--- title: "Vocoders and their types" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:vocoders-types" source: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoders-types site: BCT Engineering --- # Vocoders and their types Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2081 Bhadra, 3+5 marks; 2073 Magh, 2+3 marks (asked 2 times) - Video: [Vocoder and it's type chapter6 part2](https://www.youtube.com/watch?v=E6am20hAzyo) — IOE Electronics and communication engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoders-types - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question Describe vocoders with block diagram. Briefly explain different kind of vocoders. ## Short answer A vocoder extracts parameters of a speech production model (vocal-tract filter, excitation, pitch, gain) and transmits them for synthesis at low bit rates. Types include channel vocoder using filter banks, formant vocoder tracking resonances, cepstrum vocoder, voice-excited vocoder, and LPC which uses an all-pole filter, enabling rates as low as 2.4 kbps. ## Answer #### Vocoders A **vocoder** (voice coder) is a source coder that does not try to reproduce the speech waveform. It extracts the parameters of a model of human speech production – vocal-tract filter, excitation type (voiced/unvoiced), pitch and gain – sends only these, and the receiver synthesizes speech from them. This gives very low bit rates (about 1.2–4.8 kbps) at the cost of natural quality. ```text Analyzer (Tx) speech -+-> [spectral analysis] ----+ +-> [pitch, V/UV detector] -+-> MUX -> channel Synthesizer (Rx) channel -> DEMUX -+-> pitch, V/UV | -> [pulse gen | noise gen] | | +-> spectrum ->[synthesis filter] | v speech ``` The model: speech = excitation (pulse train for voiced, random noise for unvoiced) passed through a time-varying filter (vocal tract). #### Kinds of vocoders 1. **Channel vocoder** – A bank of band-pass filters (about 16–19 bands) measures the energy in each band; the band envelopes plus pitch/V-UV are sent. The receiver modulates a matching filter bank with the excitation. Earliest vocoder. 2. **Formant vocoder** – Sends the frequencies and amplitudes of the first three or four **formants** (vocal-tract resonances) plus pitch. Very low rate (< 1.2 kbps) but formants are hard to track accurately. 3. **Cepstrum vocoder** – Uses the cepstrum (inverse FFT of log spectrum) to separate the excitation (high quefrency) from the vocal-tract response (low quefrency); low cepstral coefficients and pitch are sent. 4. **Voice-excited vocoder** – Sends a narrowband (baseband) portion of the actual speech as excitation instead of a pitch detector output, avoiding pitch-detection errors. 5. **Linear predictive coder (LPC)** – Models the vocal tract as an all-pole filter with ~10 coefficients; LPC-10 runs at 2.4 kbps. Its improved forms (multi-pulse, RELP, CELP) are used in GSM and IS-95. **Example of bit rate saving:** 64 kbps PCM vs 2.4 kbps LPC-10 is a reduction of about 27 times, which is why vocoder principles (in the form of hybrid coders like CELP) are used in mobile systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a vocoder send? A. Model parameters of speech production, not the waveform B. Compressed waveform samples C. Only the pitch D. The raw PCM bits Answer: A. A vocoder sends vocal-tract filter, excitation type, pitch and gain, giving 1.2-4.8 kbps. (Key point: Vocoder principle) 2. What does the channel vocoder transmit about the spectrum? A. Formant frequencies B. Cepstral coefficients C. Narrowband speech baseband D. Energy envelopes of 16-19 band-pass filter outputs plus pitch and V/UV Answer: D. It measures band energies with a filter bank and sends the envelopes. (Key point: Channel vocoder) 3. How does the cepstrum vocoder separate excitation from vocal tract? A. Excitation is at low quefrency, tract at high B. Excitation is at high quefrency, tract at low quefrency C. By using formant trackers D. By band-pass filtering Answer: B. In the cepstrum the vocal-tract response is at low quefrency and excitation at high quefrency. (Key point: Cepstrum vocoder) 4. What is different about the voice-excited vocoder? A. It uses formant tracking B. It uses 64 kbps PCM C. It sends no excitation D. It sends a baseband portion of real speech as excitation, avoiding pitch-detection errors Answer: D. Using actual narrowband speech as excitation avoids pitch detector errors. (Key point: Voice-excited vocoder) ## Related in Wireless Communication - [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× - [OFDM transmitter side, advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-advantages) — Wireless Communication, chapter 4 - [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4 - [Digital vs analog modulation; GMSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adigital-modulation-gmsk) — Wireless Communication, chapter 4 - [MSK, GMSK and OFDM modulator/demodulator](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-ofdm-modem) — Wireless Communication, chapter 4 - [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — Wireless Communication, chapter 5 ## Related in other subjects - [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14× - [Block diagram of digital communication system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ablock-diagram-digital-comm) — Data Communication, chapter 1, asked 8× - [Communication system and block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomm-system-block-diagram) — Basic Electronics Engineering, chapter 5, asked 7× - [DMA controller block diagram and transfer](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adma-controller) — Computer Organization and Architecture, chapter 7, asked 6×