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Compression reduces the file size of data so that it takes up less storage space and can be transmitted more quickly. OCR J277 Section 2.6 requires you to understand why compression is needed and the difference between lossy and lossless compression, including specific techniques such as Huffman coding and run-length encoding (RLE).
| Benefit | Explanation |
|---|---|
| Reduced storage | Compressed files take up less space on a hard drive or SSD |
| Faster transfer | Smaller files can be sent over a network more quickly |
| Lower bandwidth | Less network capacity is needed to transmit compressed data |
| Cost savings | Less storage and bandwidth reduces costs |
Lossy compression permanently removes some data from the file to achieve a smaller size. The removed data cannot be recovered — the original file cannot be perfectly reconstructed.
Lossy compression removes data that is considered least important or least noticeable to human perception:
| Advantage | Disadvantage |
|---|---|
| Much smaller file sizes | Some quality is lost permanently |
| Good for media (photos, music, video) | Cannot recover the original data |
| Adjustable quality/size trade-off | Not suitable for text, code, or data where accuracy is critical |
| Format | Type | Used for |
|---|---|---|
| JPEG | Image | Photographs |
| MP3 | Audio | Music |
| MP4 / H.264 | Video | Video streaming |
| AAC | Audio | iTunes, streaming |
OCR Exam Tip: Lossy compression is suitable for media files where small quality reductions are acceptable. It is NOT suitable for text documents, program files, or medical/scientific data where every bit matters.
Lossless compression reduces file size without losing any data. The original file can be perfectly reconstructed from the compressed version.
| Advantage | Disadvantage |
|---|---|
| No data is lost — perfect reconstruction | File sizes are not reduced as much as lossy |
| Suitable for all file types | Still larger than lossy compressed files |
| Essential for text, code, and important data | Compression and decompression may be slower |
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