---
title: "Data compression techniques"
subject: "Multimedia System"
chapter: 5
key: "semester_8/multimedia_system:data-compression-techniques"
source: https://www.bctengineer.com/q/semester_8/multimedia_system/data-compression-techniques
site: BCT Engineering
---

# Data compression techniques

Multimedia System · Chapter 5: Data Compression

- Asked in: 2071 Bhadra, 4 marks; 2072 Asoj, 6 marks (asked 2 times)
- Video: [Compression Techniques | Multimedia](https://www.youtube.com/watch?v=lOEgwB6SxlE) — bcakgcasofficial
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/multimedia_system/data-compression-techniques
- Whole chapter: https://www.bctengineer.com/md/semester_8/multimedia_system/chapter-5

## Question

What do you mean by data compression? Explain the data compression techniques (methods).

## Short answer

Data compression encodes data with fewer bits to save storage and bandwidth. Lossless methods (RLE, Huffman, arithmetic, LZW) recover the original exactly; lossy methods (DPCM, DCT/wavelet, vector quantization, perceptual coding) discard information; hybrid methods like JPEG and MPEG combine both.

## Answer

Data compression is the process of encoding data using fewer bits than the original representation, to reduce storage space and transmission time. Decompression restores the data.

#### Classification of techniques

**1. Lossless compression:** the original data is recovered exactly. Used for text, programs, medical images.
- **Entropy encoding (statistical):**
  - *Run-length encoding (RLE):* a run of repeated symbols is replaced by the symbol and its count. "AAAABBB" becomes 4A3B.
  - *Huffman coding:* frequent symbols get short codes, rare symbols get long codes.
  - *Arithmetic coding:* the entire message is encoded as one number in the range 0 to 1.
- **Dictionary based:** LZW and LZ77 replace repeated strings with dictionary references. Used in GIF, ZIP.

**2. Lossy compression:** some information is permanently discarded, which gives much higher compression. Used for images, audio, video.
- *Prediction based:* DPCM, delta modulation store differences between predicted and actual values.
- *Transform based:* DCT or wavelet transform concentrates energy in few coefficients, then small ones are quantized away. Used in JPEG.
- *Vector quantization:* blocks of samples are replaced by the nearest code word from a codebook.
- *Perceptual coding:* uses psychoacoustic or psychovisual models to drop what humans cannot notice (MP3).

**3. Hybrid compression:** combines both, for example JPEG (DCT + quantization + RLE + Huffman) and MPEG (motion compensation + DCT + Huffman).

| Standard | Media | Type |
|---|---|---|
| JPEG | Still images | Lossy (hybrid) |
| MPEG-1/2/4 | Video and audio | Lossy (hybrid) |
| H.261/H.263 | Video conferencing | Lossy |
| MP3 | Audio | Lossy |
| ZIP, GIF, PNG | General, images | Lossless |

## Check yourself

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

1. What is data compression?
   A. Encoding data using fewer bits than the original to save storage and transmission time
   B. Encrypting data so others cannot read it
   C. Copying data to several disks
   D. Increasing the bit depth of data
   Answer: A. Compression reduces the bits needed; decompression restores the data. (Key point: Meaning)

2. Which technique is lossless?
   A. DCT with quantization
   B. Vector quantization
   C. Perceptual coding (MP3)
   D. Huffman coding
   Answer: D. Huffman, RLE, arithmetic and LZW are lossless entropy/dictionary methods; the others discard information. (Key point: Lossless vs lossy classes)

3. How is 'AAAABBB' encoded with RLE?
   A. A4B3 where A4 means the letters A and 4 total
   B. 4A3B
   C. AB
   D. 7AB
   Answer: B. Each run is replaced by its count and the symbol: 4A3B. (Key point: Run-length encoding)

4. What is hybrid compression?
   A. Using only lossless entropy coding
   B. Using only a dictionary
   C. Combining lossy and lossless steps, e.g. JPEG (DCT + quantization + RLE + Huffman)
   D. Compressing the same file twice with ZIP
   Answer: C. JPEG and MPEG combine transform/quantization with entropy coding. (Key point: Hybrid compression)

## Related in Multimedia System

- [Need for data compression](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Aneed-for-data-compression) — Multimedia System, chapter 5
- [Meaning of lossy compression](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Alossy-compression-meaning) — Multimedia System, chapter 5
- [Lossless versus lossy compression](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Alossy-vs-lossless) — Multimedia System, chapter 5
- [Why lossy compression is preferred](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Awhy-lossy-preferred) — Multimedia System, chapter 5

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