---
title: "Meaning of lossy compression"
subject: "Multimedia System"
chapter: 5
key: "semester_8/multimedia_system:lossy-compression-meaning"
source: https://www.bctengineer.com/q/semester_8/multimedia_system/lossy-compression-meaning
site: BCT Engineering
---

# Meaning of lossy compression

Multimedia System · Chapter 5: Data Compression

- Asked in: 2076 Bhadra, 4 marks
- Video: [Lossy vs Lossless Compression | A Beginner’s Guide](https://www.youtube.com/watch?v=X88vxU2o4f0) — Kinsta
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/multimedia_system/lossy-compression-meaning
- Whole chapter: https://www.bctengineer.com/md/semester_8/multimedia_system/chapter-5

## Question

What do you mean by lossy compression?

## Short answer

Lossy compression permanently discards some original information, so decompressed data is an approximation, not identical. It exploits human perception limits and uses quantization to achieve high compression ratios. Examples include JPEG, MP3, and MPEG; it is unsuitable where exactness is required.

## Answer

Lossy compression is a data compression method in which some of the original information is permanently discarded, so the decompressed data is not identical to the original but is close enough for the intended use. It gives much higher compression ratios than lossless methods.

**How it works**
- It removes information that humans barely notice: fine colour detail in images (we are less sensitive to colour than brightness), high-frequency sounds hidden by louder ones, or small differences between video frames.
- The main lossy step is **quantization**: values are rounded to fewer levels, so many small values become zero and compress well.
- The loss is irreversible; the removed detail cannot be restored.

**Examples**
- JPEG for images (DCT, then quantization).
- MP3 and AAC for audio (psychoacoustic model).
- MPEG and H.264 for video.

**Advantages:** very small files, fast transmission, suitable for streaming.
**Disadvantages:** quality drops, especially at high compression, and repeated compression adds artefacts such as blocking in JPEG. It is unsuitable for text, program code and data where exactness is required.

## Check yourself

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

1. What is lossy compression?
   A. Compression where the data is recovered exactly
   B. Compression that permanently discards some information, giving data close but not identical to the original
   C. Compression that only works on text
   D. Encryption of the data
   Answer: B. The discarded detail cannot be restored, but results are close enough for the intended use. (Key point: Definition)

2. What is the main lossy step?
   A. Huffman coding
   B. Run-length coding
   C. Copying
   D. Quantization, which rounds values to fewer levels so many become zero
   Answer: D. Quantization produces many zeros which compress well; the loss is irreversible. (Key point: Main lossy step)

3. For which data is lossy compression unsuitable?
   A. Streaming video
   B. Text and program code where exactness is required
   C. Music files
   D. Photographs for the web
   Answer: B. Exactness is needed for text and code; repeated lossy compression also adds artefacts like JPEG blocking. (Key point: Suitability)

## Related in Multimedia System

- [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
- [Data compression techniques](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Adata-compression-techniques) — Multimedia System, chapter 5, asked 2×

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