Operation of a vocoder

A vocoder is a speech coder that analyses speech into the parameters of a speech-production model and transmits only those parameters. The receiver synthesizes speech from them, giving bit rates of about 1.2–4.8 kbps.

Speech model: excitation source (pulse train at the pitch frequency for voiced sounds, random noise for unvoiced sounds) → time-varying vocal-tract filter → speech.

 ANALYZER (transmitter)
 speech->[frame]-+->[vocal tract analysis]-> a_k
                 +->[V/UV decision]      -> flag
                 +->[pitch estimator]    -> T0
                 +->[energy measure]     -> G
           a_k, flag, T0, G -> quantize -> MUX

 SYNTHESIZER (receiver)
 [pulse gen @ T0]--+
                   +-[V/UV]-(×G)->[vocal tract
 [noise gen]-------+               filter a_k]->speech

Operation

1. Framing: sampled speech is cut into short frames (10–30 ms) in which it is nearly stationary.
2. Spectral analysis: the vocal-tract response is estimated, as band energies (channel vocoder), formant frequencies (formant vocoder) or predictor coefficients (LPC vocoder).
3. Excitation analysis: a detector decides whether the frame is voiced or unvoiced and measures the pitch period.
4. Gain: the frame energy is computed.
5. These parameters are quantized, multiplexed and transmitted (e.g. LPC-10: 2.4 kbps).
6. Synthesis: at the receiver the V/UV flag selects either the pulse generator (at the received pitch) or the noise generator. The excitation is scaled by the gain and passed through a filter set by the received spectral parameters to recreate speech.

Merits: very low bit rate, good intelligibility, suits secure and narrow channels. Demerits: synthetic quality, sensitive to pitch errors and background noise.