Speech characteristics; formant and channel vocoder

Characteristics of speech signals

• Bandwidth mainly 300–3400 Hz.
• Non-uniform amplitude pdf: small amplitudes are most probable.
• High correlation between adjacent samples (predictable).
• Non-flat spectrum with resonant peaks called formants.
• Voiced sounds are quasi-periodic (pitch 80–350 Hz); unvoiced sounds are noise-like.
• Quasi-stationary over 10–30 ms frames.
• About 40–60% silence in conversation.

Channel vocoder

The channel vocoder is the oldest vocoder (Dudley, 1939). It represents the short-time spectrum by the energy in a set of frequency bands.

 Tx: speech-+-[BPF1]-[rectify+LPF]-> env 1 -+
            +-[BPF2]-[rectify+LPF]-> env 2 -+-> MUX
            +-[BPFn]-[rectify+LPF]-> env n -+
            +-[pitch & V/UV detector]-------+
 Rx: [pulse/noise]-+-(×env1)-[BPF1]-+
                   +-(×env2)-[BPF2]-+-(Σ)-> speech
                   +-(×envn)-[BPFn]-+

1. Speech passes through a bank of about 16–19 band-pass filters covering 200–3200 Hz.
2. Each filter output is rectified and low-pass filtered to give its slowly varying spectral envelope.
3. Envelopes are sampled (about every 20 ms) and sent with pitch and voiced/unvoiced information.
4. The receiver drives an identical filter bank with a pulse train (voiced) or noise (unvoiced), scales each band by its envelope and sums the bands.

Formant vocoder

The formant vocoder uses the fact that the speech spectrum is described mainly by its first few formants.

1. A formant tracker estimates the frequencies (and amplitudes) of the first three or four formants in each frame.
2. These values, plus pitch and V/UV decision, are transmitted.
3. The receiver uses resonators tuned to the received formant frequencies, excited by pulses or noise, to synthesize speech.

It needs fewer parameters than a channel vocoder, so the bit rate is very low (often below 1.2 kbps), but formants are hard to track accurately, especially in noise, so it is less used in practice.