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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.
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
Introduction to SpeechWhat 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
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
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.
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.
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
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.
MATLAB CODE IMPLIMENTING THE SUBBAND CODERFunction 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
1.ANALYSISBLOCK DIAGRAM OF A SUB BAND SPEECH ENCODER WITHTHREE FREQUENCY SUBDIVISION
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
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.
BLOCK DIAGRAM OF SUB BAND SPEECH DECODER WITH THREEFREQUENCY SUBDIVISION
The decoding process for the sub band encodedspeech signal is basically the reverse of the encodingprocess.The signal in adjacent lowpass and high passfrequency bands are interpolated, filterd,andcombined
Quadrature Mirror Filter (QMF)A quadrature mirror filter is a filter most commonly used toimplement a filter bank that splits an input signal into twobands. The resulting high-pass and low-pass signals are oftenreduced by a factor of 2, giving a critically sampled two-channel representation of the original signal.
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.
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.
2.Quantization and CodingSelection of the compression schemeAllocation of bits between the subbandsAllocate the available bits among the subbands according to measure of the information content in each subband.
Bit AllocationMinimizing 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
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
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.
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
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 audiocoding.” IEEE Trans. Speech and Audio Processing. 1999.
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. . Oppenheim A. V. and Schafer R. W. Discrete-Time Signal Processing. Prentice Hall. Englewood Cliffs, New Jersey, 2007.