1. Dhatchaini Rajendran Student ID: 1000636681 Date : 11-23-2010
EE5359 Multimedia Processing Project Study and Comparison of AC3, AAC and HE-AAC Audio Codecs
Dhatchaini Rajendran
Student ID:
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2. Introduction CD Audio: 16 bit encoding 2 Channels (Stereo)
Analog Audio Input
Analog Audio Output
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

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
Digital Audio Encoded
Input Output
Analysis Filter Bank
Quantization and Coding
Encoding of Bitstream
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. AC3 – Encoder
Block Diagram of AC-3 Encoder [2]

11. AC3 – Decoder
Block Diagram of AC-3 Decoder [2]

12. 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

13. 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)

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

15. 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

16. ADTS profile bits in header
Profile bits in ADTS header[11]

17. 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.

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

19. 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

20. 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
3.51
Tested at bit rate = 64.2 kbps

21. 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
184
45:1 24
140
59:1

22. 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
263
31:1

23. Comparison of the codecs
AC-3
AAC
HE-AAC
Audio compression format
Lossy
lossy
Forward transform
Time division aliasing cancelling filter banks by Princen and Bradley
MDCT
Number of channels 6
Perceptual audio coding
Yes
Sampling rates
Upto 48kHz
8-96 kHz
24 – 96 kHz
Profiles
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

24. 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.

25. 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

26. 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
[2] A/52 B ATSC Digital Audio Compression Standard:
 [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

27. [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:
[9] ISO/IEC IS , “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 ,Dec
[12] C.C.Todd, G.A. Davidson, M.F. Davis et. al,” AC-3: Flexible perceptual coding for audio transmission and storage”, Dolby laboratories

28. Thank You!