---
title: "Operation of a vocoder"
subject: "Wireless Communication"
chapter: 6
key: "semester_7/wireless_communication:vocoder-operation"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoder-operation
site: BCT Engineering
---

# Operation of a vocoder

Wireless Communication · Chapter 6: Speech and Channel Coding

- Asked in: 2081 Baisakh, 6 marks
- Video: [Voice Coders ( Vocoder )](https://www.youtube.com/watch?v=EJa6rh3c2cE) — Networking Made Easy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoder-operation
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6

## Question

With the help of a block diagram, explain the operation of a vocoder.

## Short answer

A vocoder analyzes 10–30 ms frames into vocal-tract parameters, V/UV flag, pitch and gain, then multiplexes them. The receiver selects pulse or noise excitation, scales by gain, and passes it through the synthesis filter to recreate speech, achieving 1.2–4.8 kbps with synthetic quality.

## Answer

A **vocoder** is a speech coder that analyses speech into the parameters of a speech-production model and transmits only those parameters. The receiver synthesizes speech from them, giving bit rates of about 1.2–4.8 kbps.

**Speech model:** excitation source (pulse train at the pitch frequency for voiced sounds, random noise for unvoiced sounds) → time-varying vocal-tract filter → speech.

```text
 ANALYZER (transmitter)
 speech->[frame]-+->[vocal tract analysis]-> a_k
                 +->[V/UV decision]      -> flag
                 +->[pitch estimator]    -> T0
                 +->[energy measure]     -> G
           a_k, flag, T0, G -> quantize -> MUX

 SYNTHESIZER (receiver)
 [pulse gen @ T0]--+
                   +-[V/UV]-(×G)->[vocal tract
 [noise gen]-------+               filter a_k]->speech
```

**Operation**

1. **Framing:** sampled speech is cut into short frames (10–30 ms) in which it is nearly stationary.
2. **Spectral analysis:** the vocal-tract response is estimated, as band energies (channel vocoder), formant frequencies (formant vocoder) or predictor coefficients (LPC vocoder).
3. **Excitation analysis:** a detector decides whether the frame is voiced or unvoiced and measures the pitch period.
4. **Gain:** the frame energy is computed.
5. These parameters are quantized, multiplexed and transmitted (e.g. LPC-10: 2.4 kbps).
6. **Synthesis:** at the receiver the V/UV flag selects either the pulse generator (at the received pitch) or the noise generator. The excitation is scaled by the gain and passed through a filter set by the received spectral parameters to recreate speech.

**Merits:** very low bit rate, good intelligibility, suits secure and narrow channels.
**Demerits:** synthetic quality, sensitive to pitch errors and background noise.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is speech divided into frames of 10-30 ms in a vocoder?
   A. The ear needs 10 ms delay
   B. Speech is nearly stationary over such short segments
   C. The channel only carries 20 ms bursts
   D. To reduce the sampling rate
   Answer: B. The vocal tract changes slowly, so parameters are stable within a frame. (Key point: Why frames)

2. At the receiver, what selects between the pulse generator and the noise generator?
   A. The gain G
   B. The pitch value alone
   C. The voiced/unvoiced flag
   D. The synthesis filter
   Answer: C. The V/UV flag chooses a pulse train at pitch for voiced frames or noise for unvoiced frames. (Key point: Excitation selection)

3. What is the purpose of the gain parameter?
   A. Scales the excitation to match frame energy
   B. Sets the filter order
   C. Selects voiced or unvoiced
   D. Sets the sampling rate
   Answer: A. Frame energy is measured at the analyzer and the excitation is scaled by G at the synthesizer. (Key point: Gain role)

4. Which is a demerit of vocoders?
   A. Very high bit rate
   B. Poor intelligibility always
   C. Sensitive to pitch errors and background noise, with synthetic quality
   D. Cannot be secured
   Answer: C. Pitch errors and noise degrade the model-based synthesis, and quality is synthetic. (Key point: Vocoder demerit)

## 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
- [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — Wireless Communication, chapter 5
- [Self jamming, FHMA and hybrid SS techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aself-jamming-fhma-hybrid) — Wireless Communication, chapter 7

## 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×
- [Data logger block diagram and operation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adl-block) — Instrumentation II, chapter 5, asked 6×
- [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4×
- [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3×