---
title: "Vocoder and predictive coders"
subject: "Wireless Communication"
chapter: 6
key: "semester_7/wireless_communication:vocoder-predictive-coders"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoder-predictive-coders
site: BCT Engineering
---

# Vocoder and predictive coders

Wireless Communication · Chapter 6: Speech and Channel Coding

- Asked in: 2070 Bhadra, 2+6 marks
- Video: [W 1.11  Parametric coding -- Channel Vocoder and LPC Vocoder](https://www.youtube.com/watch?v=uC2j6p-wySU) — Ramanikumar animations -Telecom and Wireless
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/vocoder-predictive-coders
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6

## Question

What is vocoder? Explain any two predictive coders.

## Short answer

A vocoder analyses speech into model parameters (vocal-tract filter, excitation type, pitch, gain) and transmits only these for low-bit-rate synthetic speech. Two predictive coders are Multi-pulse excited LPC (MPE-LPC), which uses several analysis-by-synthesis pulses per sub-frame, and Code-excited LPC (CELP), which selects the best excitation from a codebook minimizing weighted error.

## Answer

#### Vocoder

A **vocoder** (voice coder) is a speech source coder that analyses speech into the parameters of a speech-production model – vocal-tract filter, excitation type (voiced/unvoiced), pitch and gain – and transmits only these. The receiver synthesizes speech from them. Bit rates are very low (1.2–4.8 kbps) but quality is synthetic. Examples: channel, formant, cepstrum and LPC vocoders.

#### Predictive coders

Predictive coders use **linear prediction**: each sample is estimated from past samples, ŝ(n) = Σₖ₌₁ᵖ aₖ s(n − k), and the vocal tract is modelled by the all-pole filter 1/A(z). Two widely used predictive coders:

**1. Multi-pulse excited LPC (MPE-LPC)**

```text
 [pulse generator]->[1/A(z)]-> s^(n) --(-)<-- s(n)
        ^                               |
        +--[error minimisation]<-[weighting filter]
```

- LPC coefficients are computed for each frame as usual.
- Instead of a single pitch pulse or noise, the excitation has several pulses per sub-frame (e.g. 4–8 per 5 ms).
- Pulse **positions and amplitudes** are found one at a time by **analysis-by-synthesis**, minimising the perceptually weighted error between original and synthesized speech.
- No voiced/unvoiced decision or pitch detection errors; good quality at about 9.6 kbps. GSM's RPE-LTP is a variant with regularly spaced pulses.

**2. Code-excited LPC (CELP)**

```text
 [codebook index i]->(×G)->[pitch predictor]->[1/A(z)]
                                                 |
                       s(n) --(-)<--- s^(n) -----+
                               |
                [perceptual weighting] -> choose i, G
```

- A **codebook** of, say, 1024 stored random excitation vectors is kept at both ends.
- For each 5 ms sub-frame the encoder tries the codevectors through a long-term (pitch) predictor and short-term LPC filter, and selects the one giving minimum weighted error.
- Only the **codebook index**, gain, pitch parameters and LPC coefficients are sent.
- High quality at 4.8–8 kbps; used in IS-95 (QCELP), FS-1016 (4.8 kbps) and ACELP (GSM EFR, AMR).

(A third type, **residual excited LPC (RELP)**, sends the low-pass filtered prediction residual as excitation.)

## Check yourself

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

1. What does a vocoder transmit?
   A. The speech waveform samples
   B. Parameters of a speech-production model such as filter, voiced/unvoiced, pitch and gain
   C. Only the codebook
   D. Only the signal envelope in analogue form
   Answer: B. Vocoders send model parameters, giving very low bit rates but synthetic quality. (Key point: Vocoder definition)

2. How are the pulses in multi-pulse excited LPC chosen?
   A. By a voiced/unvoiced decision
   B. By analysis-by-synthesis, minimising the perceptually weighted error
   C. Randomly from a codebook
   D. By low-pass filtering the residual
   Answer: B. Pulse positions and amplitudes are found one at a time to minimise weighted error. (Key point: MPE-LPC excitation)

3. Which set is sent by a CELP coder?
   A. Codebook index, gain, pitch parameters and LPC coefficients
   B. Only LPC coefficients
   C. Only pulse positions
   D. Formant frequencies
   Answer: A. The codebook is stored at both ends so only the index and parameters are sent. (Key point: CELP parameters sent)

4. What is an advantage of MPE-LPC over basic LPC vocoder?
   A. It needs no analysis at the encoder
   B. No voiced/unvoiced decision or pitch detection errors
   C. It works at 1.2 kbps
   D. It uses no filter
   Answer: B. Several optimised pulses replace the single pitch pulse or noise, avoiding V/UV errors. (Key point: Advantage of MPE)

## Related in Wireless Communication

- [Source coders used in speech coding](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-source-coders) — Wireless Communication, chapter 6