Vocoder and predictive coders

Vocoder

A vocoder (voice coder) is a speech source coder that analyses speech into the parameters of a speech-production model – vocal-tract filter, excitation type (voiced/unvoiced), pitch and gain – and transmits only these. The receiver synthesizes speech from them. Bit rates are very low (1.2–4.8 kbps) but quality is synthetic. Examples: channel, formant, cepstrum and LPC vocoders.

Predictive coders

Predictive coders use linear prediction: each sample is estimated from past samples, ŝ(n) = Σₖ₌₁ᵖ aₖ s(n − k), and the vocal tract is modelled by the all-pole filter 1/A(z). Two widely used predictive coders:

1. Multi-pulse excited LPC (MPE-LPC)

 [pulse generator]->[1/A(z)]-> s^(n) --(-)<-- s(n)
        ^                               |
        +--[error minimisation]<-[weighting filter]

• LPC coefficients are computed for each frame as usual.
• Instead of a single pitch pulse or noise, the excitation has several pulses per sub-frame (e.g. 4–8 per 5 ms).
• Pulse positions and amplitudes are found one at a time by analysis-by-synthesis, minimising the perceptually weighted error between original and synthesized speech.
• No voiced/unvoiced decision or pitch detection errors; good quality at about 9.6 kbps. GSM's RPE-LTP is a variant with regularly spaced pulses.

2. Code-excited LPC (CELP)

 [codebook index i]->(×G)->[pitch predictor]->[1/A(z)]
                                                 |
                       s(n) --(-)<--- s^(n) -----+
                               |
                [perceptual weighting] -> choose i, G

• A codebook of, say, 1024 stored random excitation vectors is kept at both ends.
• For each 5 ms sub-frame the encoder tries the codevectors through a long-term (pitch) predictor and short-term LPC filter, and selects the one giving minimum weighted error.
• Only the codebook index, gain, pitch parameters and LPC coefficients are sent.
• High quality at 4.8–8 kbps; used in IS-95 (QCELP), FS-1016 (4.8 kbps) and ACELP (GSM EFR, AMR).

(A third type, residual excited LPC (RELP), sends the low-pass filtered prediction residual as excitation.)