EE5359 Multimedia Processing Project
Study and Comparison of AC3, AAC
and HE-AAC Audio Codecs
Dhatchaini Rajendran
Student ID: 1000636681
Date : 11-23-2010
Introduction
Analog
Audio
Input
Analog to
Digital
Conversion
Digital Signal
Processing
Digital to
Analog
Conversion
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
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 kHz
Redundancy in the audio signal
Perceptual audio coding
Digital Audio
Input
Analysis
Filter Bank
Quantization
and Coding
Perceptual
Model
Masking Thresholds
Block Diagram for Perceptual Audio Coding [2]
Encoding of
Bitstream
Encoded
Output
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
00 (0)
ID 1 (MPEG-2 profile)
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 Files
Original File
Size : 8.23 MB
AAC encoded file
Size: 385 KB
HE-AAC encoded file
Size: 272 KB
AC-3 encoded file
Size: 264 KB
Performance of AAC codec
Results: File
format
Bit rate after
encoding
(kbps)
Encoding
time
(seconds)
ADTS
64.2
8.7
ADIF
64.2
8.7
Decoding
time
(seconds)
Original Size
(MB)
Compressed
Size (kB)
Compression
Ratio
3.09
8.23
353
23:1
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 codecs
AC-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!