Formant vocoder and speech characteristics

Formant vocoder

The formant vocoder is based on the fact that the short-time spectrum of speech is mainly described by its formants, the resonant frequencies of the vocal tract. Normally the first three or four formants (F1 ≈ 300–800 Hz, F2 ≈ 800–2500 Hz, F3 ≈ 2–3 kHz) carry most of the information.

 Tx: speech-+->[formant tracker]-> F1,F2,F3 (+ amplitudes)
            +->[pitch & V/UV detector] -> pitch, flag
            +->[energy] -> gain        -> MUX -> channel

 Rx: [pulse gen | noise gen] (by V/UV, pitch)
            |
            v (×gain)
     [resonator F1]->[resonator F2]->[resonator F3]
            |
            v speech

Operation

1. Speech is divided into frames of 10–30 ms.
2. A formant tracker finds the frequencies (and sometimes bandwidths/amplitudes) of the first 3–4 formants in each frame, e.g. by peak-picking the spectrum or from LPC poles.
3. A pitch detector decides voiced/unvoiced and measures pitch.
4. Formants, pitch, V/UV flag and gain are coded and transmitted.
5. The receiver selects a pulse train (voiced) or noise (unvoiced), scales it and passes it through resonators tuned to the received formant frequencies, which rebuild the spectral envelope.

Merits: very few parameters, so very low bit rate (below about 1.2 kbps). Demerits: formants are hard to track reliably (they merge or are weak, especially in noise), so quality is poor; hence formant vocoders are rarely used commercially.

Characteristics of speech signal

• Main energy in 300–3400 Hz.
• Non-uniform amplitude pdf (small values most likely).
• High correlation between successive samples.
• Non-flat spectrum with formant peaks.
• Voiced sounds periodic (pitch 80–350 Hz), unvoiced sounds noise-like.
• Quasi-stationary over 10–30 ms.
• Silence for about 40–60% of the time in a conversation.