Need for speech coding; vocoder concept

Need for speech coding

Speech coding compresses digitized speech into fewer bits while keeping acceptable quality. It is needed because:

• Plain 64 kbps PCM uses too much of the scarce, expensive radio spectrum; low-rate coders (e.g. 13 kbps GSM, 8 kbps CELP) let more users share a channel and raise system capacity.
• Lower bit rate means narrower bandwidth, less transmit power and longer battery life.
• Coders can include protection of important bits and work well with channel coding, keeping quality high over noisy, fading links.
• It allows voice activity detection and discontinuous transmission, reducing interference.
• Digital speech can be encrypted for privacy and multiplexed easily with data.

Basic concept of VOCODER

A vocoder is a parametric speech coder based on a model of how humans produce speech:

• Source (excitation): the vocal cords produce a periodic pulse train for voiced sounds (pitch 80–350 Hz) or turbulent noise for unvoiced sounds.
• Filter: the vocal tract (throat, mouth, nose) acts as a slowly varying filter whose resonances are the formants.

Instead of sending the waveform, the vocoder analyses each 10–30 ms frame and sends only:

1. Vocal-tract filter parameters (band energies, formants or LPC coefficients),
2. Voiced/unvoiced decision,
3. Pitch period, and
4. Gain (loudness).

 Tx: speech -> [analysis] -> {filter params, V/UV,
                              pitch, gain} -> encode
 Rx: pitch pulses --+
                    +--[V/UV]--(×gain)--[vocal tract
     noise ---------+                    filter]--> speech

The receiver uses a pulse generator or noise generator as the excitation and drives a synthesis filter set by the received parameters.

Waveform coder vs vocoder

Result: rates of 1.2–4.8 kbps, good intelligibility but synthetic quality. Types include channel, formant, cepstrum, voice-excited and linear predictive vocoders.