Types of Vocoders
This sections explains the classification of the vocoders
Classification of Vocoders:
- Channel Vocoders
- Formant Vocoders
- Cepstrum Vocoders
- Voice-Excited Vocoders
- The channel Vocoders was the first analysis-synthesis systems of speech demonstrated practically.
- Channel Vocoders are frequency domain Vocoders that determine the envelope of the speech signal for a number of frequency bands and then sample, encode, and multiplex these samples with the encoded outputs of the other filters.
- Thesampling is done synchronously every 10 ms to30 ms.
- Along with the energy information about each band, the voiced/unvoiced decision, and the pitch frequency for voiced speech are also transmitted.
- The formant vocoder is similar in concept to the channel vocoder.
- The formant vocoder can operate at lower bit rates than the channel vocoder because it uses fewer control signals
- The formant vocoder attempts to transmit the positions of the peaks (formants) of the spectral envelope, instead of sending samples
- A formant vocoder must be able to identify at least three formants for representing the speech sounds
- It must control the intensities of the formants.
- Formant vocoder can reproduce speech at bit rates lower than 1200 bits/s.
- The Cepstrum vocoder separates the excitation and vocal tract spectrum by inverse Fourier transforming of the log magnitude spectrum to produce the Cepstrum of the signal.
- The low frequency coefficients in the cepstrum correspond tothe vocal tract spectral envelope
- The high frequency excitation coefficients forming a periodic pulse train at multiples of the sampling period
- Linear filtering is performed to separate the vocal tract cepstral coefficients from the excitation coefficients
- In the receiver, the vocal tract cepstral coefficients are Fourier transformed to produce the vocal tract impulse response.
- By convolving this impulse response with a synthetic excitation signal (random noise or periodic pulse train), the original speech is reconstructed
- Voice-excited Vocoders eliminate the need for pitch extraction and voicing detection operation.
- This system uses a hybrid combination of PCM transmission for the low frequency band of speech, combined with channel vocoding of higher frequency bands.
- A pitch signal is generated at the synthesizer by rectifying, band pass filtering, and clipping the baseband signal.
- Voice excited vocoder have been designed for operation at 7200 bits/s to 9600 bits/s