1. VOCODERS

2. Vocoders Speech Coding Systems Implemented in the transmitter for analysis of the voice signal Complex than waveform coders High economy in transmission bit rate Less robust Performance tends to be talker dependent

3. Speech Generation Model

4. Channel Vocoder First analysis-synthesis systems Frequency domain vocoders Determines the envelope of speech signal Samples, encodes & multiplexes these samples with encoded output Synchronous sampling along with energy information

5. Formant Vocoders Similar to channel vocoder Operates at lower bit rates compared to channel vocoder Positions of peaks alone are transmitted Capable of reproducing speech at lower bit rates like 1200 bits/s Not very successful

6. Cepstrum Vocoders Separates excitation and vocal tract spectrum by inverse Fourier transform Low frequency coefficients correspond to vocal tract cepstral coefficients High frequency excitation coefficients form a periodic pulse train at multiples of sampling frequency Convolution of impulse response with original synthetic response function leads to reconstructed speech

7. Voice-Excited Vocoder The need for pitch extraction and Voicing detection Uses hybrid combination of PCM transmission for low frequency Pitch signal is generated after performing operations to base signal Operation range 7200 bits/s to 9600 bits/s

8. Linear Predictive Coding LPC belongs to the time domain class of vocoders Extracts significant features from the speech from time waveform Good voice quality at 4.8 kbps

9. Linear Predictive Coding Determination of Predictor Coefficients

10. Linear Predictive Coding Block Diagram of a LPC coding System

11. Multipulse Excited LPC – More than one pulse is used for excitation – Pulse positions & amplitudes are adjusted to reduce error – Speech quality is improved Code-Excited LPC (CELP Coders) – Predetermined code book of Stochastic excitation signals is available in coder & decoder – Extremely Complex Linear Predictive Coding

14. Residual Excited LPC – Related to DPCM technique – Speech is synthesized at the transmitter and subtracted from original speech Linear Predictive Coding

15. Choosing Speech Codecs for Mobile Communication Since Bandwidth is limited speech compression is necessary End-to-end encoding delay must be considered Speech Codec should be robust to transmission errors Low bit rate Vocoder type codecs are not suitable Speech coder depends on the cell size Type of multiple access technique to be used is also an important factor Modulation used should be bandwidth efficient

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