A part of speech coding system which analyzes the voice signal at the transmitter is known as Vocoders. By using those parameters the transmit parameters are analysis and then synthesizeMuch more complex than waveform coders and achieve very high economy in transmission rate.But they are less robust, and their performance tend to be talker dependent.
- Linear predictive coder (LPC)
- Channel vocoder
- Formant vocoder
- Voice excited vocoder
Speech Generation Model:
Figure 7.5 shows the traditional speech generation model that is the basis of all vocoding systems
- The sound generating mechanism forms the source and is linearly separated from the intelligence modulating vocal tract filter which forms the system.
- The speech signal is assumed to be of two types: voiced and unvoiced
- Voiced sounds are a result of quasiperiodic vibrations of the vocal chord and unvoiced sounds are fricatives produced by turbulent air flow through a constriction
- The pitch frequency for most speakers is below 300 Hz.
- The pole frequencies correspond to the resonant frequencies of the vocal tract and are often called the formants of the speech signal.