology and its application in broadcasting”, International Broadcasting Convention, 2003. • [6]M. Dietz and S. Meltzer, “ CT-aacplus – a state of the art audio coding scheme”, Coding Tecnologies, EBU Technical review, Jul. 2002.

  16. [7]P. Ekstrand, “ Bandwidth extension of audio signals by spectral band replication”, IEEE Benelux Workshop on Model based Processing and Coding of Audio (MPCA-2002), Nov.15, 2002. • [8] International Standard ISO/IEC 11172-3:1993, “Information technology – Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbit/s – Part 3: Audio,” ISO/IEC, 1993.  • [9] ISO/IEC IS 13818-7, “Information technology – Generic coding of moving pictures and associated audio information Part 7: advanced audio coding (AAC)”, 1997. •  [10] M. Bosi and R.E. Goldberg, “ Introduction to digital audio coding standards”, Norwell. MA: Kluwer, 2003. •  [11] H.S. Malvar, “Signalprocessing with lapped transforms”, Artech House: Norwood MA, 1992.

  17. Thank You!

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