EE5359 Multimedia Processing Project

Study and Comparison of AC3, AAC and HE-AAC Audio Codecs

Dhatchaini RajendranStudent ID: 1000636681

Date : 11-23-2010

IntroductionIntroduction

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

Analog Audio Input

Analog Audio Output

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

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 kHzRedundancy in the audio signal

Perceptual audio coding

Digital Audio EncodedInput OutputAnalysis

Filter BankQuantization

and CodingEncoding of Bitstream

Perceptual Model

Block Diagram for Perceptual Audio Coding [2]

Masking Thresholds

Psychoacoustic parameters

• Absolute hearing thresholds• Simultaneous masking• Critical band frequency analysis• Temporal masking• Spread of masking along the basilar

membrane

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

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

AC3 – Encoder

Block Diagram of AC-3 Encoder [3]

AC3 – Encoder

Block Diagram of AC-3 Encoder [2]

AC3 – Decoder

Block Diagram of AC-3 Decoder [2]

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

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)

AAC

Block Diagram of MPEG 2 – AAC [4]

AAC file formats

• ADIF (audio data interchange format) One header at the beginning followed by raw data

blocks Simple local storing purposes

• ADTS (audio data transport stream) One header for each frame followed by a block of raw

data Better error robustness

ADTS profile bits in header

Profile bits

ID 1 (MPEG-2 profile)

00 (0)

Main profile

01 (1)

Low complexity profile (LC)

10 (2)

Scalable sample rate profile (SSR)

11 (3)

(reserved)

Profile bits in ADTS header[11]

HE-AAC

• HE-AAC = AAC + SBR• 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.

AAC Codec with SBR technology

Block Diagram of HE-AAC [5]

Audio FilesOriginal FileSize : 8.23 MB

AAC encoded fileSize: 385 KB

encodeop.3gpHE-AAC encoded fileSize: 272 KB

bu4548.ac3

AC-3 encoded fileSize: 264 KB

encodeaac.aac

Performance of AAC codec

Results: File format

Bit rate after encoding

(kbps)

Encoding time

(seconds)

Decoding time

(seconds)

Original Size (MB)

Compressed Size (kB)

Compression Ratio

ADTS 64.2 8.7 3.09 8.23 353 23:1

ADIF 64.2 8.7 3.51 8.23 353 23:1

Tested at bit rate = 64.2 kbps

Performance of HE-AAC codec

Bit rate after encoding

(kbps)

Encoding time

(seconds)

Decoding time

(seconds)

Original Size (MB)

Compressed Size (kB)

Compression Ratio

48 3.0 2.0 8.23 272 30:1

32 3.0 2.0 8.23 184 45:1

24 3.0 2.0 8.23 140 59:1

Performance of AC-3 codec

Bit rate after encoding

(kbps)

Encoding time (seconds)

Original Size (MB)

Compressed Size (kB)

Compression Ratio

32 0.53 8.23 175 47:1

48 0.41 8.23 263 31:1

Comparison of the codecsAC-3 AAC HE-AAC

Audio compression format

Lossy Lossy lossy

Forward transform Time division aliasing cancelling filter banks by

Princen and Bradley

MDCT MDCT

Number of channels 6 6 6

Perceptual audio coding Yes Yes Yes

Sampling rates Upto 48kHz 8-96 kHz 24 – 96 kHz

Profiles AC-3 LC, Main, SSR,LTP

AAC-LC + SBR

File formats .ac3 .adts, .adif, .aac,.mp4,etc.

.aac, .3gp, .mp4, etc.

Applications Digital television, DVD iTunes codec Digital radio

MUSHRA test

• Multiple stimuli with hidden reference and anchor • Defined by international telecommunication union (ITU) • Subjective evaluation of audio quality. It is used to evaluate

the perceived quality of the output from lossy audio compression algorithms.

• The listener is presented with a certain number of test samples , a hidden version of the reference and one or more anchors.

Conclusion

• The performance of AC-3, AAC and HE-AAC can be compared at low bit rates.

• AAC has better performance than AC-3 at low bit rates

• HE-AAC has been tested to have excellent quality at 48 kbps by the European broadcasting union with considerable performance upto 24 kbps

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.

•[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] AAC codec download: www.audiocoding.com•[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. Kalva et al. “Implementing multiplexing, streaming and server interaction for MPEG-4”, IEEE Transactions on circuits and systems for video technology, vol. 9, No.8, pp 1299-1311,Dec. 1999.•[12] C.C.Todd, G.A. Davidson, M.F. Davis et. al,” AC-3: Flexible perceptual coding for audio transmission and storage”, Dolby laboratories. http://www.dolby.com/uploadedFiles/English_(US)/Professional/Technical_Library/Technologies/Dolby_Digital_(AC-3)/37_ac3-flex.pdf

Thank You!