---
title: "Need for speech coding; vocoder concept"
subject: "Wireless Communication"
chapter: 6
key: "semester_7/wireless_communication:speech-coding-need-vocoder"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/speech-coding-need-vocoder
site: BCT Engineering
---

# Need for speech coding; vocoder concept

Wireless Communication · Chapter 6: Speech and Channel Coding

- Asked in: 2078 Chaitra, 2+6 marks; 2070 Magh, 4+4 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/speech-coding-need-vocoder
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6

## Question

Why do we need speech coding techniques? Explain the basic concept of VOCODER.

## Short answer

Speech coding compresses digitized speech to save scarce radio spectrum, increase capacity, reduce power and allow encryption and channel coding. A vocoder is a parametric coder that transmits only vocal-tract filter parameters, voiced/unvoiced decision, pitch and gain, typically at 1.2–4.8 kbps, giving synthetic quality.

## Answer

#### Need for speech coding

**Speech coding** compresses digitized speech into fewer bits while keeping acceptable quality. It is needed because:

- Plain 64 kbps PCM uses too much of the scarce, expensive **radio spectrum**; low-rate coders (e.g. 13 kbps GSM, 8 kbps CELP) let more users share a channel and raise **system capacity**.
- Lower bit rate means narrower bandwidth, less transmit power and longer **battery life**.
- Coders can include protection of important bits and work well with **channel coding**, keeping quality high over noisy, fading links.
- It allows **voice activity detection** and discontinuous transmission, reducing interference.
- Digital speech can be **encrypted** for privacy and multiplexed easily with data.

#### Basic concept of VOCODER

A **vocoder** is a parametric speech coder based on a model of how humans produce speech:

- **Source (excitation):** the vocal cords produce a periodic pulse train for voiced sounds (pitch 80–350 Hz) or turbulent noise for unvoiced sounds.
- **Filter:** the vocal tract (throat, mouth, nose) acts as a slowly varying filter whose resonances are the formants.

Instead of sending the waveform, the vocoder analyses each 10–30 ms frame and sends only:

1. Vocal-tract filter parameters (band energies, formants or LPC coefficients),
2. Voiced/unvoiced decision,
3. Pitch period, and
4. Gain (loudness).

```text
 Tx: speech -> [analysis] -> {filter params, V/UV,
                              pitch, gain} -> encode
 Rx: pitch pulses --+
                    +--[V/UV]--(×gain)--[vocal tract
     noise ---------+                    filter]--> speech
```

The receiver uses a pulse generator or noise generator as the excitation and drives a synthesis filter set by the received parameters.

**Waveform coder vs vocoder**

| Point | Waveform coder | Vocoder |
|---|---|---|
| What is sent | Samples of waveform | Model parameters |
| Bit rate | 16–64 kbps | 1.2–4.8 kbps |
| Quality | Natural | Synthetic |
| Example | PCM, ADPCM | Channel, LPC-10 |

**Result:** rates of 1.2–4.8 kbps, good intelligibility but synthetic quality. Types include channel, formant, cepstrum, voice-excited and linear predictive vocoders.

## Check yourself

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

1. What is the main reason speech coding is needed in mobile systems?
   A. To make speech sound synthetic
   B. To save scarce spectrum so more users share a channel
   C. To increase transmit power
   D. To remove silence from the language
   Answer: B. Lower bit rates than 64 kbps PCM use less bandwidth and raise capacity. (Key point: Why code speech)

2. In the vocoder model, what is the source for voiced sounds?
   A. Turbulent noise
   B. The vocal tract filter
   C. A periodic pulse train at the pitch
   D. A constant DC level
   Answer: C. Vocal cords give a pulse train for voiced sounds and turbulent noise for unvoiced; the vocal tract is the filter. (Key point: Source-filter model)

3. Which set of parameters does the vocoder send per frame?
   A. Filter parameters, V/UV decision, pitch period and gain
   B. Raw samples and gain
   C. Only formants
   D. Only pitch and silence flag
   Answer: A. Those four sets let the receiver synthesize speech. (Key point: Parameters sent)

4. How does a vocoder compare with a waveform coder?
   A. Higher bit rate and natural quality
   B. Same rate, lower complexity
   C. Lower bit rate (1.2-4.8 kbps) but synthetic quality
   D. Higher bit rate, synthetic quality
   Answer: C. Vocoders send parameters, giving very low rates but synthetic quality. (Key point: Waveform vs vocoder)

## 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
- [Formant vocoder and speech characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aformant-vocoder-speech) — Wireless Communication, chapter 6
- [Source coders used in speech coding](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-source-coders) — Wireless Communication, chapter 6
- [Need for equalization and basic technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aequalization-need-basic) — Wireless Communication, chapter 5
- [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5
- [Waveform/voice coding and GSM codec](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Awaveform-voice-coding-gsm-codec) — Wireless Communication, chapter 6

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