Compression: Lossy and Lossless Techniques

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).

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Why Compress Data?

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

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Lossy Compression

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.

How It Works

Lossy compression removes data that is considered least important or least noticeable to human perception:

• In images (JPEG): removes fine colour details that the human eye cannot easily distinguish
• In audio (MP3): removes frequencies that the human ear cannot easily hear (very high/low frequencies)
• In video (MP4): removes details between similar frames

Advantages and Disadvantages

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

Examples of Lossy Formats

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.

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Lossless Compression

Lossless compression reduces file size without losing any data. The original file can be perfectly reconstructed from the compressed version.

Advantages and Disadvantages

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

Examples of Lossless Formats