Upload
sabina-lyons
View
214
Download
2
Embed Size (px)
Citation preview
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!