--- title: "Formant vocoder and speech characteristics" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:formant-vocoder-speech" source: https://www.bctengineer.com/q/semester_7/wireless_communication/formant-vocoder-speech site: BCT Engineering --- # Formant vocoder and speech characteristics Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2071 Bhadra, 4+4 marks - Video: [vocoder synthesis and vocoder analysis](https://www.youtube.com/watch?v=dxVoxYi0iQg) — Nitin Bhopale - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/formant-vocoder-speech - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question Explain the operation of formant vocoder. What are the characteristics of speech signal? ## Short answer Formant vocoder tracks the first three or four formant frequencies plus pitch, voiced/unvoiced flag and gain; the receiver excites resonators tuned to those formants with pulses or noise. It runs below 1.2 kbps but formant tracking is unreliable. Speech has 300–3400 Hz bandwidth, non-uniform pdf, high correlation, formants, voiced/unvoiced sounds. ## Answer #### 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. On what idea is the formant vocoder based? A. Speech waveform is sent sample by sample B. The short-time spectrum is mainly described by formants, the resonant frequencies of the vocal tract C. Pitch alone carries all the information D. Speech is stored in a codebook of random sequences Answer: B. The first three or four formants carry most of the information, so only they are transmitted. (Key point: What a formant is) 2. How does the formant vocoder receiver rebuild speech? A. A pulse train (voiced) or noise (unvoiced) is scaled and passed through resonators tuned to the received formants B. It looks up a codebook index C. It adds the low-pass residual back to the signal D. It de-hops the signal with a PN code Answer: A. The excitation selected by the V/UV flag passes through formant resonators to recreate the spectral envelope. (Key point: Receiver synthesis) 3. Why are formant vocoders rarely used commercially despite very low bit rate? A. They need more than 64 kbps B. Formants are hard to track reliably, especially in noise, so quality is poor C. They cannot transmit pitch D. They need a codebook of 1024 vectors Answer: B. Formants merge or are weak, so tracking errors give poor quality. (Key point: Drawback of formant vocoder) 4. Which statement about speech signal is correct? A. Samples are uncorrelated and amplitude is uniform B. Main energy is in 300-3400 Hz and successive samples are highly correlated C. Voiced sounds are noise-like D. Speech is stationary over several seconds Answer: B. Speech has most energy in 300-3400 Hz, high sample correlation, and is quasi-stationary only over 10-30 ms. (Key point: Characteristics of speech) ## Related in Wireless Communication - [Speech characteristics; formant and channel vocoder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-formant-channel-vocoder) — Wireless Communication, chapter 6 - [Characteristics of speech signal](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-characteristics) — Wireless Communication, chapter 6 - [Need for speech coding; vocoder concept](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-coding-need-vocoder) — Wireless Communication, chapter 6, asked 2× - [Speech characteristics and linear predictive coder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-lpc) — Wireless Communication, chapter 6, asked 5× - [Operation of a vocoder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Avocoder-operation) — Wireless Communication, chapter 6 - [GSM signal processing (speech to radio)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-signal-processing) — Wireless Communication, chapter 8, asked 2×