3. PROBLEMS
speech coding systems is to transmit speech with the
highest possible quality using the least possible
channel capacity.
To save bandwidth in telecoms applications and to
reduce memory storage requirements.
Maintain certain levels of complexity to reduce the
processing delay and cost of implementation.
4. PRESENTATION OUTLINE
Section I
Introduction to speech
Sub-band coding (SBC)
Filter Banks
Section II
Sub band coder implimentation
QMF design
Simulation and result
Section III
Conclusion
Applications
5. Introduction to Speech
What is the Speech?
o Speech is the primary method of human
communication.
o To transmit/store a speech waveform using as
few bits as possible while retaining high quality
6. Speech Process
Production
Propagation:
Perception:
The incoming sounds are deciphered by the listener
into a received message, thereby completing the chain
of events that culminated in the transfer of
information from the speaker to the listener
7. SUB BAND CODING
Divides the speech signal into many smaller sub-bands
and encodes each sub-band separately according to
some perceptual significance.
Speech is typically divided into 4 or 8 sub-bands by a
bank of filters.
Can be used for coding speech at bit rates in the range
9.6 kbps to 32 kbps.
8. A compression approach where digital filters
are used to separate the source output into
different bands of frequencies.
Each part then can be encoded separately.
9. FILTERS
A system that isolates a constituent part corresponding to
certain frequency is called a filter.
If it isolates the low frequency components, it is called a
low- pass filter.
Similarly, we have high-pass or band –pass filters.
In general, a filter can be called a subband filter if it isolates
a number of bands
10. FILTER BANKS
Filter banks are essentially a cascade of stages, where each
stage consists of a low-pass filter and a high-pass filter
The source output is passed through a bank of filters.
This filter bank covers the range of frequencies that make
up the source output.
The passband of each filter specifies each set of
frequencies that can pass through.
13. MATLAB CODE IMPLIMENTING THE SUBBAND CODER
Function y=subband(x,h0,bits)
subband decomposition
y=subband(x, h0, [bits])
x=input signal vector
h0=basic QMF filter
bits= a vector of 2 entries giving the number of bits
y=output signal vector
16. The speech signal is to be sampled at a rate fs samples
per second.
The first frequency subdivision is splits the signal
spectrum into two equal width segments,low pass
signal and a high pass signal
The second frequency subdivision split the first
lowpass signal into two equal bands ,a lowpass
signal ,,,and a highpass signal
17. Finally, the third frequency subdivision splits the
lowpass signal from the second stage into two equal
bandwidth signals .
Thus the signal is subdivided into four frequency
bands,covering three octaves.
18. BLOCK DIAGRAM OF SUB BAND SPEECH DECODER WITH THREE
FREQUENCY SUBDIVISION
19. The decoding process for the sub band encoded
speech signal is basically the reverse of the encoding
process.
The signal in adjacent lowpass and high pass
frequency bands are interpolated, filterd,and
combined
20. Quadrature Mirror Filter (QMF)
A quadrature mirror filter is a filter most commonly used to
implement a filter bank that splits an input signal into two
bands. The resulting high-pass and low-pass signals are often
reduced by a factor of 2, giving a critically sampled two-
channel representation of the original signal.
21.
22.
23. DECIMATION
Downsampling (or "subsampling") is the process of redusing
the sampling rate of asignal. This is usually done to reduce
the data rate or the size of the data.
24. INTERPOLATOR
Upsampling is the process of increesing the sampling rate
of a signal.
The upsampling factor (commonly denoted by L) is
usually an integer or a rational fraction greater than unity.
25. 2.Quantization and Coding
Selection of the compression scheme
Allocation of bits between the subbands
Allocate the available bits among the subbands
according to measure of the information content in
each subband.
26. Bit Allocation
Minimizing the distortion i.e. minimizing the
reconstruction error drives the bit allocation
procedure.
Bit allocation procedure can have a significant
impact on the quality of the final reconstruction
27. 3.Synthesis
Quantized and Coded coefficients are used to reconstruct a
representation of the original signal at the decoder.
Encoded samples from each subband decoded
upsampled bank of reconstruction filters outputs
combined Final reconstructed output
29. CONCLUSION
Subband coding is another approach to decompose the
source output into components based on frequency.
A structure of two channel QMF with lowpass
filter,highpass filter,decimators and interpolators has
been proposed to perform subband coding of speech
signal in the digital domain.
30. The general subband encoding procedure can be summarized
as follows:
• Select a set of filters for decomposing the source.
• Using the filters, obtain the subband signals.
• Decimate the output of the filters.
• Encode the decimated output.
The decoding procedure is the inverse of the encoding
procedure
32. REFERENCES
YUE Dongjian “The Study of Speech Coding Technology
Based on Code Excited Linear Predictive Coding”
Ph.D.thesis, Tongji University, 2000.
B. Carnero and A. Drygajlo. “Perceptual speech coding
and enhancement using frame synchronized fast wavelet
packet transform algorithms.” IEEE Trans. Signal
Processing Vol.47 No.6 ,June 1999.
P. Philippe, F. Moreau de Saint-Martin and M. Lever.
“Wavelet packet filterbanks for low time delay audio
coding.” IEEE Trans. Speech and Audio Processing. 1999.
33. John G. Proakis and Dimitris G. Manolakis, “Digital
Signal Processing: Principles,Algorithms and
Applications”, Third Edition.
Roberts R. A. and Mullis C. T. Digital Signal Processing.
Addison-Wesley, Reading.
Mass, 2006.
[3]. Oppenheim A. V. and Schafer R. W. Discrete-Time
Signal Processing. Prentice Hall.
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