Vocoders and their types

Vocoders

A vocoder (voice coder) is a source coder that does not try to reproduce the speech waveform. It extracts the parameters of a model of human speech production – vocal-tract filter, excitation type (voiced/unvoiced), pitch and gain – sends only these, and the receiver synthesizes speech from them. This gives very low bit rates (about 1.2–4.8 kbps) at the cost of natural quality.

 Analyzer (Tx)
 speech -+-> [spectral analysis] ----+
         +-> [pitch, V/UV detector] -+-> MUX -> channel

 Synthesizer (Rx)
 channel -> DEMUX -+-> pitch, V/UV
                   |    -> [pulse gen | noise gen]
                   |              |
                   +-> spectrum ->[synthesis filter]
                                  |
                                  v speech

The model: speech = excitation (pulse train for voiced, random noise for unvoiced) passed through a time-varying filter (vocal tract).

Kinds of vocoders

1. Channel vocoder – A bank of band-pass filters (about 16–19 bands) measures the energy in each band; the band envelopes plus pitch/V-UV are sent. The receiver modulates a matching filter bank with the excitation. Earliest vocoder.
2. Formant vocoder – Sends the frequencies and amplitudes of the first three or four formants (vocal-tract resonances) plus pitch. Very low rate (< 1.2 kbps) but formants are hard to track accurately.
3. Cepstrum vocoder – Uses the cepstrum (inverse FFT of log spectrum) to separate the excitation (high quefrency) from the vocal-tract response (low quefrency); low cepstral coefficients and pitch are sent.
4. Voice-excited vocoder – Sends a narrowband (baseband) portion of the actual speech as excitation instead of a pitch detector output, avoiding pitch-detection errors.
5. Linear predictive coder (LPC) – Models the vocal tract as an all-pole filter with ~10 coefficients; LPC-10 runs at 2.4 kbps. Its improved forms (multi-pulse, RELP, CELP) are used in GSM and IS-95.

Example of bit rate saving: 64 kbps PCM vs 2.4 kbps LPC-10 is a reduction of about 27 times, which is why vocoder principles (in the form of hybrid coders like CELP) are used in mobile systems.