telecommunications switching systems 14
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APC
Adaptive predictive coding
Ratio of input signal to the power of thedifference signal is prediction gain.
APC involves periodic determination andtransmission of the predictor coefficients.
Adaptive Predictive Encoder
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Adaptive Predictive Decoder
Adaptive Predictive Encoder/Decoder
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Subband Coding
Input spectrum is divided into separate bands.
q is proportional to the energy levels in eachband.
High energy levels can be encoded with coarsequantization.
Speech signal masks the quantization noise.
Larger no. of bits per sample for lowfrequencies.
At higher frequencies, fewer bits per sample canbe used.
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Subband Coder
Subband coding example: G.722 7-kHz audio codec
The upper band does not require as high a data rate because it isnot as important perceptually and has significant less energy.
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Average MOS vs Bit Rate of G.722 7-kHz audio codec
Vocoders
Voice-Coders
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Vocoders
The basic goal of a vocoder is to encodeonly the perceptually important aspects ofspeech with fewer bits than the moregeneral waveform encoders.
Intelligble voice, quality < telephonestandards.
Vocoding techniques
Channel vocoder
Formant vocoder
Linear predictive coding (LPC)
The emphasis of a vocoder is in reproducing theshort-term power spectrum of the input.
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Channel Vocoder
Short term signal spectrum as a function of time
Encodes individual power levels and transmits.
Ignores phase information
Frequency domain model of voice tract transferfunction
Pitch-excited vocoder
Spectrum channel vocoders
Intelligible, synthetic sound
Channel Vocoder
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Channel Vocoder
Channel Vocoder
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Formant vocoder
Speech energy tends to be concentratedat three or four peaks called formants
Location and amplitude of these spectralpeaks are determined and transmitted.
Most significant short term components
are encoded.
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Linear predictive coding
Extracts perceptually significant featuresof speech directly from a time waveform.
Time varying model of the vocal tractexcitation and transfer function.
Speech generation model of LPC
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LPC
LPC uses prediction parameters based onthe actual input segments to which theparameters are applied (forwardestimation).
LPC provide more natural soundingspeech than purely frequency domainbased vocoders
Enhanced-Excitation Linear PredictiveCoding
Pulse Excited LPC (PELP)
Multipulse LPC (MPLPC)
Residual excited LPC (RELP)
Mixed-excitation LPC (MELP)
Code-excited LPC (CELP)
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PELP
Voiced signals are modeled by simplepulse generator, an LPC coder issometimes referred to as a pulse excitedlinear prediction (PELP)
LPC vs MPLPC
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RELP
residual waveform acts as the excitation ofprediction loop.
Residual consists of fundamentalfrequency and harmonics
Fundamental frequency is encoded.
When subband, MPLPC and RELP
compared, MPLPC provided bestperformance.
CELP
Assumes residuals are not randomwaveforms with independent samples butrather that a block of residue samples canbe represented by one of a manageablenumber of waveform templates.
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CELP
Encoder/Decoder Selection Considerations
Voice Quality
Transparency for non-voice signals
Tolerance of transmission errors
Delay