telecommunications switching systems 14

7/29/2019 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

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