This document summarizes a seminar presentation on audio compression techniques. It introduces common audio compression methods like PCM, DPCM, adaptive DPCM, linear predictive coding, perceptual coding, and MPEG audio coders. Specific techniques covered include third order predictive DPCM, backward and forward adaptive bit allocation used in Dolby AC-1. Applications of audio compression include conferencing, broadcasting radio programs by satellite, and saving memory space in sound cards.

1. SDM COLLEGE OF ENG & TECHNOLOGY
DHARWAR,KARNATAKA
SEMINAR ON
AUDIO COMPRESIION
CARRIED OUT BY
ACHYUTANAND
1

2. OUTLINE
• INTRODUCTION
• DIFFERENT AUDIO COMPRESSION TECHIQUES
• APPLICATION
• REFERENCES
• ACKNOWLEDGEMENT

3. INTRODUCTION
• Like text and image both audio and video are continuously varying analog
signals .
• Digitization process of different types of signals is known as PULSE
CODE MODULATION(PCM).
• If lower sampling rate is taken then quality of decoded signal is reduced.
• If we use fewer bits for sampling then it will introduce higher quantization
noise.
• Band limited signal: sampling rate is determined by bandwidth of
communication channel.[1]
• In PCM, each sample of the waveform is encoded independently.[2]

4. Different Techniques
• DPCM(Differential Pulse Code Modulation)
R: stores previously
digitized sample of
analog input signal.[1]
Value in the register is
updated by adding
subtracted signal to the
current register content
R1= R0+ DPCM
DPCM = PCM- R0

5. Third order predictive DPCM
•For more accurate values we require
predictor coefficient.
•Coefficients are calculated by
varying the proportions of last 3
predicted values from current
digitized value of o/p of ADC.

6. Adaptive DPCM
Principle : depends on varying no
of bits for the difference signal
which depends on the amplitude.
•Input speech is extended to
50 hz to 7 khz.
•This wider bandwidth
produces high fidelity speech
signal .
•Frequency component of
the lower sub-band has
higher perceptual
importance than higher sub
band.
•Operating bit rate: 64kbps,
56 0r 48 kbps.[3]

7. Linear predictive coding
•Analyzing the audio waveform to
determine the perceptual features,
which is then quantized and sent ,
destination uses synthesizer to
regenerate sound
•Features determining perception
of a signal
1. Pitch 2. period 3. loudness
In addition to this
1. Voiced sounds
2. unvoiced sounds [4]
•Block of digitized samples called
segment is analyzed to determine
the perceptual parameters of
speech

8. Perceptual coding
• Perceptual coding is used for compression of general audio
such as digital TV broadcast.
•Ear is sensitive to signals, in
multiple signal the strong signal
may reduce the level of
sensitivity of the ear to other
signal , in frequency called
frequency masking.
After the ear hears the loud sound , it
takes further shorter time before it can
hear quieter sound , called temp masking

9. MPEG AUDIO CODERS
•MPEG was formed by ISO to
formulate various standards related to
multimedia application
•Uses perceptual coding
•Duration of sampled segment is
equal to time to accumulate 12
successive set of 32 PCM.
•Analysis filter bank determines
the maxm amplitude of 12 sub
band samples in each sub band
•Bit allocations are done to
determine those frequency
components that are in the region
of highest sensitivity[1]
6 bits : peak amplitude in each sub
band , 4 bits : for 12 frequency
component.

10. Dolby Audio Coders
a) Forward adaptive bit allocation
•Mode of operation of perceptual
coder is called FABA.
Disadvantage : each encoded frame
contains bit allocation information
which leads to inefficient use of
available bit rate.
• Therefore it is advised to use
fixed bit allocation for each sub
band which is then used by
both encoder and decoder,
called DOLBY AC-1.
b) Fixed bit allocation

11. Contd..
•In addition to quantized samples
it also contains frequency
component that is present in the
sampled waveform , called
encoded spectral envelope , mode
of operation is called backward
Backward adaptive bit allocation adaptive bit allocation.[1]
Hybrid bit allocation

12. APPLICATION
• Used in conferencing application .
• Used in satellite to relay FM radio programs.
• Compression associated with the audio of a number of sound
cards.
• Used to save memory space.

13. References
[1] Fred Halsall, Multimedia communications : applications, network,
protocols and standards(2nd edition, Pearson education -2002)
[2] K. Giridhar, Digital communication(New 2009-10 edition)
[3] http: www.wilipedia.org/adpcm
[4]http: www.googlebooks.com/halsall

14. Acknowledgement
• The successful completion of this seminar was made possible with
the help and guidance received by me from my beloved faculty. I
would like to avail this opportunity to express my sincere thanks
and gratitude to all of them.
• I express my sincere gratitude to Mr. Mahendra M Dixit, Asst. Prof
,SDMCET for providing me an opportunity to present my seminar.
• I am thankful to my classmates for attending the seminar and
listening me from the last few minutes.
ACHYUTANAND