Source coders used in speech coding

Speech source coders remove redundancy in speech to reduce the bit rate. Two widely used ones are described below.

1. Adaptive Differential PCM (ADPCM) – a waveform coder

ADPCM uses the high correlation between speech samples. It predicts each sample from past samples and quantizes only the prediction error, using a quantizer whose step size adapts to the signal level.

 s(n)->(+)-e(n)->[adaptive quantizer]-+-> e_q(n) out
        ^ -                  ^         |
        |             [step adapt]     v
        +-- s^(n) <--[predictor]<-----(+)<-- s^(n)
               s~(n) = s^(n) + e_q(n) (reconstructed)

• The predictor forms ŝ(n) from previous reconstructed samples.
• Error e(n) = s(n) − ŝ(n) has a much smaller range than s(n), so fewer bits are needed.
• Step size and predictor coefficients adapt to the changing speech level.
• The decoder contains the same predictor, adding e_q(n) to ŝ(n).
• ITU G.726 ADPCM gives toll quality at 32 kbps (4 bits/sample) – half of 64 kbps PCM. Used in DECT.

2. Linear Predictive Coder (LPC) – a vocoder

LPC models speech as an excitation passed through an all-pole vocal-tract filter, and sends only model parameters.

 Tx: speech->[20 ms frames]->[LPC analysis a1..a10, G]
                           ->[pitch, V/UV] -> encode
 Rx: [pulses | noise]->(×G)->[1/A(z) filter]->speech

• Each frame: about 10 predictor coefficients aₖ (from minimising prediction error, ŝ(n) = Σ aₖs(n−k)), gain G, pitch and voiced/unvoiced flag.
• The receiver excites the filter 1/A(z) with a pulse train (voiced) or noise (unvoiced).
• LPC-10 works at 2.4 kbps; quality is synthetic. Hybrid versions (CELP, RPE-LTP) send a better excitation and are used in mobile systems.