Ch2-Speech_Coding-old - Speech Processing Speech Coding...

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Speech Processing Speech Coding
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February 11, 2012 Veton Këpuska 2 Speech Coding Definition: Speech Coding  is a process that leads to the representation of  analog  waveforms  with  sequences of binary digits . Even though availability of high-bandwidth communication channels has  increased, speech coding for bit reduction has retained its importance. Reduced bit-rates transmissions required for cellular networks Voice over IP Coded speech  Is less sensitive than analog signals to transmission noise Easier to:  protect against (bit) errors Encrypt Multiplex, and Packetize Typical Scenario depicted in next slide (Figure 12.1) 
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February 11, 2012 Veton Këpuska 3 Digital Telephone Communication  System
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February 11, 2012 Veton Këpuska 4 Categorization of Speech Coders Waveform Coders: Used to quantize speech samples directly and operate  at high-bit rates in the range of 16-64 kbps (bps - bits  per second) Hybrid Coders Are partially waveform coders and partly speech model- based coders and operate in the mid bit rate range of  2.4-16 kbps. Vocoders Largely model-based and operate at a low bit rate  range of 1.2-4.8 kbps. Tend to be of lower quality than waveform and hybrid  coders.
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February 11, 2012 Veton Këpuska 5 Quality Measurements Quality of coding can is viewed as the  closeness of the processed speech to the  original speech or some other desired  speech waveform. Naturalness Degree of background artifacts Intelligibility Speaker identifiability Etc.
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February 11, 2012 Veton Këpuska 6 Quality Measurements Subjective Measurement: Diagnostic Rhyme Test  (DRT) measures  intelligibility. Diagnostic Acceptability Measure  and  Mean  Opinion Score  (MOS) test provide a more  complete quality judgment. Objective Measurement: Segmental Signal to Noise Ratio (SNR) – average  SNR over a short-time segments Articulation Index – relies on an average SNR  across frequency bands. 
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February 11, 2012 Veton Këpuska 7 Quality Measurements A more complete list and definition of  subjective and objective measures can be  found at: J.R. Deller, J.G. Proakis, and J.H.I Hansen,  “Discrete-Time Processing of Speech”,  Macmillan Publishing Co., New York, NY, 1993 S.R. Quackenbush, T.P. Barnwell, and M.A.  Clements, “Objective Measures of Speech  Quality. Prentice Hall, Englewood Cliffs, NJ.  1988
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Statistical Models
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February 11, 2012 Veton Këpuska 9 Statistical Models Speech waveform is viewed as a random process.
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