--- title: "Speech characteristics; formant and channel vocoder" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:speech-formant-channel-vocoder" source: https://www.bctengineer.com/q/semester_7/wireless_communication/speech-formant-channel-vocoder site: BCT Engineering --- # Speech characteristics; formant and channel vocoder Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2082 Baisakh, 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/speech-formant-channel-vocoder - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question What are the characteristics of speech signals? Explain the basic working principle of formant and channel vocoder. ## Short answer Speech is 300–3400 Hz, quasi-stationary, with formants and voiced/unvoiced segments. A channel vocoder sends band energies from 16–19 filters plus pitch and V/UV; a formant vocoder sends three or four formant frequencies plus pitch and V/UV, giving below 1.2 kbps but difficult formant tracking. ## Answer #### 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. ```text 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. | Point | Channel vocoder | Formant vocoder | |---|---|---| | Spectrum sent as | Band energies (16–19) | 3–4 formant frequencies | | Bit rate | ~2.4 kbps | < 1.2 kbps | | Main difficulty | Many channels | Accurate formant tracking | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the channel vocoder get each band envelope? A. Rectifying and low-pass filtering the band-pass filter output B. By tracking formants C. Taking the FFT of the speech D. Using a pitch detector Answer: A. Each BPF output is rectified and low-pass filtered to give a slowly varying spectral envelope. (Key point: Channel vocoder envelope) 2. How does the channel vocoder receiver synthesize speech? A. Resonators tuned to formants B. Delaying the input C. Driving a filter bank with pulses or noise, scaling each band by its envelope and summing D. Subtracting the bands Answer: C. An identical filter bank is excited by pulse/noise and scaled per band by the received envelopes. (Key point: Channel vocoder receiver) 3. What does the formant vocoder transmit? A. Band energies of 16-19 channels B. Frequencies and amplitudes of the first 3-4 formants plus pitch and V/UV C. Predictor coefficients only D. The speech waveform Answer: B. Formants describe the spectrum with few parameters, giving a rate below 1.2 kbps. (Key point: Formant vocoder parameters) 4. Why is the formant vocoder rarely used in practice? A. Its bit rate is too high B. It cannot represent pitch C. It needs a training sequence D. Formants are hard to track accurately, especially in noise Answer: D. Accurate formant tracking is difficult, particularly with noise. (Key point: Formant vocoder limitation) ## Related in Wireless Communication - [Formant vocoder and speech characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aformant-vocoder-speech) — 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 in coder design](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-characteristics-coder-design) — Wireless Communication, chapter 6, asked 3× - [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× - [Speech characteristics and 7-bit Hamming code](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-hamming-code) — 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 ## Related in other subjects - [N-channel JFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ajfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9× - [n-channel E-MOSFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3An-emosfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9× - [N-channel enhancement MOSFET construction, working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-n-channel-enhancement) — Basic Electronics Engineering, chapter 3, asked 9× - [N-channel depletion MOSFET construction, working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-n-channel-depletion) — Basic Electronics Engineering, chapter 3, asked 9×